ACP medicine, 3rd Edition

Infectious Disease

Infections Due to Gram-Positive Bacilli

Frederick S. Southwick M.D.¹

¹Professor and Chief, Division of Infectious Diseases, and Associate Chairman, Department of Medicine, University of Florida College of Medicine

The author has no commercial relationships with manufacturers of products or providers of services discussed in this chapter.

March 2004

Diphtheria

Diphtheria derives its name from the Greek word diphtheria, meaning leather hide, which is the character of the pharyngeal membrane that is a hallmark of this disease. Once the cause of major epidemics in Europe and the United States, diphtheria was nearly eradicated by medical discoveries in the late 1800s and early 1900s. Conditions in both developing and developed countries have led to several recent outbreaks and have raised concerns that diphtheria may again become a serious public health problem.

MICROBIOLOGY

Diphtheria is caused by Corynebacterium diphtheriae, a gram-positive rod with club-shaped swellings at each end. It is nonmotile and is capable of growing on blood agar plates, causing a narrow band of hemolysis. Most strains produce a highly lethal exotoxin, whose production requires the presence of a bacteriophage that carries a specific determinant for toxin production. The organism produces characteristic metachromatic granules that stain bluish purple with methylene blue. Their snapping division results in angular and palisade arrangements of the cells on smear that frequently take on the appearance of Chinese lettering.¹

ETIOLOGY AND EPIDEMIOLOGY

Humans are the only natural host for C. diphtheriae. The organism is most commonly spread by upper respiratory tract droplets. Persons incubating the disease, those convalescing from the infection, and healthy carriers can spread the disease to others through close contact. Asymptomatic carriage can persist for years within a population. For example, in Russia the clonal strain responsible for a large diphtheria outbreak was carried asymptomatically in the population for at least 5 years before the epidemic.² Spread is more common during the colder months of the year, when people tend to be crowded indoors.³ Persons with C. diphtheriae skin lesions can also serve as reservoirs for the organism, and contamination of the environment tends to be greater from skin infection than from upper respiratory tract infection.⁴Because the organism can survive as long as 6 months in fomites and dust, these objects and particles can serve as vehicles for transmission.

Diphtheria toxoid immunization prevents the serious complications of diphtheria, alleviating the clinical manifestations of the disease by blocking the toxin's ability to enter cells. Immunization reduces local colonization of the nasopharynx with toxin-producing strains by reducing their survival advantage. In the United States, with the advent of widespread vaccine administration in the 1940s, the carrier state has dropped to very low levels, and the incidence of diphtheria has steadily declined. Before the initiation of the vaccination program, as many as 125,000 cases and 10,000 deaths were reported annually in the United States. Subsequently, the incidence declined to zero to five cases a year from 1980 to 1990. History indicates that diphtheria outbreaks occur in cycles that may include quiescent periods of up to 100 years. Therefore, the downturn in incidence may represent a normal cycle rather than herd immunity. Occasional outbreaks have been observed in Texas, Washington, and South Dakota.⁵ Two outbreaks were associated with urban alcoholics who practiced poor hygiene and lived in crowded environments; a third occurred in a Native-American community. Day care centers can also serve as a site for the spread of diphtheria. Outbreaks in Russia (15,211 cases) and the Ukraine (2,987 cases) are thought to have resulted from the decreased immunization of infants and children, as well as from waning immunity to diphtheria in older people.⁶ The resurgence of diphtheria in developed countries raises concern that reductions in the immunization of young children and the failure to revaccinate older adults could lead to a worldwide increase in the incidence of this very serious disease. The finding that only 30% of persons older than 70 years in the United States have protective antibody levels to diphtheria further emphasizes the potential for resurgence.⁷

PATHOGENESIS

1. diphtheriaeattaches to mucosal surfaces, particularly in the nasopharynx. Ocular and genital mucosae are less often infected. The skin has become an increasingly common site of infection, particularly in the United States. The diphtheria bacillus rarely invades living tissue but generally remains in superficial layers of the mucosa and skin. The major manifestations of this often serious disease result from production of a potent exotoxin. When iron concentrations are low, diphtheria bacilli possessing the corynebacteriophage containing the toxgene produce high concentrations of the toxin. This protein has a molecular weight of 62,000 and consists of two major fragments, designated A and B. Fragment B binds to a specific host cell membrane receptor, resulting in endocytosis of the entire molecule. Fragment B then forms a membrane channel that allows fragment A to enter the cell cytoplasm, block protein synthesis, and induce cell death within hours [see Figure 1].⁸Antitoxin antibody can neutralize toxin adsorbed to cells or in the extracellular fluid. However, once the toxin penetrates cells, its toxic effects are irreversible.

Figure 1. Action of Diptheria Toxin Mechanism of action of diphtheria toxin. Diphtheria toxin production is encoded by a bacteriophage carrying the tox gene that gains entry into Corynebacterium diphtheriae. The toxin consists of an A fragment and a B fragment. The B fragment binds to the receptor (heparin-binding epidermal growth factor precursor) on the cell surface, and the whole molecule is then taken into the cell by endocytosis. In the closed environment of the endosome, acidification occurs (H+). The low pH level causes the B region to unfold and form a membrane channel, allowing the A domain to pass through the membrane into the cytoplasm. The disulfide bond linking the A and B regions is reduced, and the A subunit is then freed to bind to ADP-ribosylate elongation factor-2 (EF-2). Ribosylation interferes with the ability of EF-2 to add amino acids to a peptide chain, blocking protein synthesis and causing cell death. (A—A fragment; B—B fragment; NAD—nicotinamide adenine dinucleotide; ADP—adenosine diphosphate)

In the host, the cytotoxic effects of the toxin are most marked in regions where bacterial growth is heavy and toxin concentrations are highest. Tissue necrosis is associated with an inflammatory response, leading to the formation of an adherent membrane that is greenish-gray to black and consists of fibrin, necrotic tissue, lymphocytes, polymorphonuclear leukocytes, eryth rocytes, and bacterial colonies. All cells are susceptible to the lethal effects of the toxin; however, the heart, kidneys, and nervous system are injured most often.

DIAGNOSIS

Clinical Manifestations

The incubation period for respiratory diphtheria is generally 2 to 4 days but can be as long as 7 days. Infection is usually associated with low-grade fever (approximately 38° C [100.4° F]); early in the infection, systemic complaints often are minimal. Symptoms depend on the location and duration of the infection before treatment. Concentrations of bacteria increase over time, resulting in the release of increasing amounts of the cytotoxic exotoxin. Early recognition and treatment are therefore critical for reducing the complications associated with toxin dissemination.

Respiratory tract infection

Pharyngeal infection, the most common form of diphtheria, presents as a sore throat and malaise. Symptoms may be mild in vaccinated patients, whereas unvaccinated patients tend to have more severe disease. Initially, diphtheria pharyngitis results in the development of a patchy white exudate that can be readily removed and is indistinguishable from group A streptococcal or viral pharyngitis. However, as the toxin begins to cause cell necrosis, a thin membrane begins to form that progressively thickens and spreads over the tonsils, posterior pharynx, and uvula [see Figure 2]. Initially, the membrane is white and smooth but later becomes gray, with patches of green and black necrosis. Particularly in children, the membrane can spread from the posterior wall up into the nose or down to the larynx or even the tracheobronchial tree.⁹ As the membrane spreads, it can interfere with normal airflow and lead to suffocation. Such extensive spread is generally associated with increased release of exotoxin, resulting in myocardial and neurologic complications and greatly increasing mortality. Laryngeal involvement results in hoarseness and may serve as a warning of impending respiratory compromise.¹⁰ Less often, the membrane may be limited to the nasopharynx, causing a serosanguineous nasal discharge. Such limited involvement is less likely to be associated with generalized toxicity. Patients who have no membrane, but only pharyngeal erythema, almost always have milder, uncomplicated disease. Other clinical manifestations include tachycardia, cervical adenopathy, leukocytosis, and proteinuria. Development of the classic bull neck, the result of massive adenopathy, is now uncommon.

Figure 2. Diphtheritic Membrane Diphtheritic membrane extending across the uvula in a 47-year-old woman. Neck edema also has developed.10

Cutaneous diphtheria

Although outbreaks of cutaneous diphtheria were originally described primarily in tropical areas, outbreaks over the past 3 decades have been reported in the Pacific Northwest, Midwest, and southern United States. Cutaneous infection may result in greater environmental contamination than respiratory diphtheria and may carry a greater risk of spread to others. Unlike pharyngeal diphtheria, which usually develops in the winter months, cutaneous infections tend to peak in the late summer and early fall. Most cases are reported among residents of Seattle's Skid Road district, who practice poor hygiene and often have preexisting sores.¹¹ Skin infections have also been described in young schoolchildren. The classic punched-out ulcerative lesion often found in the tropics is rare in temperate climates. When the lesion does develop, it generally begins as a pustule that progresses to an ulcer with a gray-brown membrane at its base. More commonly, diphtheria superinfects preexisting skin lesions, including traumatic breaks in the skin, insect bites, ecthyma, and impetigo. In association with C. diphtheriae, other skin pathogens, particularly Staphylococcus and Streptococcus, are found on culture. These infections tend to be indolent and are rarely associated with signs of intoxication, probably because cutaneous infections induce high levels of antitoxin antibody.

