Enterobacteriaceae & Related Organisms
PATHOGENS BOTH WITHIN & OUTSIDE THE ENTERIC TRACT
PATHOGENS PRIMARILY WITHIN THE ENTERIC TRACT
PATHOGENS OUTSIDE THE ENTERIC TRACT
Bacteroides & Prevotella
Summaries of Organisms
Practice Questions: USMLE & Course Examinations
Gram-negative rods are a large group of diverse organisms (see Figures 18–1, 18–2, and 19–1). In this book, these bacteria are subdivided into three clinically relevant categories, each in a separate chapter, according to whether the organism is related primarily to the enteric or the respiratory tract or to animal sources (Table 18–1). Although this approach leads to some overlap, it should be helpful because it allows general concepts to be emphasized.
FIGURE 18–1 Escherichia coli—Gram stain. Arrow points to a gram-negative rod. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
FIGURE 18–2 Vibrio cholerae—Gram stain. Long arrow points to a curved gram-negative rod. Arrowhead points to a flagellum at one end of a curved gram-negative rod. (Figure courtesy of Public Health Image Library, Centers for Disease Control and Prevention.)
Gram-negative rods related to the enteric tract include a large number of genera. These genera have therefore been divided into three groups depending on the major anatomic location of disease, namely, (1) pathogens both within and outside the enteric tract, (2) pathogens primarily within the enteric tract, and (3) pathogens outside the enteric tract (Table 18–1).
TABLE 18–1 Categories of Gram-Negative Rods
The frequency with which the organisms related to the enteric tract cause disease in the United States is shown in Table 18–2. Salmonella, Shigella, and Campylobacter are frequent pathogens in the gastrointestinal tract, whereas Escherichia, Vibrio, and Yersinia are less so. Enterotoxigenic strains of Escherichia coli are a common cause of diarrhea in developing countries but are less common in the United States. The medically important gram-negative rods that cause diarrhea are described in Table 18–3. Urinary tract infections are caused primarily by E. coli; the other organisms occur less commonly. The medically important gram-negative rods that cause urinary tract infections are described in Table 18–4.
TABLE 18–2 Frequency of Diseases Caused in the United States by Gram-Negative Rods Related to the Enteric Tract
TABLE 18–3 Gram-Negative Rods Causing Diarrhea
TABLE 18–4 Gram-Negative Rods Causing Urinary Tract Infection1 or Sepsis2
Patients infected with such enteric pathogens as Shigella, Salmonella, Campylobacter, and Yersinia have a high incidence of certain autoimmune diseases such as Reiter’s syndrome (see Chapter 66). In addition, infection with Campylobacter jejuni predisposes to Guillain-Barré syndrome.
Before describing the specific organisms, it is appropriate to describe the family Enterobacteriaceae, to which many of these gram-negative rods belong.
ENTEROBACTERIACEAE & RELATED ORGANISMS
The Enterobacteriaceae is a large family of gram-negative rods found primarily in the colon of humans and other animals, many as part of the normal flora. These organisms are the major facultative anaerobes in the large intestine but are present in relatively small numbers compared with anaerobes such as Bacteroides. Although the members of the Enterobacteriaceae are classified together taxonomically, they cause a variety of diseases with different pathogenetic mechanisms. The organisms and some of the diseases they cause are listed in Table 18–5.
TABLE 18–5 Diseases Caused by Members of the Enterobacteriaceae
Features common to all members of this heterogeneous family are their anatomic location and the following four metabolic processes: (1) they are all facultative anaerobes; (2) they all ferment glucose (fermentation of other sugars varies); (3) none have cytochrome oxidase (i.e., they are oxidase-negative); and (4) they reduce nitrates to nitrites as part of their energy-generating processes.
These four reactions can be used to distinguish the Enterobacteriaceae from another medically significant group of organisms—the nonfermenting gram-negative rods, the most important of which is Pseudomonas aeruginosa.1
P. aeruginosa, a significant cause of urinary tract infection and sepsis in hospitalized patients, does not ferment glucose or reduce nitrates and is oxidase-positive. In contrast to the Enterobacteriaceae, it is a strict aerobe and derives its energy from oxidation, not fermentation.
All members of the Enterobacteriaceae, being gram-negative, contain endotoxin in their cell walls. In addition, several exotoxins are produced (e.g., E. coli and Vibrio cholerae secrete exotoxins, called enterotoxins, that activate adenylate cyclase within the cells of the small intestine, causing diarrhea) (see Chapter 7).
The antigens of several members of the Enterobacteriaceae, especially Salmonella and Shigella, are important; they are used for identification purposes both in the clinical laboratory and in epidemiologic investigations. The three surface antigens are as follows:
(1) The cell wall antigen (also known as the somatic, or O, antigen) is the outer polysaccharide portion of the lipopolysaccharide (see Figure 2–6). The O antigen, which is composed of repeating oligosaccharides consisting of three or four sugars repeated 15 or 20 times, is the basis for the serologic typing of many enteric rods. The number of different O antigens is very large (e.g., there are approximately 1500 types of Salmonella and 150 types of E. coli).
(2) The H antigen is on the flagellar protein. Only flagellated organisms, such as Escherichia and Salmonella, have H antigens, whereas the nonmotile ones, such as Klebsiella and Shigella, do not. The H antigens of certain Salmonella species are unusual because the organisms can reversibly alternate between two types of H antigens called phase 1 and phase 2. The organisms may use this change in antigenicity to evade the immune response.
(3) The capsular or K polysaccharide antigen is particularly prominent in heavily encapsulated organisms such as Klebsiella. The K antigen is identified by the quellung (capsular swelling) reaction in the presence of specific antisera and is used to serotype E. coli and Salmonella typhi for epidemiologic purposes. In S. typhi, the cause of typhoid fever, it is called the Vi (or virulence) antigen.
Specimens suspected of containing members of the Enterobacteriaceae and related organisms are usually inoculated onto two media, a blood agar plate and a selective differential medium such as MacConkey’s agar or eosin–methylene blue (EMB) agar. The differential ability of these latter media is based on lactose fermentation, which is the most important metabolic criterion used in the identification of these organisms (Table 18–6). On these media, the non–lactose fermenters (e.g., Salmonella and Shigella) form colorless colonies, whereas the lactose fermenters (e.g., E. coli) form colored colonies. On EMB agar, E. coli colonies have a characteristic green sheen. The selective effect of the media in suppressing unwanted gram-positive organisms is exerted by bile salts or bacteriostatic dyes in the agar.
TABLE 18–6 Lactose Fermentation by Members of the Enterobacteriaceae and Related Organisms
An additional set of screening tests, consisting of triple sugar iron (TSI) agar and urea agar, is performed prior to the definitive identification procedures. The rationale for the use of these media and the reactions of several important organisms are presented in the box titled “Agar Media for Enteric Gram-Negative Rods” (see later in the chapter) and in Table 18–7. The results of the screening process are often sufficient to identify the genus of an organism; however, an array of 20 or more biochemical tests is required to identify the species.
TABLE 18–7 Triple Sugar Iron (TSI) Agar Reactions
AGAR MEDIA FOR ENTERIC GRAM-NEGATIVE RODS
Triple Sugar Iron (TSI) Agar
The important components of this medium are ferrous sulfate and the three sugars glucose, lactose, and sucrose. Glucose is present in one-tenth the concentration of the other two sugars. The medium in the tube has a solid, poorly oxygenated area on the bottom, called the butt, and an angled, well-oxygenated area on top, called the slant. The organism is inoculated into the butt and across the surface of the slant.
The interpretation of the test results is as follows: (1) If lactose (or sucrose) is fermented, a large amount of acid is produced, which turns the phenol red indicator yellow both in the butt and on the slant. Some organisms generate gases, which produce bubbles in the butt. (2) If lactose is not fermented but the small amount of glucose is, the oxygen-deficient butt will be yellow, but on the slant, the acid will be oxidized to CO2 and H2O by the organism and the slant will be red (neutral or alkaline). (3) If neither lactose nor glucose is fermented, both the butt and the slant will be red. The slant can become a deeper red-purple (more alkaline) as a result of the production of ammonia from the oxidative deamination of amino acids. (4) If H2S is produced, the black color of ferrous sulfide is seen.
The reactions of some of the important organisms are presented in Table 18–7. Because several organisms can give the same reaction, TSI agar is only a screening device.
The important components of this medium are urea and the pH indicator phenol red. If the organism produces urease, the urea is hydrolyzed to NH3 and CO2. Ammonia turns the medium alkaline, and the color of the phenol red changes from light orange to reddish purple. The important organisms that are urease-positive are Proteus species and K. pneumoniae.
Another valuable piece of information used to identify some of these organisms is their motility, which is dependent on the presence of flagella. Proteus species are very motile and characteristically swarm over the blood agar plate, obscuring the colonies of other organisms. Motility is also an important diagnostic criterion in the differentiation of Enterobacter cloacae, which is motile, from Klebsiella pneumoniae, which is nonmotile.
If the results of the screening tests suggest the presence of a Salmonella or Shigella strain, an agglutination test can be used to identify the genus of the organism and to determine whether it is a member of group A, B, C, or D.
Coliforms & Public Health
Contamination of the public water supply system by sewage is detected by the presence of coliforms in the water. In a general sense, the term coliform includes not only E. coli, but also other inhabitants of the colon such as Enterobacter and Klebsiella. However, because only E. coli is exclusively a large intestine organism, whereas the others are found in the environment also, it is used as the indicator of fecal contamination. In water quality testing, E. coli is identified by its ability to ferment lactose with the production of acid and gas, its ability to grow at 44.5°C, and its characteristic colony type on EMB agar. An E. coli colony count above 4/dL in municipal drinking water is indicative of unacceptable fecal contamination. Because E. coli and the enteric pathogens are killed by chlorination of the drinking water, there is rarely a problem with meeting this standard. Disinfection of the public water supply is one of the most important advances of public health in the twentieth century.