Other sites of infection

Less often, C. diphtheriae infects the conjunctiva, eye, ear, and vagina. In Seattle, ocular infections in Skid Road residents sometimes accompanied skin involvement.

Complications

Systemic complications are caused by release of the diphtheria exotoxin, which predominantly damages the heart and nervous system.

Myocarditis

Subtle evidence of myocarditis is found in up to two thirds of patients with respiratory diphtheria.¹² Clinically significant cardiac dysfunction is observed in 10% to 20% of patients. The severity of cardiac compromise correlates with the extent and severity of respiratory tract involvement. Cardiac toxicity generally occurs within 1 to 2 weeks after onset of the illness, often when pharyngeal symptoms are improving. The electrocardiogram generally reflects the severity of myocardial involvement and should be followed closely in all cases. ST segment and T wave changes and first-degree heart block are found in less severe disease, whereas left bundle branch block and AV block are associated with high mortality. In addition to damaging the Purkinje system, the toxin causes necrosis of cardiac muscle cells that can result in acute heart failure and circulatory collapse. A poor outcome is more likely in patients with extensive pharyngeal membrane and an aspartate aminotransferase level above 80 IU/L.¹³ Recovery in more severe cases of myocarditis can result in normalization of the ECG; however, patients usually sustain permanent injury to the myocardium.

Neurologic toxicity

Neurologic complications occur in approximately 10% of respiratory cases. As with myocardial involvement, the likelihood of neurologic involvement correlates with the severity of the respiratory infection. Symptoms develop 10 to 28 days after the onset of respiratory complaints. Two types of neuropathy are seen. The first, cranial nerve involvement, is generally limited to the glossopharyngeal and vagus nerves, resulting in difficulty swallowing, aspiration, nasal regurgitation, and loss of the gag reflex. Less often, oculomotor and facial nerves become impaired. The second type of neuropathy tends to occur somewhat later in the course of the illness and resembles Guillain-Barré syndrome. These patients have quadriparesis associated with hyporeflexia. Degeneration of myelin sheaths and axon cylinders is observed in biopsy specimens. After treatment of the infection, slow but complete neurologic recovery ensues.¹⁴

Physical Examination and Laboratory Tests

Prompt recognition and treatment of respiratory diphtheria are critical for preventing complications and mortality. In the early stages, diphtheria pharyngitis mimics group A streptococcal pharyngitis and mononucleosis. As diphtheria progresses, unlike in pharyngitis and mononucleosis, the exudate changes, becoming darker and forming a membrane that cannot be removed without causing bleeding. Neurologic abnormalities, such as ninth and 10th nerve deficits or ECG changes, should also alert the clinician to the possibility of diphtheria. The microbiology laboratory staff must be notified of the possibility of C. diphtheriae because normal throat flora usually overgrow on blood agar plates. Nonnutritive, moist, reducing transport medium is helpful in preventing the overgrowth of competitors, and samples need to be inoculated on Löffler and tellurite media for proper identification. Assays for diphtheria toxin production also need to be performed.

TREATMENT

All patients with a clinical diagnosis of respiratory diphtheria should be hospitalized and isolated, because the course of illness is unpredictable. Rapid administration of antiserum is of primary importance. The sooner diphtheria antitoxin is given, the more favorable the outcome. Antitoxin is most effective if given within 4 days after the onset of illness. The antibody blocks entry of toxin into cells and therefore is effective only in neutralizing toxin in the extracellular space before entry into the cytoplasm. Dosage of antitoxin is adjusted to the severity of disease. Patients with extensive involvement of the tonsils and pharynx or larynx, who are expected to have higher concentrations of toxin, should be given at least half the treatment dose of antitoxin by intravenous infusion over 60 minutes. If extensive disease has been present for 3 or more days or if a bull neck has developed, administration of 80,000 to 120,000 units is recommended. For milder disease of shorter duration (48 hours or less), 20,000 to 40,000 units may be used. Antitoxin is given once; repeated doses provide no added benefit. Because antiserum is derived from horse serum, approximately 10% of patients have an allergic reaction. If skin or eye testing demonstrates hypersensitivity, desensitization should be attempted.

Antibiotic Therapy

Antibiotic therapy should be initiated as soon as possible and serves three purposes: (1) it shuts off toxin production; (2) it eradicates other potential pharyngeal pathogens, including group A streptococci; and (3) it eliminates the carrier state, preventing the spread of C. diphtheriae to other nonimmunized persons. Erythromycin is considered the treatment of choice. Penicillin is also effective; it is given intramuscularly until the patient is able to swallow, then orally (as penicillin V) for the remainder of the 2-week course [see Table 1]. A study of Vietnamese children found penicillin to be more effective than erythromycin.¹⁵ C. diphtheriae is generally susceptible to clindamycin and rifampin. However, there has been less experience with the use of these antimicrobial agents.

Table 1 Antibiotic Treatment of Infections Caused by Gram-Positive Bacilli

Pathogen (Disease) Drug Dosage Comment Corynebacterium     diphtheriae      (diphtheria) Erythromycin 500 mg I.V. or p.o., q.i.d. × 2 wk First choice Penicillin Penicillin G, 600,000 U I.M. b.i.d., then    penicillin V p.o. 250 mg q.i.d. × 2 wk First choice; in children, penicillin may be more effective       than erythromycin C. urealyticum Vancomycin 1 g I.V. q. 12 hr × 2–3 wk Use antibiotic sensitivity testing to guide therapy C. jeikeium Vancomycin 1 g I.V. q. 12 hr × 2–3 wk First choice; use antibiotic sensitivity testing to guide      therapy Penicillin G  + gentamicin 20 million U/day I.V. divided q. 4–6 hr 5 mg/kg/day I.V. divided q. 8 hr Alternative C. ulcerans Erythromycin 500 mg I.V. or p.o., q.i.d. × 2 wk — Rhodococcus equi Vancomycin ± rifampin ± erythromycin, imipenem, amikacin,       or ciprofloxacin 1 g I.V. q. 12 hr × 6 wk 600 mg p.o., q.d. 1 g I.V. q. 6 hr 0.5–1 g I.V. q. 6 hr 15 mg/kg I.V. q.d., or divided q. 8–12 hr 500–750 mg p.o. or 400 mg I.V. q. 12 hr Prolonged therapy often required, relapse common;     combination therapy recommended; no one regimen     has proved to be more effective Listeria       monocytogenes Ampicillin ± gentamicin 2 g I.V. q. 6 hr × 3–6 wk 5 mg I.V. q.d. or divided q. 8 hr Clinical efficacy of gentamicin has not been proved Trimethoprim-sulfamethoxazole     (TMP-SMX) 20 mg TMP, 100 mg SMX/kg/day I.V.,    divided q. 6–8 hr × 3–6 wk Use in penicillin-allergic patients Nocardia Sulfisoxazole or    sulfadiazine 1.5–2 g I.V. q. 6 hr May not be available on hospital formulary; follow by   oral sulfonamide × 10–11 mo TMP-SMX 20 mg TMP, 100 mg SMX/kg/day I.V.,    divided q. 6–8 hr × 1–2 mo Follow by oral sulfonamide × 10–11 mo Minocycline 100 mg I.V. q. 12 hr Alternative drug; may cause vertigo Imipenem ± amikacin 0.5–1 g I.V. q. 6 hr 15 mg/kg I.V. q.d. or divided q. 8–12 hr Alternative Bacillus anthracis(anthrax) Ciprofloxacin    or doxycycline    + one or two additional    antibiotics:    rifampin    vancomycin    penicillin G    ampicillin    chloramphenicol    imipenem    clindamycin    clarithromycin 400 mg I.V. q. 12 hr followed by     500 mg p.o., b.i.d. × 60 days 100 mg I.V. q. 12 hr followed by 100 mg    p.o., b.i.d. × 60 days 600 mg p.o., q.d. 1 g I.V. q. 12 hr 20 million U/day I.V. divided q. 4–6 hr 2 g I.V. q. 4 hr 1 g I.V. q. 6 hr 500–1,000 mg I.V. q. 6 hr 600 mg I.V. q. 8 hr 500 mg p.o., b.i.d. Prolonged treatment necessary because spores may     persist and later germinate; no one combination     regimen preferred; some experts favor the addition     of clindamycin, which theoretically may block     toxin production; chloramphenicol associated with     granulocytopenia B. cereus Vancomycin 1 g I.V. q. 12 hr First-line therapy; duration depends on the type of infection Imipenem 500–1,000 mg I.V. q. 6 hr Alternative Clindamycin 600 mg I.V. q. 8 hr Alternative Ciprofloxacin 400 mg I.V. q. 12 hr Alternative Other Bacillusspecies Vancomycin ± gentamicin 1 g I.V. q. 12 hr 5 mg/kg/day I.V. or divided q. 8 hr First-line therapy; duration depends on the type of       infection Clindamycin ± gentamicin 600 mg I.V. g. 8 hr 5 mg/kg/day I.V. or divided q. 8 hr Alternative Imipenem 500–1,000 mg I.V. q. 6 hr Alternative Erysipelothrix(erysipeloid) Penicillin G Benzathine form; single 600,000    U I.M. dose First-line therapy Erythromycin 250–500 mg p.o., q.i.d. × 10 days Alternative Erysipelothrix     (endocarditis) Penicillin G 20 million U/day I.V. divided q. 4–6 hr    × 4–6 wk First-line therapy; recommended for endocarditis Ceftriaxone 1 g I.V. q.d. × 4–6 wk For penicillin-allergic patient Cefazolin 1.5–2 g I.V. q. 8 hr × 4–6 wk For penicillin-allergic patient

Other Measures

Extensive pharyngeal and laryngeal membrane formation can lead to upper airway obstruction. Therefore, patients need to be closely monitored, and if signs of obstruction are detected, prompt intubation or tracheostomy must be performed. Sedatives may obscure the development of respiratory difficulties and should be avoided. Patients with myocarditis need cardiac monitoring and should initially be kept at bed rest. Treatment of heart failure with digoxin may result in further impairment of electrical conduction and lead to heart block. Experience with cardiac pacemakers is limited, but pacemakers would be expected to reduce mortality from complete heart block.

Isolation and Treatment of the Carrier State

When diphtheria is suspected, the patient should be isolated until two cultures from the infected site are negative. Cultures should be obtained from all persons who have been in close contact with the patient to determine whether they are pharyngeal carriers. All carriers need to be treated with erythromycin or penicillin for 14 days, and eradication of the carrier state must be documented by follow-up cultures.

PREVENTION

Active immunization using formalin-detoxified diphtheria toxin effectively prevents diphtheria. Preschool children (6 weeks to 7 years of age) should be immunized with three 0.5 ml intramuscular injections of diphtheria-tetanus-acellular pertussis (DTaP) vaccine spaced 4 to 8 weeks apart. A fourth dose should be given 6 to 12 months later. Immunity to the toxin is not lifelong, and if primary immunization was completed before 4 years of age, a booster is recommended at the time of school entry. Subsequently, booster injections need to be given every 10 years to maintain protective immunity. A single dose of vaccine is sufficient for most age groups, with the exception of persons 30 to 49 years of age, who may require three doses to generate protective antibody titers.¹⁶ Surveys indicate that high percentages of adults in the United States and Europe fail to demonstrate a significant immune response to diphtheria toxoid. It has been estimated that epidemic diphtheria is favored when more than 70% of the population lacks protective immunity, a condition now present in many developed countries. A toxoid booster inoculation every 10 years is strongly recommended for all adults.

Nondiphtheria Corynebacterium and Rhodococcus

In addition to the species diphtheriae, the genus Corynebacterium contains a large number of species that for decades were considered to be culture contaminants and constituents of the normal human flora. These organisms were previously termed diphtheroids. As the number of immunocompromised hosts and persons with prosthetic devices has increased, the role of nondiphtheria corynebacteria as true pathogens has become evident¹⁷ [see Table 2]. C. urealyticum and C. jeikeium are two non diphtheria strains that are particularly important nosocomial pathogens.

Table 2 Clinical Syndromes Caused by Nondiphtheria Corynebacterium Species

Species Clinical Syndrome C. ulcerans C. pseudotuberculosis     (C. ovis) C. (Arcanobacterium)     haemolyticum C. pseudodiphtheriticum     (C. hofmannii) C. urealyticum     (formerly group D2) C. jeikeium (group JK) Rhodococcus equi     (C. equi) Pharyngitis, skin ulcer, diphtheria Suppurative lymphadenitis Pharyngitis, scarlatiniform rash Endocarditis, pneumonia, tracheobronchitis,   lymphadenitis Alkaline-encrusted cystitis, urinary tract   infections Nosocomial septicemia, wound infection,   endocarditis Necrotizing pneumonia in patients with   AIDS

1. UREALYTICUM
2. urealyticumis a slow-growing, urease-positive organism that is widely distributed on the skin of hospitalized patients. C. urealyticumis one cause of alkaline-encrusted cystitis, a urinary tract infection that is severe and difficult to treat.¹⁸ The urea-splitting activity of the organism leads to alkaline urine and the formation of struvite stones. Factors that predispose an individual to this infection include previous urinary tract infections, urologic instrumentation, immunosuppression, underlying inflammation of the bladder, and bladder neoplasia. Less commonly, C. urealyticum can cause septicemia, endocarditis, osteomyelitis, and pneumonia. This organism is often resistant to most antibiotics; therefore, vancomycin is recommended for initial treatment pending sensitivity testing [see Table 1].
3. JEIKEIUM
4. jeikeiumfrequently colonizes the skin of hospitalized patients. Patients at highest risk for colonization and subsequent infection with C. jeikeiuminclude those receiving broad-spectrum antibiotics and those requiring prolonged hospitalization. In patients with neoplastic disease, other risk factors include prolonged neutropenia and breaks in the integument.¹⁹ In most patients, infection presents as bacteremia. The incidence is highest in neutropenic patients and those who have undergone cardiac surgery.¹⁷ Bacteremia is most often associated with colonization of an intravenous catheter.²⁰ Prosthetic endocarditis and native valve endocarditis have been reported, as have rare cases of extravascular infections, including cutaneous lesions, pneumonia, peritonitis, prosthetic knee infection, and ventriculostomy infection. Most isolates of this organism tend to be multiply resistant and frequently are sensitive only to vancomycin²¹ [see Table 1]. Contaminated catheter lines can often be sterilized with antibiotics alone.²⁰ In patients with prosthetic valve infection, however, the foreign material often has to be removed to control the infection.

RHODOCOCCUS EQUI

Rhodococcus equi, also known as Corynebacterium equi, primarily infects immunocompromised hosts with defects in cell-mediated immunity, particularly patients with AIDS²² and those with solid-organ transplants.²³ Cases in normal hosts have also been reported.^24,25 R. equi is found in the soil and at particularly high concentrations in horse manure. Infection is generally acquired through the lungs.

The primary manifestation in most cases is cavitary lung disease resembling tuberculosis, nocardiosis, or fungal infection. Lung consolidation without cavitation is also seen. Bronchoscopy, thoracentesis, or surgery may be required to make the diagnosis, although blood cultures are frequently positive.²⁴ R. equi organisms may be mistaken for contaminating diphtheroids, or because they are modified acid-fast positive, the infection may be misdiagnosed as tuberculosis. Extrapulmonary infections may also occur.

Macrolides and rifampin act synergistically in combination, and regimens containing these two agents are often recommended. These antibiotics achieve high intracellular levels, an important characteristic for clearing R. equi, because this pathogen primarily multiplies in cells. The organism is also sensitive to vancomyin and aminoglycosides, and vancomycin is often included in the initial regimen [see Table 1]. Cephalosporins should be avoided because of the frequent development of resistance, and multidrug resistance is becoming more common.²⁶ Short courses of therapy are associated with relapse; therefore, therapy needs to be continued for many weeks. Mortality in AIDS patients is approximately 15%; however, half of the survivors are never completely cured of R. equi infection.²²

Listeriosis

Listeria monocytogenes, a food-borne pathogen, is the cause of listeriosis, a serious and often fatal infection.

MICROBIOLOGY

1. monocytogenesis an aerobic and facultatively anaerobic, non-spore-forming, gram-positive rod. As opposed to coryne bacteria, this organism is motile, possessing one to five flagella. Listeria organisms grow at a wide range of temperatures (3° to 42° C), which explains its ability to contaminate refrigerated foods. This bacterium can also grow at acidic concentrations of a pH of 5 or higher and salt concentrations of as high as 10% to 12%. Listeriaorganisms can be readily cultured on blood agar plates, where they cause slight zones of β-hemolysis. Listeria organisms on occasion can appear somewhat coccoid and, therefore, may be mistaken for diphtheroids orStreptococcus pneumoniae on Gram stains of cerebrospinal fluid. There are at least 11 serotypes of L. monocytogenes. Serotypes 1b and 4b are most commonly associated with listeriosis.

ETIOLOGY AND EPIDEMIOLOGY

1. monocytogenesis found in soil, dust fertilizer, sewage, stream water, plants, processed foods, and the intestinal tract of many mammals. Investigations of multiple outbreaks indicate that both sporadic and common-source outbreaks of listeriosis are the result of food contamination. Outbreaks have been linked to raw vegetables, Mexican-style cheese, milk, undercooked chicken, and foods purchased in delicatessens.²⁷Prepared refrigerated foods stored for prolonged periods and requiring no further high-temperature heating are most likely to be contaminated because Listeria organisms can readily multiply on refrigerated foods.

The overall incidence of listeriosis is low: 0.7 cases per 100,000 population. This infection more often occurs in persons older than 70 years (2.1 cases per 100,000); pregnant women (12 cases per 100,000); patients with defects in cell-mediated immunity, including renal transplant recipients; patients receiving high doses of corticosteroids; and patients with AIDS (100 cases per 100,000).²⁸ At a large referral hospital, Listeria organisms were the third most common cause of community-acquired bacterial meningitis in adults (12% of cases).²⁹Despite its relatively low incidence, listeriosis concerns public health officials because this disease is associated with a high fatality rate (23%), unlike infections from other food-borne pathogens, such as Salmonella organisms, which are rarely fatal. Given the increasing numbers of elderly and immunocompromised patients, the incidence of listeriosis is likely to increase.