The appropriate treatment for infections caused by members of the Enterobacteriaceae and related organisms must be individually tailored to the antibiotic sensitivity of the organism. Generally speaking, a wide range of antimicrobial agents are potentially effective (e.g., some penicillins and cephalosporins, aminoglycosides, chloramphenicol, tetracyclines, quinolones, and sulfonamides). The specific choice usually depends on the results of antibiotic sensitivity tests.
Note that many isolates of these enteric gram-negative rods are highly antibiotic resistant because of the production of β-lactamases and other drug-modifying enzymes. These organisms undergo conjugation frequently, at which time they acquire plasmids (R factors) that mediate multiple drug resistance. For example, plasmid-encoded New Delhi metallo-β-lactamase causes resistance to penicillins, cephalosporins, monobactams, and carbapenems.
PATHOGENS BOTH WITHIN & OUTSIDE THE ENTERIC TRACT
E. coli is the most common cause of urinary tract infection and gram-negative rod sepsis. It is one of the two important causes of neonatal meningitis and the agent most frequently associated with “traveler’s diarrhea,” a watery diarrhea. Some strains of E. coli are enterohemorrhagic and cause bloody diarrhea.
E. coli is a straight gram-negative rod (Figure 18–1), in contrast to the curved gram-negative rods of the genera Vibrio, Campylobacter, and Helicobacter.
E. coli is the most abundant facultative anaerobe in the colon and feces. It is, however, greatly outnumbered by the obligate anaerobes such as Bacteroides.
E. coli ferments lactose, a property that distinguishes it from the two major intestinal pathogens, Shigella and Salmonella. It has three antigens that are used to identify the organism in epidemiologic investigations: the O, or cell wall, antigen; the H, or flagellar, antigen; and the K, or capsular, antigen. Because there are more than 150 O, 50 H, and 90 K antigens, the various combinations result in more than 1000 antigenic types of E. coli. Specific serotypes are associated with certain diseases (e.g., O55 and O111 cause outbreaks of neonatal diarrhea).
The reservoir of E. coli includes both humans and animals. The source of the E. coli that causes urinary tract infections is the patient’s own colonic flora that colonizes the urogenital area. The source of the E. coli that causes neonatal meningitis is the mother’s birth canal; the infection is acquired during birth. In contrast, the E. coli that causes traveler’s diarrhea is acquired by ingestion of food or water contaminated with human feces. Note that the main reservoir of enterohemorrhagic E. coli O157 is cattle and the organism is acquired in undercooked meat.
E. coli has several clearly identified components that contribute to its ability to cause disease: pili, a capsule, endotoxin, and three exotoxins (enterotoxins), two that cause watery diarrhea and one that causes bloody diarrhea and hemolytic–uremic syndrome.
Intestinal Tract Infection
The first step is the adherence of the organism to the cells of the jejunum and ileum by means of pili that protrude from the bacterial surface. Once attached, the bacteria synthesize enterotoxins (exotoxins that act in the enteric tract), which act on the cells of the jejunum and ileum to cause diarrhea. The toxins are strikingly cell-specific; the cells of the colon are not susceptible, probably because they lack receptors for the toxin. Enterotoxigenic strains of E. coli can produce either or both of two enterotoxins.
(1) The heat-labile toxin (LT) acts by stimulating adenylate cyclase. Both LT and cholera toxin act by catalyzing the addition of adenosine diphosphate-ribose (a process called ADP-ribosylation) to the G protein that stimulates the cyclase. This irreversibly activates the cyclase. The resultant increase in intracellular cyclic adenosine monophosphate (AMP) concentration stimulates cyclic AMP–dependent protein kinase, which phosphorylates ion transporters in the membrane. The transporters export ions, which cause an outpouring of fluid, potassium, and chloride from the enterocytes into the lumen of the gut, resulting in watery diarrhea. Note that cholera toxin has the same mode of action.
(2) The other enterotoxin is a low-molecular-weight, heat-stable toxin (ST), which stimulates guanylate cyclase.
The enterotoxin-producing strains do not cause inflammation, do not invade the intestinal mucosa, and cause a watery, nonbloody diarrhea. However, certain strains of E. coli are enteropathic (enteroinvasive) and cause disease not by enterotoxin formation but by invasion of the epithelium of the large intestine, causing bloody diarrhea (dysentery) accompanied by inflammatory cells (neutrophils) in the stool.
Certain enterohemorrhagic strains of E. coli (i.e., those with the O157:H7 serotype) also cause bloody diarrhea by producing an exotoxin called Shiga toxin, so called because it is very similar to that produced by Shigella species. Shiga toxin acts by removing an adenine from the large (28S) ribosomal RNA, thereby stopping protein synthesis. Shiga toxin is encoded by temperate (lysogenic) bacteriophages. Shiga toxin is also called verotoxin because it has a cytopathic effect on Vero (monkey) cells in culture.
These O157:H7 strains are associated with outbreaks of bloody diarrhea following ingestion of undercooked hamburger, often at fast-food restaurants. The bacteria on the surface of the hamburger are killed by the cooking, but those in the interior, which is undercooked, survive. Also, direct contact with animals (e.g., visits to farms and petting zoos) has resulted in bloody diarrhea caused by O157:H7 strains.
Some patients with bloody diarrhea caused by O157:H7 strains also have a life-threatening complication called hemolytic–uremic syndrome (HUS), which occurs when Shiga toxin enters the bloodstream. This syndrome consists of hemolytic anemia, thrombocytopenia, and acute renal failure. The hemolytic anemia and renal failure occur because there are receptors for Shiga toxin on the surface of the endothelium of small blood vessels and on the surface of kidney epithelium. Death of the endothelial cells of small blood vessels results in a microangiopathic hemolytic anemia in which the red cells passing through the damaged area become grossly distorted (schistocytes) and then lyse. Thrombocytopenia occurs because platelets adhere to the damaged endothelial surface. Death of the kidney epithelial cells leads to renal failure. Treatment of diarrhea caused by O157:H7 strains with antibiotics, such as ciprofloxacin, increases the risk of developing HUS by increasing the amount of Shiga toxin released by the dying bacteria.
Urinary Tract Infections
Certain O serotypes of E. coli preferentially cause urinary tract infections. These uropathic strains are characterized by pili with adhesin proteins that bind to specific receptors on the urinary tract epithelium. The binding site on these receptors consists of dimers of galactose (Gal-Gal dimers). Cranberry juice contains flavonoids that inhibit the binding of pili to receptors and may be useful in the prevention of recurrent urinary tract infections. The motility of E. coli may aid its ability to ascend the urethra into the bladder and ascend the ureter into the kidney.
The other two structural components, the capsule and the endotoxin, play a more prominent role in the pathogenesis of systemic, rather than intestinal tract, disease. The capsular polysaccharide interferes with phagocytosis, thereby enhancing the organism’s ability to cause infections in various organs. For example, E. coli strains that cause neonatal meningitis usually have a specific capsular type called the K1 antigen. The endotoxin of E. coli is the cell wall lipopolysaccharide, which causes several features of gram-negative sepsis such as fever, hypotension, and disseminated intravascular coagulation.
Th-17 helper T cells that produce interleukin-17 are an important host defense against sepsis caused by enteric bacteria such as E. coli and Klebsiella. Patients infected with human immunodeficiency virus (HIV) experience a loss of Th-17 cells and are predisposed to sepsis caused by E. coli and Klebsiella.
E. coli causes a variety of diseases both within and outside the intestinal tract. The main clinical findings, the major pathogenetic factors, and the main laboratory results are described in Table 18–8.
TABLE 18–8 Clinical Aspects of Escherichia coli
(1) Clinical findings within the intestinal tract:
Diarrhea caused by enterotoxigenic E. coli is usually watery, nonbloody, self-limited, and of short duration (1–3 days). It is frequently associated with travel (traveler’s diarrhea, or “turista”).2
Infection with enterohemorrhagic E. coli (EHEC), on the other hand, results in a dysentery-like syndrome characterized by bloody diarrhea, abdominal cramping, and fever similar to that caused by Shigella.
The O157:H7 strains of E. coli also cause bloody diarrhea, which can be complicated by HUS. This syndrome is characterized by kidney failure, hemolytic anemia, and thrombocytopenia. The hemolytic anemia is caused by exotoxin-induced capillary damage, which results in damage to the red cells as they pass through the capillaries. These distorted, fragmented red cells called schistocytes can be seen on blood smear and are characteristic of a microangiopathic hemolytic anemia. In 2011, an outbreak of diarrhea and HUS in Germany was caused by a Shiga toxin–producing strain of E. coli that was typed as O104:H4, not O157:H7.
HUS occurs particularly in children who have been treated with fluoroquinolones or other antibiotics for their diarrhea. For this reason, antibiotics should not be used to treat diarrhea caused by EHEC.