PATHOGENESIS

1. monocytogeneshas an unusual life cycle³⁰[see Figure 3, part a]. Several proteins (internalins³¹) on the surface of the bacterium allow attachment and subsequent ingestion of Listeria organisms by host cells. Once internalized, the bacterium is surrounded by host cell membrane, forming a phagolysosome, a closed space that is generally toxic for pathogens. Listeria organisms evade destruction by producing the exotoxin listeriolysin O, which lyses the confining membranes. All pathogenic strains of Listeria organisms produce listeriolysin O, and their escape into cytoplasm of the host cell is required for pathogenesis. Once in the growth-permissive cytoplasm, the bacteria proliferate, with doubling times of about 1 hour. The Listeria surface protein ActA possesses binding sequences to attract actin regulatory proteins that stimulate actin filament assembly.^30,32 About 2 hours after entry into the cytoplasm, actin filaments polarize at one end of the bacteria and provide the force for movement through the cytoplasm [see Figure 3, part b]. Many of the bacteria migrate to the periphery of the cytoplasm, where they push against the host cell's outer membrane to form elongated protrusions or filopods that can be ingested by adjacent cells. Once a bacterium enters the adjacent cell, the life cycle begins anew. Listeria organisms, therefore, can spread from cell to cell without directly contacting the extracellular environment. A number of other virulence factors, in addition to ActA and internalins, have been identified, and their contributions to Listeria pathogenesis are being defined.

Figure 3. Life Cycle of Listeria Monocytogenes (a) Life cycle of Listeria monocytogenes in host cells. (b) New actin filament assembly drives the bacterium through the cytoplasm. TheListeria surface protein ActA induces host cell actin to assemble into a rocket tail. The actin filament tail progressively lengthens, with new host cell actin monomers being added at the junction between the bacterium and the actin filament tail. The older regions of the actin tail attach to the host cell's cytoskeleton, providing a purchase so that the forces of actin filament lengthening can be applied to the bacterium to drive it through the cytoplasm. Bacteria are able to move at rapid speeds (0.02 to 1.4 µm/sec).30

The Listeria organism's intracellular lifestyle explains many of this pathogen's unique clinical characteristics.³⁰ Although the association between contaminated foods and the Listeria organism has been well documented, evidence of gastrointestinal disease has been absent in most cases. The Listeria organism's capacity to enter the gastrointestinal tract without causing erosive lesions is explained by the ability of this pathogen to stimulate phagocytosis by gastrointestinal cells and macrophages. Subsequently, the Listeria pathogen commandeers host cell actin regulatory proteins to spread from cell to cell and eventually enter the bloodstream either in monocytes or as free organisms after cell lysis.

The ability of the Listeria organism to avoid the extracellular environment also explains the increased incidence of listeriosis in immunocompromised patients, neonates, and pregnant women. Increased risk of listeriosis has not been associated with deficiencies of immunoglobulins or complement. However, clinical conditions and therapies (particularly corticosteroids) that lead to deficiencies in cell-mediated immunity, the primary defense for controlling intracellular pathogens, increase the risk for listeriosis. For example, treatment with fludarabine and prednisone in patients with chronic lymphocytic leukemia markedly lowers the CD4⁺ T cell counts and increases the incidence of listeriosis.³³ Patients with AIDS are most likely to contract Listeria infection when their CD4⁺ T cell counts fall below 40/mm³.³⁴

Clinical Manifestations

Listeriosis varies in its clinical presentation; primary manifestations most often are sepsis, meningitis, or both. Other extravascular infections are also reported, but they are surprisingly rare.

Infection during pregnancy

Nearly one third of individuals who contract listeriosis are pregnant women, with infection occurring most frequently in the third trimester. The symptoms tend to be relatively mild, consisting of a flulike illness with chills, fever, and muscle aches. Back pain, a less frequent complaint, suggests a urinary tract infection; however, urinalysis and urine culture results are normal. Blood cultures, although not always obtained, are positive. Symptoms usually resolve spontaneously without therapy.³⁵

Neonatal listeriosis

The Listeria organism can cross the placental barrier, probably as a result of cell-to-cell spread mediated by host cell actin. The organism may cause amnionitis and precipitate premature labor, leading to septic abortion. Transplacental transmission can also cause the unique clinical syndrome of granulomatosis infantiseptica. The organism can disseminate in utero, forming abscesses and granulomas involving the fetal liver, spleen, lungs, kidneys, brain, and skin.³⁶ Mortality is high, ranging from 35% to 55%.

Chorioamnionitis is the most common early manifestation of perinatal Listeria infection. A Gram stain of the meconium frequently reveals gram-positive bacilli, suggesting the diagnosis. In addition to causing congenital infection, the Listeria pathogen can cause meningitis in neonates 7 to 28 days of age. Listerial meningitis is the third most common form of meningitis in neonates, accounting for 5% to 15% of cases.

Adult meningitis and meningoencephalitis

Meningitis is the most common manifestation of listeriosis. The Listeria pathogen has a predilection for the central nervous system, particularly the meninges. Although the clinical presentation of listerial meningitis is similar to that of other forms of bacterial meningitis (i.e., Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis), several characteristics distinguish Listeria infections. Meningeal signs develop less frequently in patients with listerial meningitis than in patients with other forms of bacterial meningitis.³⁷However, tremor and grand mal or focal motor seizures are observed with a higher frequency, suggesting more extensive invasion of the CNS.³⁰ The CSF response may reflect the intracellular nature of Listeria organisms. Compared with the CSF cell counts in other forms of bacterial meningitis, both the total number of white blood cells and the percentage of neutrophils tend to be reduced in patients with listerial meningitis. Because Listeria primarily grows in cells, it is less commonly found in Gram stains of CSF (positive in 5% to 33% of cultures versus 80% in other forms of bacterial meningitis).³⁸

Meningoencephalitis, a direct invasion of the cerebral cortex, can also result from Listeria infection, but it is not a recognized complication of other forms of bacterial meningitis. The ability of the Listeria pathogen to cross the meninges and blood-brain barrier is also likely to be the result of endothelial cell or macrophage phagocytosis of the organisms and utilization of the host cell contractile system to migrate to and grow in the brain. Listeria organisms most commonly invade the brain stem, causing a syndrome that has been called rhomboencephalitis.³⁹ The disease is usually biphasic; CNS manifestations are preceded by 7 to 10 days of malaise, fatigue, headache, nausea or vomiting, and fever. These symptoms are followed by the development of multiple cranial nerve deficits, particularly of the sixth and seventh nerves. Brain stem damage can also cause hemiparesis, ataxia, and respiratory dysfunction, often followed by respiratory arrest. In cases of meningoencephalitis, CSF monocyte counts may reach 80% to 90%. In some cases, the CSF may contain no cells or only a few cells and have normal protein and glucose levels. Magnetic resonance imaging reveals areas of increased resonance and is the best way to visualize the brain stem. Computed tomography with contrast generally shows areas of increased uptake with or without ring enhancement.

The diagnosis of both listerial meningitis and meningoencephalitis frequently is delayed. A monocytic response in the CSF and a negative CSF Gram stain can lead the clinician to confuse listerial meningitis and meningoencephalitis with herpes and other forms of viral encephalitis, viral meningitis, tuberculous meningitis, Lyme disease, syphilis, cryptococcal meningitis, Wegener granulomatosis, or CNS sarcoidosis. Particularly in the immunocompromised host, the possibility of Listeria infection must always be considered as a possible cause of CNS infection.

Bacteremia

Bacteremia without meningitis or focal infections occurs in 5% to 30% of adult cases. There are no distinctive clinical features except for peripheral monocytosis, which is present in a small percentage of patients. Diagnosis is based on blood culture findings.

Miscellaneous infections

Like many other pathogens that are able to enter the bloodstream, the Listeria organism can cause focal infections at many extravascular sites, including bones, normal and prosthetic joints, eyes,⁴⁰ spinal cord, pleura, peritoneum, and liver.⁴¹ Isolated brain abscesses have also been reported.³⁷ In rare cases, the Listeria organism causes endocarditis, myocarditis, and mycotic aneurysms. Although Listeria enters via the gastrointestinal tract, symptomatic gastroenteritis is uncommon. However an outbreak of febrile gastroenteritis associated with contaminated delicatessen precooked turkey was reported in Los Angeles.⁴²

Laboratory Tests

1. monocytogenescan be readily cultured, although a diphtheroid-like organism discovered in blood or CSF cultures is frequently misinterpreted as a contaminant. Diphtheroids can be rapidly differentiated from Listeriaorganisms by using a microscopic bacterial motility test. Decreasing motility is indicative of Listeria organisms. Gram stains of cerebrospinal or meconium fluid showing gram-positive rods strongly suggest Listeria infection. In an immunocompromised host, treatment of listeriosis should be initiated pending the laboratory staff's final diagnosis.

PREVENTION AND TREATMENT

Preventive Measures

It is not possible to eliminate the large reservoir of Listeria organisms found throughout the environment. Physicians can help prevent disease by instructing patients at risk on how to minimize the multiplication of Listeria organisms in foodstuffs and how to kill the organisms on potentially contaminated foods. Patients need to avoid unsterilized dairy products, undercooked meats, and prepared foods that have been refrigerated but not resterilized by high-temperature reheating. More detailed preventive instructions are provided by the Centers for Disease Control and Prevention (CDC) [see Table 3].

Table 3 Dietary Recommendations for Preventing Food-Borne Listeriosis

General recommendations   Thoroughly cook raw food from animal sources   Wash raw vegetables thoroughly   Keep uncooked meats separate from vegetables and cooked foods   Avoid consumption of unpasteurized milk or foods made with raw milk   Wash hands, knives, and cutting boards after handling uncooked foods Additional recommendations for high-risk persons*   Avoid soft cheeses (e.g., Mexican style, feta, Brie, Camembert, and blue veined); hard cheeses, cream cheese, cottage cheese, and yogurt can be eaten   Leftovers or ready-to-eat foods (e.g., hot dogs) should be reheated until they are steaming hot   Pregnant women and immunosuppressed persons should consider avoiding foods from delicatessen counters or thoroughly cooking cold cuts before eating * High-risk persons include immunocompromised persons, pregnant women, neonates, and the elderly.

Antibiotic Therapy

No clinical trials comparing various antibiotic regimens have been published. Bacteriostatic drugs, such as chloramphenicol and tetracycline, are associated with high failure rates in patients with listeriosis and cannot be recommended. Ampicillin is generally recommended as the treatment of choice, and the Listeria pathogen is generally sensitive⁴³ [see Table 1]. In immunosuppressed patients, relapse has been reported after 2 weeks of penicillin therapy. The poor response to bacteriostatic drugs and the slow response to ampicillin therapy probably result from the Listeria organism's ability to survive and grow in cells. The intracellular level of ampicillin may not be sufficient for complete sterilization. Immunosuppression reduces the host's ability to clear infected cells. The exact duration of antibiotic treatment required to prevent relapse is not known; however, 3 to 6 weeks of therapy is probably prudent for immunosuppressed patients. Antibiotics that penetrate cells poorly, such as aminoglycosides, may be synergistic in vitro but are unlikely to prove efficacious in the living host. Although some experts have recommended that an aminoglycoside be added to ampicillin, the Listeria organism grows in cells in the presence of extracellular gentamicin concentrations of 10 to 20 µg/ml. Furthermore, addition of gentamicin to ampicillin therapy has failed to improve outcome in a mouse infection model.⁴⁴ Therefore, aminoglycosides are unlikely to work in patients with listeriosis and certainly should be avoided in kidney transplant recipients and other patients with renal dysfunction. On the other hand, trimethoprim-sulfamethoxazole, a drug combination that readily enters cells and is bactericidal for Listeria organisms, may prove to be the most effective agent for treating Listeriainfection. Trimethoprim-sulfamethoxazole has proved to be effective in patients with listeriosis and penicillin hypersensitivity [see Table 1].³⁹ Tissue culture and animal studies suggest that levofloxacin may also be effective; however, treatment of human Listeria infection with this quinolone has not been reported.⁴⁵

Listeria infection in the CNS is associated with a high mortality despite appropriate antibiotic treatment. In patients with meningoencephalitis, mortality ranges from 36% to 51%. Survivors often have permanent neurologic sequelae. Mortality in patients with meningitis is somewhat lower (26%), but it is higher in patients with seizures and in those older than 65 years.³⁷ Early recognition and rapid institution of antibiotics are critical for improving outcome.³⁹

Nocardiosis

MICROBIOLOGY

Nocardia species are thin, aerobic, gram-positive bacilli that form branching filaments that tend to fragment into coccobacilli. Gram stain is often taken up, variably resulting in an irregular, beaded appearance in exudates [see Figure 4]. Nocardia asteroides is the most common Nocardia species to cause human disease in the United States (80% to 90% of cases). N. brasiliensis, N. otitidis-caviarum, and N. farcinica⁴⁶ are rarer human pathogens. All four organisms are commonly found in soil. In addition to being gram positive, they are modified acid-fast positive. This characteristic helps differentiate Nocardia species from the Actinomyces organism, a gram-positive, anaerobic pathogen that also forms branching filaments. The Nocardia organism grows slowly on blood agar plates and Sabouraud dextrose agar. In sputum specimens, other organisms often overgrow, obscuring Nocardia colonies. Colonies generally have an orange pigment and appear heaped up and folded. They can also be white or pink. Colony characteristics suggestive of the Nocardia organism may not develop for 2 to 4 weeks. Samples should be obtained when the patient is not taking antibiotics. Modified Thayer-Martin medium and buffered charcoal-yeast extract agar can be used to enhance recovery of Nocardia species.

Figure 4. Joint Fluid with Nocardia Asteroides Gram stain of joint fluid containing Nocardia asteroides.

ETIOLOGY AND EPIDEMIOLOGY

Nocardiosis is relatively rare, infecting approximately 1,000 persons a year in the United States. Often found in soil, Nocardia organisms most often infect immunocompromised hosts and primarily cause pulmonary infections, brain abscesses, and skin infections.

PATHOGENESIS

In most cases, the Nocardia organism gains entry to the host through the respiratory tract. Inhaled bacteria elicit a neutrophil response that inhibits but does not kill the organism. Nocardia organisms are phagocytosed by neutrophils and incorporated into phagolysosomes. In this closed membrane space, the organism is able to survive for prolonged periods. Like Mycobacterium tuberculosis, pathogenic Nocardiaspecies produce superoxide dismutase, a product that inactivates the toxic oxygen by-products of the neutrophils and macrophages. In addition, both M. tuberculosis and Nocardia species synthesize a second product: a mycolic acid that inhibits the fusion of lysosomes with the phagolysosomal compartment. This inhibitory activity prevents the release of toxic proteases and other antibacterial products that would otherwise reach the intracellular bacteria. The host defenses utilized to protect against nocardiosis are multifactorial and include neutrophils, macrophages, and cell-mediated and humoral immunity. Patients at risk for nocardiosis include those with chronic granulomatous disease (which compromises the ability of neutrophils to produce toxic oxygen by-products) and those with dysgammaglobulinemia. The highest percentage of cases occur in patients with impaired cell-mediated immunity, including renal and cardiac transplant patients,⁴⁷ other patients on high-dose corticosteroids, patients with Cushing disease, cancer patients,⁴⁸ and those with AIDS who have CD4⁺ T cell counts below 250 cells/mm³.⁴⁹ Patients with chronic pulmonary disorders, particularly alveolar proteinosis, are also at increased risk. Approximately one third of those who acquire nocardiosis have no identifiable predisposing condition.

DIAGNOSIS

Clinical Manifestations

Nocardiosis has no pathognomonic characteristics, and delays in diagnosis are common. This infection must always be considered in the immunocompromised host.

Pulmonary nocardiosis

Pulmonary infection is the most common manifestation of nocardiosis, occurring in approximately two thirds of cases. In most cases, pulmonary disease is subacute in onset, mimicking pulmonary tuberculosis. Patients complain of productive cough, pleuritic chest pain, dyspnea, fever, anorexia, and weight loss. Occasionally, hemoptysis develops, particularly in patients with large cavitary lung lesions; if left untreated, the disease tends to run a chronic waxing-and-waning course. Chest x-ray findings are variable and include the following, in order of most to least frequent: pulmonary nodules or mass lesions, areas of consolidation, cavitary lesions, interstitial infiltrates, and pleural effusions. In addition, CT may demonstrate areas of low attenuation in consolidations, multiple nodules, and chest wall extension of the infection. AIDS patients are more likely to have multiple pulmonary nodules, cavitary lung lesions, and upper lobe infiltrates.⁴⁹ On occasion, the infiltrate spontaneously resolves, particularly in patients with normal immune function. A brain abscess may develop later as a consequence of transient dissemination of the organism.

CNS infection

In approximately one third of patients with nocardiosis, the CNS becomes infected. Brain abscess is the most common CNS infection. The lesions are often multilocular and can occur in any region of the brain.⁵⁰ CT or MRI with contrast demonstrates ring-enhancing lesions, as observed with other causes of pyogenic brain abscess. In AIDS patients, brain abscess is often accompanied by an abnormal chest x-ray, which suggests the diagnosis.⁵¹ With other patients, abnormalities on chest x-ray are not always present. However, when abnormalities are detected, the combined findings of a lung nodule on chest x-ray and ring-enhancing CNS lesions are often mistaken for lung carcinoma with metastasis. Patients treated with surgical drainage of their abscesses have a higher survival rate than those who are not.⁵⁰ Nocardial meningitis is a less common CNS manifestation and is often associated with brain abscess (40% of meningitis cases). Patients with nocardial meningitis typically have subacute to chronic meningitis characterized by fever, stiff neck, and head ache. CSF analysis demonstrates a predominance of polymorphonuclear neutrophils, a low CSF glucose level, and an elevated protein level. CSF cultures are often negative, particularly in the first 3 days of the disease, and patients fail to fully respond to empirical antibiotic therapy. Appropriate treatment is often delayed, and mortality is high (50% to 60%).

Cutaneous infection

Cutaneous involvement is uncommon and is generally caused by N. brasiliensis. A break in the skin caused by trauma, an insect bite, a thorn bush scratch,⁵² and even a cat scratch can result in local invasion by Nocardia organisms. A pustule or a moderately erythematous, nonfluctuant nodule develops at the site of inoculation. Regional adenopathy is generally found. The presence of multiple subcutaneous nodules indicates dissemination of the organism and more often occurs in the immunocompromised patient. In the tropical regions of South and Central America, ulcerations and large tumorlike lesions called mycetomas occur on the lower legs and are caused by N. asteroides.