(2) Clinical findings outside of the intestinal tract:
E. coli is the leading cause of community-acquired urinary tract infections. These infections occur primarily in women; this finding is attributed to three features that facilitate ascending infection into the bladder, namely, a short urethra, the proximity of the urethra to the anus, and colonization of the vagina by members of the fecal flora. It is also the most frequent cause of nosocomial (hospital-acquired) urinary tract infections, which occur equally frequently in both men and women and are associated with the use of indwelling urinary catheters. Urinary tract infections can be limited to the bladder or extend up the collecting system to the kidneys. If only the bladder is involved, the disease is called cystitis, whereas infection of the kidney is called pyelonephritis. The most prominent symptoms of cystitis are pain (dysuria) and frequency of urination; patients are usually afebrile. Pyelonephritis is characterized by fever, flank pain, and costovertebral angle tenderness; dysuria and frequency may or may not occur.
E. coli is also a major cause, along with the group B streptococci, of meningitis and sepsis in neonates. Exposure of the newborn to E. coli and group B streptococci occurs during birth as a result of colonization of the vagina by these organisms in approximately 25% of pregnant women. E. coli is the organism isolated most frequently from patients with hospital-acquired sepsis, which arises primarily from urinary, biliary, or peritoneal infections. Peritonitis is usually a mixed infection caused by E. coli or other facultative enteric gram-negative rod plus anaerobic members of the colonic flora such as Bacteroides and Fusobacterium.
Specimens suspected of containing enteric gram-negative rods, such as E. coli, are grown initially on a blood agar plate and on a differential medium, such as EMB agar or MacConkey’s agar. E. coli, which ferments lactose, forms pink colonies, whereas lactose-negative organisms are colorless. On EMB agar, E. coli colonies have a characteristic green sheen. Some of the important features that help distinguish E. coli from other lactose-fermenting gram-negative rods are as follows: (1) it produces indole from tryptophan, (2) it decarboxylates lysine, (3) it uses acetate as its only source of carbon, and (4) it is motile. E. coli O157:H7 does not ferment sorbitol, which serves as an important criterion that distinguishes it from other strains of E. coli. The isolation of enterotoxigenic or enteropathogenic E. coli from patients with diarrhea is not a routine diagnostic procedure.
Treatment of E. coli infections depends on the site of disease and the resistance pattern of the specific isolate. For example, an uncomplicated lower urinary tract infection (cystitis) can be treated using oral trimethoprim-sulfamethoxazole or nitrofurantoin. Pyelonephritis can be treated with ciprofloxacin or ceftriaxone. However, E. coli sepsis requires treatment with parenteral antibiotics (e.g., a third-generation cephalosporin, such as cefotaxime, with or without an aminoglycoside, such as gentamicin). For the treatment of neonatal meningitis, a combination of ampicillin and cefotaxime is usually given. Antibiotic therapy is usually not indicated in E. coli diarrheal diseases. However, administration of trimethoprim-sulfamethoxazole or loperamide (Imodium) may shorten the duration of symptoms. Rehydration is typically all that is necessary in this self-limited disease.
There is no specific prevention for E. coli infections, such as active or passive immunization. However, various general measures can be taken to prevent certain infections caused by E. coli and other organisms. For example, the incidence of urinary tract infections can be lowered by the judicious use and prompt withdrawal of catheters and, in recurrent infections, by prolonged prophylaxis with urinary antiseptic drugs (e.g., nitrofurantoin or trimethoprim-sulfamethoxazole). The use of cranberry juice to prevent recurrent urinary tract infections appears to be based on the ability of flavonoids in the juice to inhibit the binding of the pili of the uropathic strains of E. coli to the bladder epithelium rather than to acidification of the urine, which was the previous explanation.
Some cases of sepsis can be prevented by prompt removal of or switching the site of intravenous lines. Traveler’s diarrhea can sometimes be prevented by the prophylactic use of doxycycline, ciprofloxacin, trimethoprim-sulfamethoxazole, or Pepto-Bismol. Ingestion of uncooked foods and unpurified water should be avoided while traveling in certain countries.
Salmonella species cause enterocolitis, enteric fevers such as typhoid fever, and septicemia with metastatic infections such as osteomyelitis. They are one of the most common causes of bacterial enterocolitis in the United States.
Salmonellae are gram-negative rods that do not ferment lactose but do produce H2S—features that are used in their laboratory identification. Their antigens—cell wall O, flagellar H, and capsular Vi (virulence)—are important for taxonomic and epidemiologic purposes. The O antigens, which are the outer polysaccharides of the cell wall, are used to subdivide the salmonellae into groups A–I. There are two forms of the H antigens, phases 1 and 2. Only one of the two H proteins is synthesized at any one time, depending on which gene sequence is in the correct alignment for transcription into mRNA. The Vi antigens (capsular polysaccharides) are antiphagocytic and are an important virulence factor for S. typhi, the agent of typhoid fever. The Vi antigens are also used for the serotyping of S. typhi in the clinical laboratory.
There are three methods for naming the salmonellae. Ewing divides the genus into three species: S. typhi, Salmonella choleraesuis, and Salmonella enteritidis. In this scheme there is one serotype in each of the first two species and 1500 serotypes in the third. Kaufman and White assign different species names to each serotype; there are roughly 1500 different species, usually named for the city in which they were isolated. Salmonella dublin according to Kaufman and White would be S. enteritidis serotype dublin according to Ewing. The third approach to naming the salmonellae is based on relatedness determined by DNA hybridization analysis. In this scheme, S. typhi is not a distinct species but is classified as Salmonella enterica serotype (or serovar) typhi. All three of these naming systems are in current use.
Clinically, the Salmonella species are often thought of in two distinct categories, namely, the typhoidal species (i.e., those that cause typhoid fever) and the nontyphoidal species (i.e., those that cause diarrhea [enterocolitis] and metastatic infections, such as osteomyelitis). The typhoidal species are S. typhi and S. paratyphi. The nontyphoidal species are the many serotypes of S. enterica. Of the serotypes, S. enterica serotype choleraesuis is the species most often involved in metastatic infections.
Pathogenesis & Epidemiology
The three types of Salmonella infections (enterocolitis, enteric fevers, and septicemia) have different pathogenic features.
(1) Enterocolitis is characterized by an invasion of the epithelial and subepithelial tissue of the small and large intestines. Strains that do not invade do not cause disease. The organisms penetrate both through and between the mucosal cells into the lamina propria, with resulting inflammation and diarrhea. Neutrophils limit the infection to the gut and the adjacent mesenteric lymph nodes; bacteremia is infrequent in enterocolitis. In contrast to Shigella enterocolitis, in which the infectious dose is very small (on the order of 100 organisms), the dose of Salmonella required is much higher, at least 100,000 organisms. Various properties of salmonellae and shigellae are compared in Table 18–9. Gastric acid is an important host defense; gastrectomy or use of antacids lowers the infectious dose significantly.
TABLE 18–9 Comparison of Important Features of Salmonella and Shigella
(2) In typhoid and other enteric fevers, infection begins in the small intestine, but few gastrointestinal symptoms occur. The organisms enter, multiply in the mononuclear phagocytes of Peyer’s patches, and then spread to the phagocytes of the liver, gallbladder, and spleen. This leads to bacteremia, which is associated with the onset of fever and other symptoms, probably caused by endotoxin. Survival and growth of the organism within phagosomes in phagocytic cells are a striking feature of this disease, as is the predilection for invasion of the gallbladder, which can result in establishment of the carrier state and excretion of the bacteria in the feces for long periods.
(3) Septicemia accounts for only about 5% to 10% of Salmonella infections and occurs in one of two settings: a patient with an underlying chronic disease, such as sickle cell anemia or cancer, or a child with enterocolitis. The septic course is more indolent than that seen with many other gram-negative rods. Bacteremia results in the seeding of many organs, with osteomyelitis, pneumonia, and meningitis as the most common sequelae. Osteomyelitis in a child with sickle cell anemia is an important example of this type of salmonella infection. Previously damaged tissues, such as infarcts and aneurysms, especially aortic aneurysms, are the most frequent sites of metastatic abscesses. Salmonella are also an important cause of vascular graft infections.
The epidemiology of Salmonella infections is related to the ingestion of food and water contaminated by human and animal wastes. S. typhi, the cause of typhoid fever, is transmitted only by humans, but all other species have a significant animal as well as human reservoir. Human sources are either persons who temporarily excrete the organism during or shortly after an attack of enterocolitis or chronic carriers who excrete the organism for years. The most frequent animal source is poultry and eggs, but meat products that are inadequately cooked have been implicated as well. Dogs and other pets, including turtles, snakes, lizards, and iguanas, are additional sources.
After an incubation period of 12 to 48 hours, enterocolitis begins with nausea and vomiting and then progresses to abdominal pain and diarrhea, which can vary from mild to severe, with or without blood. Usually the disease lasts a few days, is self-limited, causes nonbloody diarrhea, and does not require medical care except in the very young and very old. HIV-infected individuals, especially those with a low CD4 count, have a much greater number of Salmonella infections, including more severe diarrhea and more serious metastatic infections than those who are not infected with HIV. Salmonella typhimurium is the most common species of Salmonella to cause enterocolitis in the United States, but almost every species has been involved.
In typhoid fever, caused by S. typhi, and in enteric fever, caused by organisms such as S. paratyphi A, B, and C (S. paratyphi B and C are also known as Salmonella schottmuelleri and Salmonella hirschfeldii, respectively), the onset of illness is slow, with fever and constipation rather than vomiting and diarrhea predominating. Diarrhea may occur early but usually disappears by the time the fever and bacteremia occur. After the first week, as the bacteremia becomes sustained, high fever, delirium, tender abdomen, and enlarged spleen occur. Rose spots (i.e., rose-colored macules on the abdomen) are associated with typhoid fever but occur only rarely. Leukopenia and anemia are often seen. Liver function tests are often abnormal, indicating hepatic involvement.
The disease begins to resolve by the third week, but severe complications such as intestinal hemorrhage or perforation can occur. About 3% of typhoid fever patients become chronic carriers. The carrier rate is higher among women, especially those with previous gallbladder disease and gallstones.
Septicemia is most often caused by S. choleraesuis. The symptoms begin with fever but little or no enterocolitis and then proceed to focal symptoms associated with the affected organ, frequently bone, lung, or meninges.
In enterocolitis, the organism is most easily isolated from a stool sample. However, in the enteric fevers, a blood culture is the procedure most likely to reveal the organism during the first 2 weeks of illness. Bone marrow cultures are often positive. Stool cultures may also be positive, especially in chronic carriers in whom the organism is secreted in the bile into the intestinal tract.
Salmonellae form non–lactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. On TSI agar, an alkaline slant and an acid butt, frequently with both gas and H2S (black color in the butt), are produced. S. typhi is the major exception; it does not form gas and produces only a small amount of H2S. If the organism is urease-negative (Proteus organisms, which can produce a similar reaction on TSI agar, are urease-positive), the Salmonella isolate can be identified and grouped by the slide agglutination test into serogroup A, B, C, D, or E based on its O antigen. Definitive serotyping of the O, H, and Vi antigens is performed by special public health laboratories for epidemiologic purposes.
Salmonellosis is a notifiable disease, and an investigation to determine its source should be undertaken. In certain cases of enteric fever and sepsis, when the organism is difficult to recover, the diagnosis can be made serologically by detecting a rise in antibody titer in the patient’s serum (Widal test).
Enterocolitis caused by Salmonella is usually a self-limited disease that resolves without treatment. Fluid and electrolyte replacement may be required. Antibiotic treatment does not shorten the illness or reduce the symptoms; in fact, it may prolong excretion of the organisms, increase the frequency of the carrier state, and select mutants resistant to the antibiotic. Antimicrobial agents are indicated only for neonates or persons with chronic diseases who are at risk for septicemia and disseminated abscesses. Plasmid-mediated antibiotic resistance is common, and antibiotic sensitivity tests should be done. Drugs that retard intestinal motility (i.e., that reduce diarrhea) appear to prolong the duration of symptoms and the fecal excretion of the organisms.
The treatment of choice for enteric fevers such as typhoid fever and septicemia with metastatic infection is either ceftriaxone or ciprofloxacin. Ampicillin or ciprofloxacin should be used in patients who are chronic carriers of S. typhi. Cholecystectomy may be necessary to abolish the chronic carrier state. Focal abscesses should be drained surgically when feasible.
Salmonella infections are prevented mainly by public health and personal hygiene measures. Proper sewage treatment, a chlorinated water supply that is monitored for contamination by coliform bacteria, cultures of stool samples from food handlers to detect carriers, handwashing prior to food handling, pasteurization of milk, and proper cooking of poultry, eggs, and meat are all important.
Two vaccines are available, but they confer limited (50%–80%) protection against S. typhi. One contains the Vi capsular polysaccharide of S. typhi (given intramuscularly), and the other contains a live, attenuated strain (Ty21a) of S. typhi (given orally). The two vaccines are equally effective. The vaccine is recommended for those who will travel or reside in high-risk areas and for those whose occupation brings them in contact with the organism. A new conjugate vaccine against typhoid fever containing the capsular polysaccharide (Vi) antigen coupled to a carrier protein is safe and immunogenic in young children but is not available in the United States at this time.
PATHOGENS PRIMARILY WITHIN THE ENTERIC TRACT
Shigella species cause enterocolitis. Enterocolitis caused by Shigella is often called bacillary dysentery. The term dysentery refers to bloody diarrhea.
Shigellae are non–lactose-fermenting, gram-negative rods that can be distinguished from salmonellae by three criteria: they produce no gas from the fermentation of glucose, they do not produce H2S, and they are nonmotile. All shigellae have O antigens (polysaccharide) in their cell walls, and these antigens are used to divide the genus into four groups: A, B, C, and D.
Pathogenesis & Epidemiology
Shigellae are the most effective pathogens among the enteric bacteria. They have a very low ID50 (see page 31). Ingestion of as few as 100 organisms causes disease, whereas at least 105 V. cholerae or Salmonella organisms are required to produce symptoms. Various properties of shigellae and salmonellae are compared in Table 18–9.
Shigellosis is only a human disease (i.e., there is no animal reservoir). The organism is transmitted by the fecal–oral route. The four Fs—fingers, flies, food, and feces—are the principal factors in transmission. Foodborne outbreaks outnumber waterborne outbreaks by 2 to 1. Outbreaks occur in day care nurseries and in mental hospitals, where fecal–oral transmission is likely to occur. Children younger than 10 years account for approximately half of Shigella-positive stool cultures. There is no prolonged carrier state with Shigella infections, unlike that seen with S. typhi infections.
Shigellae, which cause disease almost exclusively in the gastrointestinal tract, produce bloody diarrhea (dysentery) by invading the cells of the mucosa of the distal ileum and colon. Local inflammation accompanied by ulceration occurs, but the organisms rarely penetrate through the wall or enter the bloodstream, unlike salmonellae. Although some strains produce an enterotoxin (called Shiga toxin), invasion is the critical factor in pathogenesis. The evidence for this is that mutants that fail to produce enterotoxin but are invasive can still cause disease, whereas noninvasive mutants are nonpathogenic. Shiga toxins are encoded by lysogenic bacteriophages. Shiga toxins very similar to those produced by Shigella are produced by enterohemorrhagic E. coli O157:H7 strains that cause enterocolitis and HUS.
After an incubation period of 1 to 4 days, symptoms begin with fever and abdominal cramps, followed by diarrhea, which may be watery at first but later contains blood and mucus. The disease varies from mild to severe depending on two major factors: the species of Shigella and the age of the patient, with young children and elderly people being the most severely affected. Shigella dysenteriae, which causes the most severe disease, is usually seen in the United States only in travelers returning from abroad. Shigella sonnei, which causes mild disease, is isolated from approximately 75% of all individuals with shigellosis in the United States. The diarrhea frequently resolves in 2 or 3 days; in severe cases, antibiotics can shorten the course. Serum agglutinins appear after recovery but are not protective because the organism does not enter the blood. The role of intestinal IgA in protection is uncertain.
Shigellae form non–lactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. On TSI agar, they cause an alkaline slant and an acid butt, with no gas and no H2S. Confirmation of the organism as Shigella and determination of its group are done by slide agglutination.
One important adjunct to laboratory diagnosis is a methylene blue stain of a fecal sample to determine whether neutrophils are present. If they are found, an invasive organism such as Shigella, Salmonella, or Campylobacter is involved rather than a toxin-producing organism such as V. cholerae, E. coli, or Clostridium perfringens. (Certain viruses also cause diarrhea without neutrophils in the stool.)
The main treatment for shigellosis is fluid and electrolyte replacement. In mild cases, no antibiotics are indicated. In severe cases, a fluoroquinolone (e.g., ciprofloxacin) is the drug of choice, but the incidence of plasmids conveying multiple drug resistance is high enough that antibiotic sensitivity tests must be performed. Trimethoprim-sulfamethoxazole is an alternative choice. Antiperistaltic drugs are contraindicated in shigellosis, because they prolong the fever, diarrhea, and excretion of the organism.
Prevention of shigellosis is dependent on interruption of fecal–oral transmission by proper sewage disposal, chlorination of water, and personal hygiene (handwashing by food handlers). There is no vaccine, and prophylactic antibiotics are not recommended.
V. cholerae, the major pathogen in this genus, is the cause of cholera. Vibrio parahaemolyticus causes diarrhea associated with eating raw or improperly cooked seafood. Vibrio vulnificus causes cellulitis and sepsis. Important features of pathogenesis by V. cholerae, C. jejuni, and Helicobacter pylori are described in Table 18–10.
TABLE 18–10 Important Features of Pathogenesis by Curved Gram-Negative Rods Affecting the Gastrointestinal Tract
Vibrios are curved, comma-shaped, gram-negative rods (Figure 18–2). V. cholerae is divided into two groups according to the nature of its O cell wall antigen. Members of the O1 group cause epidemic disease, whereas non-O1 organisms either cause sporadic disease or are nonpathogens. The O1 organisms have two biotypes, called classic and El Tor, and three serotypes, called Ogawa, Inaba, and Hikojima. (Biotypes are based on differences in biochemical reactions, whereas serotypes are based on antigenic differences.) These features are used to characterize isolates in epidemiologic investigations. Serogroup O139 organisms, which caused a major epidemic in 1992, are identified by their reaction to antisera to the O139 polysaccharide antigens (O antigen).
V. parahaemolyticus and V. vulnificus are marine organisms; they live primarily in the ocean, especially in warm salt water. They are halophilic (i.e., they require a high NaCl concentration to grow).
1. Vibrio cholerae
Pathogenesis & Epidemiology
V. cholerae is transmitted by fecal contamination of water and food, primarily from human sources. Human carriers are frequently asymptomatic and include individuals who are either in the incubation period or convalescing. The main animal reservoirs are marine shellfish, such as shrimp and oysters. Ingestion of these without adequate cooking can transmit the disease.
A major epidemic of cholera, which spanned the 1960s and 1970s, began in Southeast Asia and spread over three continents to areas of Africa, Europe, and the rest of Asia. A pandemic of cholera began in Peru in 1991 and has spread to many countries in Central and South America. The organism isolated most frequently was the El Tor biotype of O1 V. cholerae, usually of the Ogawa serotype. The factors that predispose to epidemics are poor sanitation, malnutrition, overcrowding, and inadequate medical services. Quarantine measures failed to prevent the spread of the disease because there were many asymptomatic carriers. In 1992, V. cholerae serogroup O139 emerged and caused a widespread epidemic of cholera in India and Bangladesh.