Other infections

Dissemination occurs in approximately 40% of pulmonary Nocardia infections and can result in localized infection in any organ. Septic arthritis, osteomyelitis, endophthalmitis, sinusitis, peritonitis, and purulent pericarditis have all been reported.

Laboratory Tests

Invasive procedures are generally required to obtain infected tissue samples. The histopathology of biopsy specimens usually reveals an acute inflammatory response with a predominance of neutrophils. Tissue necrosis with minimal fibrosis often results in the formation of multilocular abscesses with minimal capsular formation. Gram stain or Brown-Brenn stains often reveal gram-positive beaded branching forms. Nocardia species can also be visualized by using a modified acid-fast stain. The organism is not well seen after hematoxylin-eosin or periodic acid-Schiff stain. Culture is the definitive way to prove the diagnosis. For sputum cultures, selective media may be required to prevent the overgrowth of more rapidly growing mouth flora. For the diagnosis of meningitis, large volumes of CSF should be obtained for culture.

Nocardia organisms are slow growing and are difficult to identify on routine culture. When a potential case is encountered, it is important for the clinician to alert microbiology and pathology laboratory staffs that Nocardia is a possible pathogen, so that cultures can be incubated for a longer period.

TREATMENT

Sulfonamides alone and trimethoprim-sulfamethoxazole remain the treatments of choice. High intravenous doses of these agents are required: sulfadiazine (1.5 to 2 g every 6 hours) or trimethoprim-sulfamethoxazole (20 mg/kg/day of trimethoprim and 100 mg/kg/day of sulfamethoxazole given in three divided doses) to maintain serum sulfonamide levels in the 12 to 15 mg/dl range. Once substantial improvement is documented, oral treatment can be substituted after 1 to 2 months of intravenous therapy. For sulfa-allergic patients, possible alternatives need to be determined on the basis of sensitivity testing. Minocycline, imipenem, amoxicillin-potassium clavulanate, and amikacin alone or in various combinations have been successful in individual patients [see Table 1].⁵³ Because of the intracellular nature of nocardiosis and the organism's slow growth rate, 12 months of antibiotic therapy is required to prevent relapse. In cases of abscess formation in the brain, subcutaneous tissue, or other organs (except the lung), surgical drainage is also required for cure.

PROGNOSIS

The overall mortality from nocardiosis is approximately 25%. Otherwise healthy persons with pulmonary nocardiosis have a better prognosis (15% mortality). Fatality rates are higher in patients with bacteremia,⁵⁴ patients with acute infection (symptomatic for less than 3 weeks), patients receiving corticosteroids or cytotoxic agents, patients with disseminated disease involving two or more noncontiguous organs, and patients with meningitis.

Anthrax

Bacillus anthracis causes infections primarily in animals, particularly herbivores. However, contact with animals or animal products can produce B. anthracis infections in humans. Although B. anthracis was once the cause of severe epidemics, our understanding of this pathogen's epidemiology and the vaccination of domestic animals have resulted in a marked reduction of naturally acquired anthrax in the United States. In developing countries, however, outbreaks associated with exposure to animals and animal products continue to be reported. In October 2001, anthrax spores were used in a bioterrorist attack in the United States.⁵⁵ This attack emphasized the need for all health care personnel to be familiar with the clinical manifestations, diagnostic approach, treatment, and prevention of anthrax. The identification of a single case of B. anthracis is now a cause for alarm.⁵⁶

MICROBIOLOGY

1. anthracisis an aerobic, gram-positive rod that forms endospores. The spores are highly resistant to adverse conditions and are able to survive at extreme temperatures, at high pH and salinity levels, and in disinfectants. The organism can be readily cultured on standard blood and nutrient agar plates. For contaminated specimens (e.g., stool), selective media or decontamination methods can be used that take advantage of the spores' ability to resist heat, ethanol, and various antibiotics. The spores germinate when they are exposed to an environment rich in amino acids, nucleosides, and glucose, such as the blood or tissues of a mammalian host. Once germination occurs, the organism multiplies rapidly. On blood agar plates, vegetative bacteria form gray-white colonies and are nonhemolytic. A fluorescent antibody stain can be used to identify the organism.

ETIOLOGY AND EPIDEMIOLOGY

Anthrax is primarily a disease of herbivores (cattle, sheep, horses, goats, and swine). Humans become infected as a result of contact with infected animals (agricultural exposure) or through exposure of infected animal products (industrial exposure). The October 2001 bioterrorist attack through the United States mail system has emphasized the danger of anthrax spores as a biologic weapon. Accidental laboratory-related infections have also been reported. B. anthracis, like all Bacillus species, is a saprophyte that grows in the soil, and animals generally contract the infection through contact with soil. Because domestic animals in the United States are vaccinated against anthrax, agricultural exposure is rare, and the diagnosis of anthrax in the United States or other developed countries should immediately raise the possibility of a bioterrorist attack.

Most cases of anthrax in the United States have occurred as a result of contact with animal products imported from Asia, the Middle East, and Africa. Wool, goat hair, and animal hides are the most common sources of infection. Persons who work in the early stages of processing these materials are exposed to the highest inoculum of spores and are most likely to contract disease. Processed materials have also caused human disease. Cases have been traced to shaving-brush bristles, wool coats, yarn, goat-skin bongo drums, and heroin preparations. The largest recent outbreak of anthrax occurred in Sverdlovsk (now Yekaterinburg), Russia, in 1979, resulting in approximately 96 inhalation cases and 64 deaths. Accidental aerosol release of anthrax spores from a germ-warfare facility is suspected, and polymerase chain reaction (PCR) analysis of tissue samples from 11 victims has identified multiple strains of B. anthracis in each sample, consistent with infection by a manufactured preparation of bacterial spores.⁵⁷ In October 2001, B. anthracis spores were sent in at least five letters to Florida, Washington, D.C., and New York City, resulting in 22 cases of anthrax (11 cases of confirmed inhalational anthrax and 11 cases of cutaneous disease [seven confirmed and four suspected]). Several cases developed as a consequence of cross-contamination of mail, and a number of postal workers were infected by spores aerosolized during mail processing.⁵⁵

PATHOGENESIS

Spores gain entry into the epidermis through abrasions in the skin and can be inhaled into the lungs as 1 to 5 µm particles. Once in the host, the spores germinate, multiply, and produce toxins that cause tissue edema and necrosis. In the lungs, spores are ingested by macrophages, where many are lysed and destroyed. However, surviving spores are transported to the mediastinal lymph nodes, where they germinate, multiply rapidly, and quickly enter the vascular system, causing bacteremia. Extrapolation from monkey experiments indicates that inhalation of 2,500 to 55,000 spores is required to cause fatal disease in 50% of humans. However other experimental data suggest that the inhalation of as few as 1 to 3 spores may be sufficient to cause disease, and two cases of fatal inhalation anthrax in New York City and Connecticut suggest that in some individuals, fatal doses may be quite low.⁵⁶ Coating of anthrax spores to prevent their aggregation improves their ability to infect the lung (such spores are termed weaponized). The spores used in the United States attack were weaponized, which explains the efficiency of infection. Another major epidemiologic concern is the duration of risk for contracting disease after the inhalation of spores. In Sverdlovsk, cases occurred up to 43 days after exposure,⁵⁸ and in monkey experiments, viable spores were found in mediastinal lymph nodes 100 days after the spores were inhaled.⁵⁹

Three toxin components are synthesized on the bacterial surface and account for the major pathologic consequences of infection: protective antigen, lethal toxin, and edema toxin. Protective antigen binds host cell receptors and transports either lethal or edema toxin in the cells, which can cause cell swelling and death. Lethal toxin has a protease activity that cleaves specific kinases, which in turn may induce cell lysis.^60,61

DIAGNOSIS

Clinical Manifestations

Skin infection

Skin disease is the most common manifestation of anthrax, and half of the victims (11 of 22) in the United States bioterrorist attack presented with cutaneous disease. In the absence of exposure to animals or animal products, the diagnosis of cutaneous anthrax should immediately raise the possibility of a bioterrorist attack. One to 7 days after inoculation of spores into the skin, a small papule develops and progresses to a vesicle over the ensuing few days. Erythema and nonpitting edema often surround the vesicle. Initially, the vesicular fluid is serous and contains large numbers of organisms. Once the vesicle ruptures, a black eschar becomes evident at the base of the ulcer [seeFigure 5]. Anthrax derives its name, which is taken from the Greek word for coal, from these characteristic black lesions. Despite the erythema and swelling, lesions are not painful but may be mildly pruritic. Lymphangitis, lymphadenopathy, fever, and malaise may accompany the skin infection. After 1 to 2 weeks, the skin lesion dries and a permanent scar is formed. Lesions occur primarily on exposed regions of the body. The arms are the most frequent site of infection; the face and neck are also commonly involved. Lesions are usually single but may occur at multiple sites as a result of simultaneous inoculations.⁶²

Figure 5. Anthrax Lesion Typical dark, necrotic, painless pruritic skin lesion of anthrax on the wrist of a shepherd from Morocco.