The pathogenesis of cholera is dependent on colonization of the small intestine by the organism and secretion of enterotoxin. For colonization to occur, large numbers of bacteria must be ingested because the organism is particularly sensitive to stomach acid. Persons with little or no stomach acid, such as those taking antacids or those who have had gastrectomy, are much more susceptible. Adherence to the cells of the brush border of the gut, which is a requirement for colonization, is related to secretion of the bacterial enzyme mucinase, which dissolves the protective glycoprotein coating over the intestinal cells.
After adhering, the organism multiplies and secretes an enterotoxin called choleragen (cholera toxin). This exotoxin can reproduce the symptoms of cholera even in the absence of the Vibrio organisms. The mode of action of cholera toxin is described in the next paragraph and in Figure 7–3 in the chapter on bacterial pathogenesis.
Choleragen consists of an A (active) subunit and a B (binding) subunit. The B subunit, which is a pentamer composed of five identical proteins, binds to a ganglioside receptor on the surface of the enterocyte. The A subunit is inserted into the cytosol, where it catalyzes the addition of ADP-ribose to the Gs protein (Gs is the stimulatory G protein). This locks the Gs protein in the “on” position, which causes the persistent stimulation of adenylate cyclase. The resulting overproduction of cyclic AMP activates cyclic AMP–dependent protein kinase, an enzyme that phosphorylates ion transporters in the cell membrane, resulting in the loss of water and ions from the cell. The watery efflux enters the lumen of the gut, resulting in a massive watery diarrhea that contains neither neutrophils nor red blood cells. Morbidity and death are due to dehydration and electrolyte imbalance. However, if treatment is instituted promptly, the disease runs a self-limited course in up to 7 days.
The genes for cholera toxin and other virulence factors are carried on a single-stranded DNA bacteriophage called CTX. Lysogenic conversion of non–toxin-producing strains to toxin-producing ones can occur when the CTX phage transduces these genes. The pili that attach the organism to the gut mucosa are the receptors for the phage.
Non-O1 V. cholerae is an occasional cause of diarrhea associated with eating shellfish obtained from the coastal waters of the United States.
Watery diarrhea in large volumes is the hallmark of cholera. There are no red blood cells or white blood cells in the stool. Rice-water stool is the term often applied to the nonbloody effluent. There is no abdominal pain, and subsequent symptoms are referable to the marked dehydration. The loss of fluid and electrolytes leads to cardiac and renal failure. Acidosis and hypokalemia also occur as a result of loss of bicarbonate and potassium in the stool. The mortality rate without treatment is 40%.
The approach to laboratory diagnosis depends on the situation. During an epidemic, a clinical judgment is made and there is little need for the laboratory. In an area where the disease is endemic or for the detection of carriers, a variety of selective media3 that are not in common use in the United States are used in the laboratory.
For diagnosis of sporadic cases in this country, a culture of the diarrhea stool containing V. cholerae will show colorless colonies on MacConkey’s agar because lactose is fermented slowly. The organism is oxidase-positive, which distinguishes it from members of the Enterobacteriaceae. On TSI agar, an acid slant and an acid butt without gas or H2S are seen because the organism ferments sucrose. A presumptive diagnosis of V. cholerae can be confirmed by agglutination of the organism by polyvalent O1 or non-O1 antiserum. A retrospective diagnosis can be made serologically by detecting a rise in antibody titer in acute- and convalescent-phase sera.
Treatment consists of prompt, adequate replacement of water and electrolytes, either orally or intravenously. Glucose is added to the solution to enhance the uptake of water and electrolytes. Antibiotics such as tetracycline are not necessary, but they do shorten the duration of symptoms and reduce the time of excretion of the organisms.
Prevention is achieved mainly by public health measures that ensure a clean water and food supply. The vaccine, composed of killed organisms, has limited usefulness; it is only 50% effective in preventing disease for 3 to 6 months and does not interrupt transmission. A live vaccine is available in certain countries but not in the United States. Neither the killed nor the live vaccine is recommended for routine use in travelers. The use of tetracycline for prevention is effective in close contacts but cannot prevent the spread of a major epidemic. Prompt detection of carriers is important in limiting outbreaks.
2. Vibrio parahaemolyticus
V. parahaemolyticus is a marine organism transmitted by ingestion of raw or undercooked seafood, especially shellfish such as oysters. It is a major cause of diarrhea in Japan, where raw fish is eaten in large quantities, but is an infrequent pathogen in the United States, although several outbreaks have occurred aboard cruise ships in the Caribbean. Little is known about its pathogenesis, except that an enterotoxin similar to choleragen is secreted and limited invasion sometimes occurs.
The clinical picture caused by V. parahaemolyticus varies from mild to quite severe watery diarrhea, nausea and vomiting, abdominal cramps, and fever. The illness is self-limited, lasting about 3 days. V. parahaemolyticus is distinguished from V. cholerae mainly on the basis of growth in NaCl: V. parahaemolyticus grows in 8% NaCl solution (as befits a marine organism), whereas V. cholerae does not. No specific treatment is indicated, because the disease is relatively mild and self-limited. Disease can be prevented by proper refrigeration and cooking of seafood.
3. Vibrio vulnificus
V. vulnificus is also a marine organism (i.e., it is found in warm salt waters such as the Caribbean Sea). It causes severe skin and soft tissue infections (cellulitis), especially in shellfish handlers, who often sustain skin wounds. It can also cause a rapidly fatal septicemia in immunocompromised people who have eaten raw shellfish containing the organism. Hemorrhagic bullae in the skin often occur in patients with sepsis caused by V. vulnificus. Chronic liver disease (e.g., cirrhosis) predisposes to severe infections. The recommended treatment is doxycycline.
C. jejuni is a frequent cause of enterocolitis, especially in children. C. jejuni infection is a common antecedent to Guillain-Barré syndrome. Other Campylobacter species are rare causes of systemic infection, particularly bacteremia.
Campylobacters are curved, gram-negative rods that appear either comma- or S-shaped. They are microaerophilic, growing best in 5% oxygen rather than in the 20% present in the atmosphere. C. jejuni grows well at 42°C, whereas Campylobacter intestinalis4 does not—an observation that is useful in microbiologic diagnosis.
Pathogenesis & Epidemiology
Domestic animals such as cattle, chickens, and dogs serve as a source of the organisms for humans. Transmission is usually fecal–oral. Food and water contaminated with animal feces are the major sources of human infection. Foods, such as poultry, meat, and unpasteurized milk, are commonly involved. Puppies with diarrhea are a common source for children. Human-to-human transmission occurs but is less frequent than animal-to-human transmission. C. jejuni is a major cause of diarrhea in the United States; it was recovered in 4.6% of patients with diarrhea, compared with 2.3% and 1% for Salmonella and Shigella, respectively.
Features of pathogenesis by Campylobacter are described in Table 18–10. Inflammation of the intestinal mucosa often occurs, accompanied by blood in stools. Systemic infections (e.g., bacteremia) occur most often in neonates or debilitated adults.
Enterocolitis, caused primarily by C. jejuni, begins as watery, foul-smelling diarrhea followed by bloody stools accompanied by fever and severe abdominal pain. Systemic infections, most commonly bacteremia, are caused more often by C. intestinalis. The symptoms of bacteremia (e.g., fever and malaise) are associated with no specific physical findings.
Gastrointestinal infection with C. jejuni is associated with Guillain-Barré syndrome, the most common cause of acute neuromuscular paralysis. Guillain-Barré syndrome is an autoimmune disease attributed to the formation of antibodies against C. jejuni that cross-react with antigens on neurons (see Chapter 66). Infection with Campylobacter is also associated with two other autoimmune diseases: reactive arthritis and Reiter’s syndrome. These are also described in Chapter 66.
If the patient has diarrhea, a stool specimen is cultured on a blood agar plate containing antibiotics5 that inhibit most other fecal flora.
The plate is incubated at 42°C in a microaerophilic atmosphere containing 5% oxygen and 10% carbon dioxide, which favors the growth of C. jejuni. It is identified by failure to grow at 25°C, oxidase positivity, and sensitivity to nalidixic acid. Unlike Shigella and Salmonella, lactose fermentation is not used as a distinguishing feature. If bacteremia is suspected, a blood culture incubated under standard temperature and atmospheric conditions will reveal the growth of the characteristically comma- or S-shaped, motile, gram-negative rods. Identification of the organism as C. intestinalis is confirmed by its failure to grow at 42°C, its ability to grow at 25°C, and its resistance to nalidixic acid.
Erythromycin or ciprofloxacin is used successfully in C. jejuni enterocolitis. The treatment of choice for C. intestinalis bacteremia is an aminoglycoside.
There is no vaccine or other specific preventive measure. Proper sewage disposal and personal hygiene (handwashing) are important.
Helicobacter pylori causes gastritis and peptic ulcers. Infection with H. pylori is a risk factor for gastric carcinoma and is linked to mucosal-associated lymphoid tissue (MALT) lymphomas.
Helicobacters are curved gram-negative rods similar in appearance to campylobacters, but because they differ sufficiently in certain biochemical and flagellar characteristics, they are classified as a separate genus. In particular, helicobacters are strongly urease-positive, whereas campylobacters are urease-negative.
Pathogenesis & Epidemiology
H. pylori attaches to the mucus-secreting cells of the gastric mucosa. The production of large amounts of ammonia from urea by the organism’s urease, coupled with an inflammatory response, leads to damage to the mucosa. Loss of the protective mucus coating predisposes to gastritis and peptic ulcer (Table 18–10). The ammonia also neutralizes stomach acid, allowing the organism to survive. Epidemiologically, most patients with these diseases show H. pylori in biopsy specimens of the gastric epithelium.
The natural habitat of H. pylori is the human stomach, and it is probably acquired by ingestion. However, it has not been isolated from stool, food, water, or animals. Person-to-person transmission probably occurs because there is clustering of infection within families. The rate of infection with H. pylori in developing countries is very high—a finding that is in accord with the high rate of gastric carcinoma in those countries.
MALT lymphomas are B-cell tumors located typically in the stomach, but they occur elsewhere in the gastrointestinal tract as well. H. pylori is often found in the MALT lesion, and the chronic inflammation induced by the organism is thought to stimulate B-cell proliferation and eventually a B-cell lymphoma. Antibiotic treatment directed against the organism often causes the tumor to regress.
Gastritis and peptic ulcer are characterized by recurrent pain in the upper abdomen, frequently accompanied by bleeding into the gastrointestinal tract. No bacteremia or disseminated disease occurs.
The organism can be seen on Gram-stained smears of biopsy specimens of the gastric mucosa. It can be cultured on the same media as campylobacters. In contrast to C. jejuni, H. pylori is urease-positive. Urease production is the basis for a noninvasive diagnostic test called the “urea breath” test. In this test, radiolabeled urea is ingested. If the organism is present, urease will cleave the ingested urea, radiolabeled CO2 is evolved, and the radioactivity is detected in the breath.
A test for Helicobacter antigen in the stool can be used for diagnosis and for confirmation that treatment has eliminated the organism. The presence of IgG antibodies in the patient’s serum can also be used as evidence of infection.
Treatment & Prevention
The concept that underlies the choice of drugs is to use antibiotics to eliminate Helicobacter plus a drug to reduce gastric acidity. A combination of two antibiotics is used because resistance, especially to metronidazole, has emerged. Treatment of duodenal ulcers with antibiotics (e.g., amoxicillin and metronidazole) and bismuth salts (Pepto-Bismol) results in a greatly decreased recurrence rate. Tetracycline can be used instead of amoxicillin. There is no vaccine or other specific preventive measure.
PATHOGENS OUTSIDE THE ENTERIC TRACT
These organisms are usually opportunistic pathogens that cause nosocomial infections, especially pneumonia and urinary tract infections. K. pneumoniae is an important respiratory tract pathogen outside hospitals as well.
K. pneumoniae, Enterobacter cloacae, and Serratia marcescens are the species most often involved in human infections. They are frequently found in the large intestine but are also present in soil and water. These organisms have very similar properties and are usually distinguished on the basis of several biochemical reactions and motility. K. pneumoniae has a very large polysaccharide capsule, which gives its colonies a striking mucoid appearance. S. marcescens produces red-pigmented colonies (Figure 18–3).
FIGURE 18–3 Serratia marcescens—red-pigmented colonies. Arrow points to a red-pigmented colony of S. marcescens. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Pathogenesis & Epidemiology
Of the three organisms, K. pneumoniae is most likely to be a primary, nonopportunistic pathogen; this property is related to its antiphagocytic capsule. Although this organism is a primary pathogen, patients with K. pneumoniae infections frequently have predisposing conditions such as advanced age, chronic respiratory disease, diabetes, or alcoholism. The organism is carried in the respiratory tract of about 10% of healthy people, who are prone to pneumonia if host defenses are lowered.
Enterobacter and Serratia infections are clearly related to hospitalization, especially to invasive procedures such as intravenous catheterization, respiratory intubation, and urinary tract manipulations. In addition, outbreaks of Serratia pneumonia have been associated with contamination of the water in respiratory therapy devices. Prior to the extensive use of these procedures, S. marcescens was a harmless organism most frequently isolated from environmental sources such as water.
Serratia also causes endocarditis in users of injection drugs. As with many other gram-negative rods, the pathogenesis of septic shock caused by these organisms is related to the endotoxins in their cell walls.
Urinary tract infections and pneumonia are the usual clinical entities associated with these three bacteria, but bacteremia and secondary spread to other areas such as the meninges and liver occur. It is difficult to distinguish infections caused by these organisms on clinical grounds, with the exception of pneumonia caused by Klebsiella, which produces a thick, bloody sputum (“currant-jelly” sputum) and can progress to necrosis and abscess formation.
There are two other species of Klebsiella that cause unusual human infections rarely seen in the United States. Klebsiella ozaenae is associated with atrophic rhinitis, and Klebsiella rhinoscleromatis causes a destructive granuloma of the nose and pharynx.
Organisms of this group produce lactose-fermenting (colored) colonies on differential agar such as MacConkey’s or EMB, although Serratia, which is a late lactose fermenter, can produce a negative reaction. These organisms are differentiated by the use of biochemical tests.
Because the antibiotic resistance of these organisms can vary greatly, the choice of drug depends on the results of sensitivity testing. Isolates from hospital-acquired infections are frequently resistant to multiple antibiotics. Carbapenem-resistant strains are an important cause of hospital-acquired infections and are resistant to almost all known antibiotics. An aminoglycoside (e.g., gentamicin) and a cephalosporin (e.g., cefotaxime) are used empirically until the results of testing are known. In severe Enterobacter infections, a combination of imipenem and gentamicin is often used.
Some hospital-acquired infections caused by gram-negative rods can be prevented by such general measures as changing the site of intravenous catheters, removing urinary catheters when they are no longer needed, and taking proper care of respiratory therapy devices. There is no vaccine.
These organisms primarily cause urinary tract infections, both community- and hospital-acquired.
These gram-negative rods are distinguished from other members of the Enterobacteriaceae by their ability to produce the enzyme phenylalanine deaminase. In addition, they produce the enzyme urease, which cleaves urea to form NH3 and CO2. Certain species are very motile and produce a striking swarming effect on blood agar, characterized by expanding rings (waves) of organisms over the surface of the agar (Figure 18–4).
FIGURE 18–4 Proteus species—swarming motility on blood agar. Arrowhead points to the site where Proteus bacteria were placed on the blood agar. Short arrow points to the edge of the first ring of swarming motility. Long arrow points to the edge of the second ring of swarming motility. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
The cell wall O antigens of certain strains of Proteus, such as OX-2, OX-19, and OX-K, cross-react with antigens of several species of rickettsiae. These Proteus antigens can be used in laboratory tests to detect the presence of antibodies against certain rickettsiae in patients’ serum. This test, called the Weil-Felix reaction after its originators, is being used less frequently as more specific procedures are developed.
In the past, there were four medically important species of Proteus. However, molecular studies of DNA relatedness showed that two of the four were significantly different. These species have been renamed: Proteus morganii is now Morganella morganii, and Proteus rettgeri is now Providencia rettgeri. In the clinical laboratory, these organisms are distinguished from Proteus vulgaris and Proteus mirabilis on the basis of several biochemical tests.
Pathogenesis & Epidemiology
The organisms are present in the human colon as well as in soil and water. Their tendency to cause urinary tract infections is probably due to their presence in the colon and to colonization of the urethra, especially in women. The vigorous motility of Proteus organisms may contribute to their ability to invade the urinary tract.
Production of the enzyme urease is an important feature of the pathogenesis of urinary tract infections by this group. Urease hydrolyzes the urea in urine to form ammonia, which raises the pH, producing an alkaline urine. This encourages the formation of stones (calculi) called “struvite” composed of magnesium ammonium phosphate. Stones in the urinary tract obstruct urine flow, damage urinary epithelium, and serve as a nidus for recurrent infection by trapping bacteria within the stone. Because alkaline urine also favors growth of the organisms and more extensive renal damage, treatment involves keeping the urine at a low pH.
The signs and symptoms of urinary tract infections caused by these organisms cannot be distinguished from those caused by E. coli or other members of the Enterobacteriaceae. Proteus species can also cause pneumonia, wound infections, and septicemia. P. mirabilis is the species of Proteus that causes most community- and hospital-acquired infections, but P. rettgeri is emerging as an important agent of nosocomial infections.
These organisms usually are highly motile and produce a “swarming” overgrowth on blood agar, which can frustrate efforts to recover pure cultures of other organisms. Growth on blood agar containing phenylethyl alcohol inhibits swarming, thus allowing isolated colonies of Proteus and other organisms to be obtained. They produce non–lactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. P. vulgaris and P. mirabilis produce H2S, which blackens the butt of TSI agar, whereas neither M. morganii nor P. rettgeri does. P. mirabilis is indole-negative, whereas the other three species are indole-positive—a distinction that can be used clinically to guide the choice of antibiotics. These four medically important species are urease-positive. Identification of these organisms in the clinical laboratory is based on a variety of biochemical reactions.
Most strains are sensitive to aminoglycosides and trimethoprim-sulfamethoxazole, but because individual isolates can vary, antibiotic sensitivity tests should be performed. P. mirabilis is the species most frequently sensitive to ampicillin. The indole-positive species (P. vulgaris, M. morganii, and P. rettgeri) are more resistant to antibiotics than is P. mirabilis, which is indole-negative. The treatment of choice for the indole-positive species is a cephalosporin (e.g., cefotaxime). P. rettgeri is frequently resistant to multiple antibiotics.
There are no specific preventive measures, but many hospital-acquired urinary tract infections can be prevented by prompt removal of urinary catheters.