Respiratory infection (Woolsorters' disease)

Unlike cutaneous anthrax, which is rarely fatal, inhalational anthrax is usually a fulminant disease with a high mortality. The index case in the United States bioterrorist attack presented as typical inhalational anthrax, followed quickly by sepsis and meningitis.⁶³ The pulmonary form of disease is usually biphasic in its presentation.⁶⁴ From 1 to 5 days after inhalation of spores, the patient has symptoms suggestive of a viral syndrome: nonproductive cough, malaise, fatigue, myalgia, and mild fever. Occasionally, the sensation of chest heaviness is reported. Other manifestations reported in the United States bioterrorist cases included sweats, often drenching; nausea and vomiting; abdominal pain; headache; confusion; and sore throat.⁶⁵ Rhonchi may be heard on pulmonary auscultation. Within 2 to 4 days, symptoms temporarily resolve but are rapidly followed by the second, more severe stage of the disease. This stage involves the sudden onset of severe respiratory distress with dyspnea, cyanosis, and diffuse diaphoresis, accompanied by fever, tachycardia, and tachypnea. On pulmonary auscultation, moist, crepitant rales are evident, and findings are consistent with pleural effusion. Chest x-ray often demonstrates a widened mediastinum. Infiltrates or consolidation may also be seen, and pleural effusions are commonly found.⁶⁵ Death often occurs within 24 hours and may be preceded by septic shock. A Russian patient was described as dying suddenly, in midsentence. Recent experience in the United States suggests that early aggressive antibiotic therapy may modify the outcome, and death rates were reduced from above 85% to below 50%.⁵⁶ Autopsy usually reveals hemorrhagic necrosis of the thoracic lymph nodes, drainage from the lungs, and hemorrhagic mediastinitis. A high index of suspicion is critical because any delay in diagnosis and treatment greatly increases the likelihood of a fatal outcome.

Gastrointestinal infection

Gastrointestinal anthrax has not been reported in the United States. It occurs primarily in developing countries, usually after ingestion of contaminated meat. The incubation period is usually 3 to 5 days. Patients initially have nausea, vomiting, anorexia, and fever. Acute abdominal pain, hematemesis, and bloody diarrhea follow rapidly. Findings on examination suggest an acute surgical abdomen, and there is moderate leukocytosis with immature band forms. Rapid progression to toxemia and shock leads to death within 2 to 5 days after the initial onset of symptoms.

An oropharyngeal form of anthrax has also been described. Inflammatory lesions that resemble the cutaneous lesions develop on the posterior pharynx, hard palate, or tonsils. Tissue necrosis and edema are accompanied by sore throat, dysphagia, fever, regional lymphadenopathy, and toxemia.

Meningitis

Anthrax meningitis can result from bacteremia precipitated by cutaneous, respiratory, or gastrointestinal infection. This complication is relatively rare, occurring in fewer than 5% of patients. In the index case in the United States bioterrorist attack, the patient presented to the emergency department with confusion, and gram-positive rods were identified in his CSF.⁶³ The onset of meningeal symptoms usually occurs simultaneously with the primary lesion or within several days after its onset. Meningitis is hemorrhagic and rapidly fatal (within 6 days).

Patient History and Laboratory Tests

A careful epidemiologic history is the single most important means of suggesting the diagnosis. A history of contact with herbivores or products from these animals, particularly if the products come from outside the United States, should raise the possibility of anthrax. The sudden appearance of several cases of severe acute febrile illness with a fulminant fatal outcome should raise the possibility of a bioterrorist attack. Certain occupational groups are at higher risk of being exposed to anthrax spores disseminated in the mail: post-office workers, members of the news media, and politicians and their staffs. Therefore, the occupational history can provide a critical clue for recognizing early inhalational and cutaneous anthrax.

The physical appearance of the skin lesions is characteristic, and Gram stains and cultures of the ulcer base are frequently positive. A history of exposure to dust from a contaminated animal product can usually be obtained in the prodromal phase of the respiratory illness, when symptoms are mild. In the absence of a thorough history, the disease is initially mistaken for a viral respiratory illness or bronchitis. Patients with gastrointestinal disease have a history of eating undercooked, often spoiled, meat.

Gram stain of the peripheral blood may reveal gram-positive bacilli, and in cases of meningitis, the CSF Gram stain is often positive. Blood cultures are positive in most cases of inhalational anthrax, and specimens for culture should be drawn immediately. The microbiology laboratory must be alerted to the possibility of anthrax, or the organism may be identified only as a Bacillus species and the pathogen misinterpreted as a contaminant. In the United States, the public health Laboratory Response Network, consisting of 81 clinical laboratories, has been established to specifically identify bioweapon pathogens, and all suspected samples should be referred to one of these laboratories for confirmatory diagnosis.⁵⁶

In addition to microbiologic-study results, a chest x-ray showing a widened mediastinum, infiltrates, and pleural effusions suggests inhalational anthrax. A chest CT scan is also helpful in this disease, revealing hyperdense hilar and mediastinal nodes, mediastinal edema, and infiltrates and pleural effusions. Thoracentesis may reveal hemorrhagic pleural fluid.

A variety of rapid-assay kits are available to detect B. anthracis spores on environmental surfaces. However, none of these kits has been independently evaluated or endorsed by the CDC or the Food and Drug Administration, and many false positive results were reported during the United States bioterrorist attack.⁵⁶

Enzyme-linked immunosorbent assays are available that measure antibody titers against lethal and edema toxins. A fourfold rise in titers over 4 weeks or a single titer of 1:32 is considered positive. This assay is not helpful during the acute illness. In patients receiving antibiotic prophylaxis, the antibody response may be blunted.

TREATMENT AND PROGNOSIS

Antibiotic therapy should be immediately initiated in all patients deemed at high risk who have fever or a systemic illness consistent with inhalational anthrax. Any delay in therapy increases the risk of a fatal outcome. First-line antibiotics are intravenous ciprofloxacin or doxycycline combined with one or two other antibiotics with activity against the pathogen [see Table 1]. Because anthrax strains may have constitutive and inducible β-lactamases, monotherapy with penicillin or ampicillin is not recommended. When meningitis is suspected, doxycycline should not be used, because of its poor CNS penetration. Once the patient is stable, oral antibiotics can be given, with ciprofloxacin (500 mg twice daily) or doxycycline (100 mg twice daily) being the treatment of choice. Because of the risk of delayed germination of spores in the host, therapy should be continued for 60 days. Oral ciprofloxacin or doxycycline for 60 days is recommended for cutaneous disease. Excision of skin lesions is contraindicated because of the increased risk of precipitating bacteremia. However, after appropriate antibiotic therapy, excision and skin grafting may be necessary.⁶⁶

Before antibiotics became available, cutaneous disease was associated with a mortality of 10% to 20%. With appropriate antibiotic treatment, fewer than 1% of patients die. Despite appropriate antibiotics and respiratory support, inhalational anthrax in the past was almost always fatal. However, experiences in Russia and the United States demonstrated that with early systemic antibiotic therapy, mortality can be reduced to approximately 50%. Gastrointestinal disease also is associated with a high mortality (25% to 100%).

PREVENTION

Postexposure prophylaxis is critical for the prevention of secondary cases. The selection of patients for prophylaxis depends on the environmental setting and the conditions of the spore release. Nasal swab cultures are insensitive and should not be used to determine whether an individual should receive prophylactic antibiotics. Nasal swab cultures are recommended only as an epidemiologic tool to determine the extent of exposure in a specific area or building. In cases of suspected exposure to B. anthracis, the preventive regimens of choice are oral fluoroquinolones (e.g., ciprofloxacin, 500 mg twice daily; levofloxacin, 500 mg a day; or ofloxacin, 400 mg twice daily) or, if fluoroquinolones are contraindicated, doxycycline (100 mg twice daily). Prophylaxis should be continued until exposure is excluded. If exposure is confirmed, prophylaxis should be continued for 60 days. In persons thought to be heavily exposed, prophylaxis for 100 days may be considered.⁶⁷

A killed vaccine derived from a component of the anthrax exotoxin is available and is recommended for all industrial workers at risk for exposure to contaminated animal products. As a result of increased concerns about biologic warfare and bioterrorism, military personnel are now vaccinated. Postexposure vaccination, although not approved by the FDA, has been shown to provide additional protection in animal studies and is recommended as an option by the CDC. In such cases, the vaccine is considered an investigational new drug and should be administered with informed consent.⁶⁸

Anecdotal reports in the lay press have associated anthrax vaccine with a high incidence of serious reactions. Nevertheless, although mild localized reactions occur in up to 30% of anthrax vaccine recipients, to date, with the exception of two cases of optic neuritis,⁶⁹ and one of delayed anaphylaxis,⁷⁰ serious adverse reactions have not been reported.⁷¹

For decontamination after anthrax exposure, exposed skin should be washed extensively with soap and water. Because of the potential danger of secondary aerosolization, decontamination of exposed environments is recommended. Decontamination is technically difficult and requires expert analysis that takes into account the nature of the exposure and the environmental conditions.⁵⁶

Infections and Disorders Due to Other Bacillus Species

Bacillus organisms are found in soil, dust, decaying organic matter, and water. Some species are part of the normal flora, particularly in patients who have had prolonged hospitalizations. Despite their widespread distribution, these organisms rarely cause infection and are more often isolated as a contaminant. Risk factors associated with serious Bacillus infections include use of intravascular catheters; intravenous drug abuse; sickle cell disease; and immunosuppression caused by malignancy, neutropenia, corticosteroid therapy, or AIDS.⁷²Because these organisms are frequently resistant to third-generation cephalosporins, prolonged antibiotic treatment regimens that include these agents may select out for Bacillus organisms.

1. cereusis the most frequent Bacillusspecies to cause invasive infection, followed by B. subtilis [see Table 4].

Table 4 Clinical Syndromes Caused by Bacillus Species

Species Clinical Syndrome B. alvei B. cereus B. sphaericus B. subtilis Meningitis, pneumonia, empyema, bacteremia Ophthalmitis, bacteremia, pneumonia, osteomyelitis,   endocarditis, soft tissue infection Peritonitis, pleuritis, lung infection, meningitis,   bacteremia Meningitis, otitis, mastolditis, urinary tract infection,   bacteremia, endocarditis, ventricular shunt   infection

DIAGNOSIS

Clinical Manifestations

With the exception of B. cereus eye infections, Bacillus infections are rare and cannot be clinically distinguished from those caused by other pyogenic organisms. Pneumonia can develop in the compromised host, and necrotizing pneumonia caused by B. cereus has been reported in patients with acute leukemia and hepatic malignancy. Fatal pseudomembranous tracheobronchitis and pneumonia have been reported in a patient with aplastic anemia.⁷³

Bacterial endocarditis caused by Bacillus species is a well-recognized complication of intravenous drug abuse. The tricuspid valve is most often involved, and the course of illness tends to be indolent. Bacillus species are among many pathogens that infect prosthetic valves.⁷⁴Indwelling catheters may become colonized with Bacillus organisms, resulting in positive blood culture results.

High-grade bacteremia can be complicated by fatal meningoencephalitis in the immunocompromised host.⁷⁵ Necrotizing fasciitis can be caused by B. cereus.

In rare cases, meningitis may result from Bacillus bacteremia, but more often, it has followed inadvertent inoculation of organisms into the CSF during spinal anesthesia.⁷⁶ Because of their hardy growth characteristics, Bacillus species are common laboratory contaminants. Pseudobacteremia caused by contamination of alcohol swabs and rubber stoppers on blood culture bottles and pseudomeningitis caused byBacillus contamination of commercial culture media have been reported.⁷⁷

Motor vehicle accidents in which injuries result from direct contact with the road may result in B. cereus soft tissue and bone infections that necessitate extensive surgical debridement and amputation for cure. Close-range gunshot wounds and injection of contaminated heroin have also been complicated by B. cereus soft tissue infection.^78,79

Ocular infections

1. cereusis a primary pathogen for ocular infections. This species is one of the most common agents associated with posttraumatic endophthalmitis. When B. cereusis the causative agent, an intraocular foreign body is often present. Injuries caused by metal fragments and injuries associated with contamination by soil and dust increase the risk of infection by B. cereus. The onset of infection is rapid and leads to destruction of the vitreous and retinal tissue, causing loss of vision 12 to 48 hours after inoculation. Patients frequently become systemically ill, with fever and leukocytosis. Endophthalmitis and panophthalmitis can also be related to intravenous drug abuse. Early diagnosis and treatment are critical for preventing permanent structural changes and blindness.⁸⁰

Food poisoning

1. cereusproduces two enterotoxins, diarrheal toxin and emetic toxin, that are responsible for two food-poisoning syndromes. The emetic form is associated primarily with the ingestion of contaminated fried rice.⁸¹From 1 to 6 hours after ingestion, the person experiences vomiting and symptoms identical to those of staphylococcal food poisoning. A case of fulminant fatal hepatic failure and rhabdomyolysis associated with ingestion of B. cereus emetic toxin has also been reported.⁸² The diarrheal form has a longer incubation period (10 to 12 hours), and manifestations are related to lower rather than upper gastrointestinal symptoms. Symptoms include abdominal pain, profuse watery diarrhea, tenesmus, and nausea. This syndrome is usually self-limited and lasts 12 to 24 hours. Outbreaks are generally associated with ingestion of contaminated meat, vegetables, and mayonnaise.⁸³

TREATMENT

1. cereusis resistant to penicillin and other β-lactam antibiotics, including cephalosporins; it is sensitive to vancomycin, gentamicin, imipenem, and ciprofloxacin. Clindamycin, erythromycin, chloramphenicol, and tetracycline have also been shown to be active. OtherBacillusspecies are susceptible to penicillins and cephalosporins. Pending speciation and susceptibility testing, vancomycin or clindamycin with or without gentamicin is the empirical treatment of choice when infection with Bacillus species is suspected [see Table 1].

If Bacillus bacteremia develops in immunocompromised patients with long-term indwelling catheters, antibiotic therapy must be instituted and the catheter removed to prevent recurrence of the infection.⁸⁴ In addition to antibiotic treatment, the deep-seated soft tissue infection associated with necrotizing fasciitis requires aggressive surgical debridement.⁸⁵

For intraocular infections, systemic therapy is usually supplemented with intravitreous clindamycin and gentamicin administered by an ophthalmologist. Intravitreous dexamethasone and early vitrectomy have been recommended for the management of sight-threatening endophthalmitis by B. cereus.⁸⁰

For patients with food poisoning, antibiotics are not required, because the disease is self-limited. Supportive care may include intravenous fluids if the patient becomes severely dehydrated.

Infections Due to the Erysipelothrix Organism

Human infections by the Erysipelothrix organism are rare and are always caused by E. rhusiopathiae (formerly E. insidiosa). This aerobic gram-positive bacillus grows on routine nonselective media and is nonhemolytic, nonmotile, and catalase negative. Because of the cell wall's high lipid content (about 30%), E. rhusiopathiae is resistant to desiccation and may tolerate salting, pickling, and smoking. The organism is capable of growing over a broad temperature range (4° to 42° C) and is widespread in nature. E. rhusiopathiae can infect mammals, birds, fish, shellfish, and insects. Human infection results from handling dead infected animal parts. Infection can also result from cat bites.⁸⁶ Most cases of skin infection occur during the summer and early fall. Infection is almost always the result of occupational exposure of slaughterhouse workers, butchers, fishers, farmers, and veterinarians. The organism is usually traumatically inoculated. Bacilli remain extracellular and are often located deep in the skin near capillaries, where the organism can gain entry into the bloodstream.⁸⁷

DIAGNOSIS

Clinical Manifestations

Erysipeloid

Between 2 and 7 days after skin inoculation, a purplish-red, nonvesicular area arises with a sharply defined, raised, serpiginous border. Lesions most often develop on the face and hands. Proximal regions of the hand and fingers are involved, whereas the distal phalanges are usually spared. Lesions spread peripherally at a slow pace (1 cm/day). Over time, the central part of the lesion begins to heal, resulting in a pale center surrounded by a fiery-red outer border. Lesions itch or burn and are rarely associated with lymphangitis or lymphadenitis. Fever and systemic complaints are rare.

Endocarditis

Endocarditis is rare but may occur on both deformed and normal heart valves. The onset is acute or subacute and most often involves the aortic valve. In approximately 40% of cases, a skin lesion is noted just before or at the time of diagnosis, suggesting the possibility of E. rhusiopathiae.⁸⁸ Often, the skin lesion has healed by the time endocarditis becomes apparent. Mental-status changes associated with multiple cerebral hemorrhages may also accompany this form of endocarditis.⁸⁹

Patient History and Laboratory Tests

An appropriate epidemiologic history will suggest the diagnosis. Morphologically, the skin lesions resemble erysipelas (caused by group A streptococci). However, the rate of progression of Erysipelothrix infection is considerably slower, and unlike erysipelas, this skin lesion is not associated with tenderness or lymphadenopathy. Injection and culturing of nonbacteriostatic normal saline is rarely successful. Biopsy of a full-thickness skin specimen from the advancing border of the lesion and culturing in glucose-containing broth result in the highest yields. PCR assays for erysipelas have proved to be useful in swine and have been applied to humans.⁹⁰ Diagnosis of endocarditis depends on isolation of the organism from blood cultures.

TREATMENT

The treatment of choice for erysipeloid is penicillin; a single injection of 600,000 units of penicillin G benzathine generally is curative. For penicillin-allergic patients, oral erythromycin (250 to 500 mg every 6 hours) is effective. This skin infection is usually self-limited and lasts about 3 weeks; however, antibiotic treatment hastens the healing. Bacterial endocarditis is best treated with intravenous penicillin G. For penicillin-allergic patients, intravenous cefazolin or ceftriaxone is recommended [see Table 1].^90,91 Most strains are resistant to vancomycin, which therefore should not be used for empirical therapy for endocarditis if E. rhusiopathiae is a possibility. Despite appropriate therapy, the mortality associated with endocarditis is 30% to 40%.

Acknowledgments

Figure 1 R. J. Collier, M.D., Harvard Medical School. Adapted by Dimitry Schidlovsky.

Figure 3 Dimitry Schidlovsky.

Figure 4 Courtesy of Professor Jean Hilarie Saurat, Geneva University Hospital, Geneva, Switzerland.

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Editors: Dale, David C.; Federman, Daniel D.