Pseudomonas aeruginosa causes infections (e.g., sepsis, pneumonia, and urinary tract infections) primarily in patients with lowered host defenses. It also causes chronic lower respiratory tract infections in patients with cystic fibrosis, wound infections (cellulitis) in burn patients (Figure 18–5), and malignant otitis externa in diabetic patients. It is the most common cause of ventilator-associated pneumonia. (P. aeruginosa is also known as Burkholderia aeruginosa.) Pseudomonas cepacia (renamed Burkholderia cepacia) and Pseudomonas maltophilia (renamed Xanthomonas maltophilia and now called Stenotrophomonas maltophilia) also cause these infections, but much less frequently. Pseudomonas pseudomallei (also known as Burkholderia pseudomallei), the cause of melioidosis, is described in Chapter 27.
FIGURE 18–5 Cellulitis caused by Pseudomonas aeruginosa. Note the blue-green color of the pus in the burn wound infection. (Courtesy of Dr. Robert L. Sheridan.)
Pseudomonads are gram-negative rods that resemble the members of the Enterobacteriaceae but differ in that they are strict aerobes (i.e., they derive their energy only by oxidation of sugars rather than by fermentation). Because they do not ferment glucose, they are called nonfermenters, in contrast to the members of the Enterobacteriaceae, which do ferment glucose. Oxidation involves electron transport by cytochrome c (i.e., they are oxidase-positive).
Pseudomonads are able to grow in water containing only traces of nutrients (e.g., tap water), and this favors their persistence in the hospital environment. P. aeruginosa and B. cepacia have a remarkable ability to withstand disinfectants; this accounts in part for their role in hospital-acquired infections. They have been found growing in hexachlorophene-containing soap solutions, in antiseptics, and in detergents.
P. aeruginosa produces two pigments useful in clinical and laboratory diagnosis: (1) pyocyanin, which can color the pus in a wound blue, and (2) pyoverdin (fluorescein), a yellow-green pigment that fluoresces under ultraviolet light, a property that can be used in the early detection of skin infection in burn patients. In the laboratory, these pigments diffuse into the agar, imparting a blue-green color that is useful in identification. P. aeruginosa is the only species of Pseudomonas that synthesizes pyocyanin (Figure 18–6).
FIGURE 18–6 Pseudomonas aeruginosa—blue-green pigment. Blue-green pigment (pyocyanin) produced by P. aeruginosa diffuses into the agar. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Strains of P. aeruginosa isolated from cystic fibrosis patients have a prominent slime layer (glycocalyx), which gives their colonies a very mucoid appearance. The slime layer mediates adherence of the organism to mucous membranes of the respiratory tract and prevents antibody from binding to the organism.
Pathogenesis & Epidemiology
P. aeruginosa is found chiefly in soil and water, although approximately 10% of people carry it in the normal flora of the colon. It is found on the skin in moist areas and can colonize the upper respiratory tract of hospitalized patients. Its ability to grow in simple aqueous solutions has resulted in contamination of respiratory therapy and anesthesia equipment, intravenous fluids, and even distilled water.
P. aeruginosa is primarily an opportunistic pathogen that causes infections in hospitalized patients (e.g., those with extensive burns), in whom the skin host defenses are destroyed; in those with chronic respiratory disease (e.g., cystic fibrosis), in whom the normal clearance mechanisms are impaired; in those who are immunosuppressed; in those with neutrophil counts of less than 500/mL; and in those with indwelling catheters. It causes 10% to 20% of hospital-acquired infections and, in many hospitals, is the most common cause of gram-negative nosocomial pneumonia, especially ventilator-associated pneumonia.
Pathogenesis is based on multiple virulence factors: endotoxin, exotoxins, and enzymes. Its endotoxin, like that of other gram-negative bacteria, causes the symptoms of sepsis and septic shock. The best known of the exotoxins is exotoxin A, which causes tissue necrosis. It inhibits eukaryotic protein synthesis by the same mechanism as diphtheria exotoxin, namely, ADP-ribosylation of elongation factor-2. It also produces enzymes, such as elastase and proteases, that are histotoxic and facilitate invasion of the organism into the bloodstream. Pyocyanin damages the cilia and mucosal cells of the respiratory tract.
Strains of P. aeruginosa that have a “type III secretion system” are significantly more virulent than those that do not. This secretion system transfers the exotoxin from the bacterium directly into the adjacent human cell, which allows the toxin to avoid neutralizing antibody. Type III secretion systems are mediated by transport pumps in the bacterial cell membrane. Of the four exoenzymes known to be transported by this secretion system, Exo S is the one most clearly associated with virulence. Exo S has several modes of action, the most important of which is ADP-ribosylation of a Ras protein, leading to damage to the cytoskeleton.
P. aeruginosa can cause infections virtually anywhere in the body, but urinary tract infections, pneumonia (especially in cystic fibrosis patients), and wound infections (especially burns) (Figure 18–5) predominate. It is an important cause of hospital-acquired pneumonia, especially in those undergoing mechanical ventilation (ventilator-associated pneumonia). From these sites, the organism can enter the blood, causing sepsis. The bacteria can spread to the skin, where they cause black, necrotic lesions called ecthyma gangrenosum (Figure 18–7). Patients with P. aeruginosa sepsis have a mortality rate of greater than 50%. It is an important cause of endocarditis in intravenous drug users.
FIGURE 18–7 Ecthyma gangrenosum. Necrotic skin lesion caused by Pseudomonas aeruginosa. (Reproduced with permission from Wolff K, Johnson R, Saavedra A (eds): Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology. 7th ed. New York: McGraw-Hill, 2013. Copyright © 2013 by The McGraw-Hill Companies, Inc.)
Severe external otitis (malignant otitis externa) and other skin lesions (e.g., folliculitis) occur in users of swimming pools and hot tubs (hot tub folliculitis) in which the chlorination is inadequate. P. aeruginosa is the most common cause of osteochondritis of the foot in those who sustain puncture wounds through the soles of gym shoes. Corneal infections caused by P. aeruginosa are seen in contact lens users.
P. aeruginosa grows as non–lactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. It is oxidase-positive. A typical metallic sheen of the growth on TSI agar, coupled with the blue-green pigment on ordinary nutrient agar (Figure 18–6), and a fruity aroma are sufficient to make a presumptive diagnosis. The diagnosis is confirmed by biochemical reactions. Identification for epidemiologic purposes is done by bacteriophage or pyocin6 typing.
Because P. aeruginosa is resistant to many antibiotics, treatment must be tailored to the sensitivity of each isolate and monitored frequently; resistant strains can emerge during therapy. The treatment of choice is an antipseudomonal penicillin (e.g., piperacillin/tazobactam or ticarcillin/clavulanate) plus an aminoglycoside (e.g., gentamicin or amikacin). For infections caused by highly resistant strains, colistin (polymyxin E) is useful. The drug of choice for urinary tract infections is ciprofloxacin. The drug of choice for infections caused by B. cepacia and S. maltophilia is trimethoprim-sulfamethoxazole.
Prevention of P. aeruginosa infections involves keeping neutrophil counts above 500/mL, removing indwelling catheters promptly, taking special care of burned skin, and taking other similar measures to limit infection in patients with reduced host defenses.
BACTEROIDES & PREVOTELLA
Members of the genus Bacteroides are the most common cause of serious anaerobic infections (e.g., sepsis, peritonitis, and abscesses). Bacteroides fragilis is the most frequent pathogen. Prevotella melaninogenica is also an important pathogen. P. melaninogenica was formerly known as Bacteroides melaninogenicus, and both names are still encountered.
Bacteroides and Prevotella organisms are anaerobic, non–spore-forming, gram-negative rods. Of the many species of Bacteroides, two are human pathogens: B. fragilis7 and Bacteroides corrodens.
Members of the B. fragilis group are the predominant organisms in the human colon, numbering approximately 1011/g of feces, and are found in the vagina of approximately 60% of women. P. melaninogenica and B. corrodens occur primarily in the oral cavity.
Pathogenesis & Epidemiology
Because Bacteroides and Prevotella species are part of the normal flora, infections are endogenous, usually arising from a break in a mucosal surface, and are not communicable. These organisms cause a variety of infections, such as local abscesses at the site of a mucosal break, metastatic abscesses by hematogenous spread to distant organs, or lung abscesses by aspiration of oral flora.
Predisposing factors such as surgery, trauma, and chronic disease play an important role in pathogenesis. Local tissue necrosis, impaired blood supply, and growth of facultative anaerobes at the site contribute to anaerobic infections. The facultative anaerobes, such as E. coli, utilize the oxygen, thereby reducing it to a level that allows the anaerobic Bacteroides and Prevotella strains to grow. As a result, many anaerobic infections contain a mixed facultative and anaerobic flora. This has important implications for therapy; both the facultative anaerobes and the anaerobes should be treated.
The polysaccharide capsule of B. fragilis is an important virulence factor. The host response to the capsule plays a major role in abscess formation. Note also that the endotoxin of B. fragilis contains a variant lipid A that is missing one of the fatty acids and consequently is 1000-fold less active than the typical endotoxin of bacteria such as Neisseria meningitidis.
Enzymes such as hyaluronidase, collagenase, and phospholipase are produced and contribute to tissue damage. Enterotoxin-producing strain of B. fragilis can cause diarrhea in both children and adults.
The B. fragilis group of organisms is most frequently associated with intra-abdominal infections, either peritonitis or localized abscesses. Pelvic abscesses, necrotizing fasciitis, and bacteremia occur as well. Abscesses of the mouth, pharynx, brain, and lung are more commonly caused by P. melaninogenica, a member of the normal oral flora, but B. fragilis is found in about 25% of lung abscesses. In general, B. fragilis causes disease below the diaphragm, whereas P. melaninogenica causes disease above the diaphragm. Prevotella intermedia is an important cause of gingivitis, periodontitis, and dental abscess.
Bacteroides species can be isolated anaerobically on blood agar plates containing kanamycin and vancomycin to inhibit unwanted organisms. They are identified by biochemical reactions (e.g., sugar fermentations) and by production of certain organic acids (e.g., formic, acetic, and propionic acids), which are detected by gas chromatography. P. melaninogenica produces characteristic black colonies (Figure 18–8).
FIGURE 18–8 Prevotella melaninogenica—black pigmented colonies. Arrow points to a black pigmented colony of P. melaninogenica. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Members of the B. fragilis group are resistant to penicillins, first-generation cephalosporins, and aminoglycosides, making them among the most antibiotic-resistant of the anaerobic bacteria. Penicillin resistance is the result of β-lactamase production. Metronidazole is the drug of choice, with cefoxitin, clindamycin, and chloramphenicol as alternatives. Aminoglycosides are frequently combined to treat the facultative gram-negative rods in mixed infections. The drug of choice for P. melaninogenica infections is either metronidazole or clindamycin. β-Lactamase–producing strains of P. melaninogenica have been isolated from patients. Surgical drainage of abscesses usually accompanies antibiotic therapy, but lung abscesses often heal without drainage.
Prevention of Bacteroides and Prevotella infections centers on perioperative administration of a cephalosporin, frequently cefoxitin, for abdominal or pelvic surgery. There is no vaccine.
1. Your patient is a 75-year-old man with an indwelling urinary catheter following prostatectomy for prostate cancer. He now has the sudden onset of fever to 40°C, blood pressure of 70/40, and a pulse of 140. You draw several blood cultures, and the laboratory reports that all are positive for a gram-negative rod that forms red pigmented colonies. Which one of the following bacteria is the most likely cause of this infection?
(A) Escherichia coli
(B) Klebsiella pneumoniae
(C) Proteus mirabilis
(D) Pseudomonas aeruginosa
(E) Serratia marcescens
2. You’re a public health epidemiologist who is called to investigate an outbreak of bloody diarrhea in 16 people. You find that it is associated with eating rare hamburgers in a particular fast-food restaurant. A culture of the remaining uncooked hamburger grows a gram-negative rod that produces a dark purple colony on EMB agar, which is evidence that it ferments lactose. Which one of the following bacteria is the most likely cause of this outbreak?
(A) Escherichia coli
(B) Salmonella enterica
(C) Salmonella typhi
(D) Shigella dysenteriae
(E) Vibrio cholerae
3. Your patient has third-degree burns over most of his body. He was doing well until 2 days ago, when he spiked a fever, and his dressings revealed pus that had a blue-green color. Gram stain of the pus revealed a gram-negative rod that formed colorless colonies on EMB agar. Which one of the following bacteria is the most likely cause of this infection?
(A) Campylobacter jejuni
(B) Escherichia coli
(C) Haemophilus influenzae
(D) Pseudomonas aeruginosa
(E) Salmonella enterica
4. Regarding the patient in Question 3, which one of the following is the best combination of antibiotics to treat the infection?
(A) Azithromycin plus gentamicin
(B) Doxycycline plus gentamicin
(C) Metronidazole plus gentamicin
(D) Piperacillin/tazobactam plus gentamicin
(E) Vancomycin plus gentamicin
5. Regarding the members of the family Enterobacteriaceae, which one of the following is the most accurate?
(A) All members of the family are anaerobic, which means they must be cultured in the absence of oxygen.
(B) All members of the family ferment lactose, which is an important diagnostic criterion in the clinical laboratory.
(C) All members of the family have endotoxin, an important pathogenetic factor.
(D) All members of the family produce an enterotoxin, which ADP-ribosylates a G protein in human enterocytes.
6. You’re on a summer program working in a clinic in a small village in Ecuador. There is an outbreak of cholera, and your patient has massive diarrhea and a blood pressure of 70/40. Which one of the following would be the most appropriate action to take?
(A) Administer antimotility drugs to diminish the diarrhea.
(B) Administer intravenous saline to replenish volume.
(C) Administer tetracycline to kill the organism.
(E) Perform stool cultures and fecal leukocyte tests to make an accurate diagnosis.
7. Your patient is a 20-year-old woman with diarrhea. She has just returned to the United States from a 3-week trip to Peru, where she ate some raw shellfish at the farewell party. She now has severe watery diarrhea, perhaps 20 bowel movements a day, and is feeling quite weak and dizzy. Her stool is guaiac-negative, a test that determines whether there is blood in the stool. A Gram stain of the stool reveals curved gram-negative rods. Culture of the stool on MacConkey’s agar shows colorless colonies. Which one of the following bacteria is the most likely cause of this infection?
(A) Escherichia coli
(B) Helicobacter pylori
(C) Proteus mirabilis
(D) Pseudomonas aeruginosa
(E) Vibrio cholerae
8. Your patient is a 6-year-old boy with bloody diarrhea for the past 2 days accompanied by fever to 40°C and vomiting. He has a pet corn snake. Blood culture and stool culture from the boy and stool culture from the snake (taken very carefully!) revealed the same organism. The cultures grew a gram-negative rod that formed colorless colonies on EMB agar. Which one of the following bacteria is the most likely cause of this infection?
(A) Helicobacter pylori
(B) Proteus mirabilis
(C) Salmonella enterica
(D) Shigella dysenteriae
(E) Vibrio cholerae
9. Your patient is a 25-year-old woman with pain on urination and cloudy urine but no fever or flank pain. She has not been hospitalized. You think she probably has cystitis, an infection of the urinary bladder. A Gram stain of the urine reveals gram-negative rods. Culture of the urine on EMB agar shows colorless colonies, and a urease test was positive. Swarming motility was noted on the blood agar plate. Which one of the following bacteria is the most likely cause of this infection?
(A) Escherichia coli
(B) Helicobacter pylori
(C) Proteus mirabilis
(D) Pseudomonas aeruginosa
(E) Serratia marcescens
10. Your patient has abdominal pain, and a mass is discovered in the left lower quadrant. Upon laparotomy (surgical opening of the abdomen), an abscess is found. Culture of the pus revealed Bacteroides fragilis. Regarding this organism, which one of the following is the most accurate?
(A) A stage in the life cycle of B. fragilis involves forming spores in the soil.
(B) B. fragilis is an anaerobic gram-negative rod whose natural habitat is the human colon.
(C) B. fragilis produces black colonies when grown on blood agar.
(D) Pathogenesis by B. fragilis involves an exotoxin that increases cyclic AMP by ADP-ribosylation of a G protein.
(E) The toxoid vaccine should be administered to prevent disease caused by B. fragilis.
11. Regarding the patient in Question 10, which one of the following is the best antibiotic to treat the infection?
(D) Penicillin G
12. Your patient in the gastrointestinal clinic is a 50-year-old insurance salesman with what he describes as a “sour stomach” for several months. Antacids relieve the symptoms. After taking a complete history and doing a physical examination, you discuss the case with your resident, who suggests doing a urea breath test, which tests for the presence of urease. Which one of the following bacteria does the resident think is the most likely cause of the patient’s disease?
(A) Helicobacter pylori
(B) Proteus mirabilis
(C) Salmonella enterica
(D) Serratia marcescens
(E) Shigella dysenteriae
13. Your patient is a 35-year-old woman with epilepsy who had a grand-mal seizure about 2 months ago. She comes to see you now because she has been coughing up foul-smelling sputum for the past week. Chest X-ray reveals a cavity with an air-fluid level. Gram stain of the sputum reveals gram-negative rods, and culture reveals black colonies that grow on blood agar only in the absence of air. Which one of the following bacteria is the most likely cause of this infection?
(A) Bacteroides fragilis
(B) Campylobacter jejuni
(C) Klebsiella pneumoniae
(D) Prevotella melaninogenica
(E) Proteus mirabilis
SUMMARIES OF ORGANISMS
Brief summaries of the organisms described in this chapter begin on page 663. Please consult these summaries for a rapid review of the essential material.
PRACTICE QUESTIONS: USMLE & COURSE EXAMINATIONS
Questions on the topics discussed in this chapter can be found in the Clinical Bacteriology section of PART XIII: USMLE (National Board) Practice Questions starting on page 693. Also see PART XIV: USMLE (National Board) Practice Examination starting on page 731.
1The other less frequently isolated organisms in this group are members of the following genera: Achromobacter, Acinetobacter, Alcaligenes, Eikenella, Flavobacterium, Kingella, and Moraxella; see Chapter 27.
2Enterotoxigenic E. coli is the most common cause of traveler’s diarrhea, but other bacteria (e.g., Salmonella, Shigella, Campylobacter, and Vibrio species), viruses such as Norwalk virus, and protozoa such as Giardia and Cryptosporidium species are also involved.
3Media such as thiosulfate-citrate-bile salts agar or tellurite-taurocholate-gelatin are used.
4Also known as Campylobacter fetus subsp. fetus.
5For example, Skirrow’s medium contains vancomycin, trimethoprim, cephalothin, polymyxin, and amphotericin B.
6A pyocin is a type of bacteriocin produced by P. aeruginosa. Different strains produce various pyocins, which can serve to distinguish the organisms.
7B. fragilis is divided into five subspecies, the most important of which is B. fragilis subsp. fragilis. The other four subspecies are B. fragilis subsp. distasonis, ovatus, thetaiotamicron, and vulgatus. It is proper, therefore, to speak of the B. fragilis group rather than simply B. fragilis.