Infectious diseases are caused by microbes or by microbial products. Antimicrobial drugs are intended to eliminate foreign organisms or abnormal cells from healthy tissues of the patient without comparable effects on the normal tissue cells of the host. This essential property of these drugs is called selective toxicity.
28.1 Classification of Antimicrobial Agents
Chemical Structure and Mechanism of Action
The main classification of antimicrobial agents is based on chemical structure (e.g., β-lactams and aminoglycosides) and mechanism of action (see Table 28.1).
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Table 28.1 |
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Mechanism of Action |
Drugs |
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Inhibition of bacterial cell wall synthesis |
β-lactams: penicillins, cephalosporins, and carbapenems Others: cycloserine, vancomycin, and bacitracin |
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Reversible inhibition of protein synthesis by disrupting the function of 30S or 50S ribosomal subunits |
Bacteriostatic: chloramphenicol, tetracyclines, erythromycin, clindamycin, streptogramins, and linezolid Bactericidal: aminoglycosides |
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Inhibition of nucleic acid metabolism by inhibiting RNA polymerase |
Rifampin and rifabutin |
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Inhibition of nucleic acid metabolism by inhibiting DNA gyrase or topoisomerase |
The quinolones |
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Inhibition of essential enzymes of folate metabolism (antimetabolites) |
Trimethoprim and sulfonamides |
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Bacteriostatic or Bactericidal
Antimicrobial agents are also classified according to whether they are bacteriostatic or bactericidal (Fig. 28.1).
Fig. 28.1 
Bacteriostatic versus bactericidal antibacterial agents.
Bacteria are able to multiply in vitro in a growth medium if conditions are favorable. If the growth medium contains an antibiotic, the bacteria may be killed (bactericidal effect), or the bacteria may survive but are unable to multiple (bacteriostatic effect).
— Bacteriostatic agents primarily inhibit bacterial growth. Killing of the organism is then dependent upon host defense mechanisms. The disadvantage of these agents is that in the setting of inadequate host defense mechanisms, any partially inhibited organisms may survive, replicate, and produce recurrent disease when the antibiotic is discontinued.
– Bactericidal agents are capable of killing the bacteria and are preferable if the patient has neutropenia or immunosuppression.
Anatomy of bacteria
Bacteria are single-cell organisms 0.3 to 5 μm in size and are typically spherical (cocci), straight (bacilli), curved, or spiral rods. They lack a nuclear membrane and have no true nucleus. The chromosome in bacteria is typically a single, closed circle DNA that is concentrated in a nucleoid region. Some bacteria possess smaller extrachromosomal pieces of DNA called plasmids. The cytoplasmic membrane is surrounded by a cell wall. The cell wall of gram-negative bacteria have an outer membrane that is absent in gram-positive bacteria.
Oxygen levels and bacterial growth
Different bacteria require different oxygen levels for optimal growth and cell division. There are obligate aerobes that require a high level of oxygen for growth, microaerophiles that require oxygen but at a reduced level, facultative anaerobes that can grow in the presence or absence of oxygen, aero-tolerant anaerobes that can tolerate some oxygen, and obligate anaerobes that grow only in the absence of oxygen.
Spectra of Antimicrobial Agents
Antimicrobial agents are further classified into spectra depending on the range of microorganisms on which they act:
– Narrow-spectrum agents are effective against a limited range of microorganisms.
– Extended-spectrum agents are principally effective against gram-positive bacteria, but they are also effective against a significant range of gram-negative bacteria.
– Broad-spectrum agents are effective against a wide range of microorganisms.
The use of broad-spectrum antibiotics should be limited, as they predispose patients to superinfection (the appearance of a new infection during treatment) by disrupting the body's natural bacterial flora.
Superinfections
Antibiotic drugs alter the normal microbial population of the intestinal, upper respiratory, and genitourinary tracts. This alteration of the normal flora may lead to the development of a superinfection, which is defined as the appearance of a new infection during therapy of the primary infection. This phenomenon is relatively common and may be dangerous because the superinfecting microbes are frequently drug resistant. Superinfections are more likely to occur with broad-spectrum antibiotics and with longer treatment durations.
28.2 Selection of Antimicrobial Agents
The selection of antimicrobial agents involves the consideration of many factors relating to the microorganisms involved, patient (host) factors, and pharmacology of the agents themselves.
Microorganism Factors
Species of Microorganism
Successful treatment of an infection requires knowledge of the pathogen(s) i nvolved. Rapid tests are available to confirm the presence of some common infections prior to the initiation of antibiotic therapy. Examples include a dipstick test for the presence of bacteria in the urine and a throat swab for strep throat. Empiric therapy can then be initiated. In more severe infections, especially if the pathogen has shown antibiotic resistance, definitive identification of the infectious microorganism and its susceptibility to various antibiotics by laboratory testing is required.
Bacterial identification typically involves characterization by Gram staining, cell shape, and media requirements for growth. More advanced tests involve binding of specific antibodies and genetic analysis by polymerase chain reaction (PCR) or gene sequencing.
Gram staining
Gram staining is a laboratory test that allows bacteria to be classified in two groups, gram positive and gram negative, based on the composition of their cell walls. Gram-positive bacteria cell walls are rich in proteoglycan but have no lipopolysaccharide and stain purple, whereas gram-negative bacteria have little proteoglycan but are rich in lipopolysaccharide and stain pink. Gram staining is an important tool in helping to determine the species of bacteria responsible for infections so that the most appropriate antimicrobial agent is selected for treatment.
Susceptibility to Antimicrobial Agents
Bacterial strains, even of the same species, may vary widely in antibiotic sensitivity. Several tests are available for determination of bacterial sensitivity to antimicrobial agents to allow for optimal selection. The standard tests are disk diffusion tests and agar or broth dilution tests. Other quantitative tests are used to determine minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). The MIC is the concentration of an antibiotic necessary to inhibit microbial growth under standardized conditions; the MBC is the concentration of antibiotic required to kill the microorganism. The results of these tests can then be used to determine the antibiotic dose required.
Disk diffusion test
Sensitivity to various antibiotics can be determined with the disk diffusion method. Microorganisms are cultured over paper disks on an agar surface. The disks contain antibiotic drugs. After 18 to 24 hours of incubation, the size of the clear zone of inhibition around the disk is measured. The diameter of the zone depends on the activity of the drug against the test strain. Newer methods measure bacterial gene expression by the polymerase chain reaction (PCR) to identify specific pathogens.
Broth dilution tests
In dilution tests, the concentration of antibiotics is serially diluted in either solid agar or liquid broth containing a culture of the test microorganism. The lowest concentration of the agent that prevents growth after 18 to 24 hours of incubation is known as the minimal inhibitory concentration (MIC). Automated systems also use a broth dilution method. Bacterial growth is measured as the optical density of culture of the organism in liquid (broth) in various concentrations of drug. The MIC is the concentration at which the optical density remains below a threshold.
Bacterial enzymes
Enzymes are produced by many organisms and serve to promote or enhance the infection by breaking down tissues to produce foodstuffs and allowing the spread of the organism within tissue. Mucinase is produced by Entamoeba histolytica and acts to dissolve the protective mucoid coating on intestinal epithelial cells. Many clostridial organisms, including Clostridium perfringens, produce collagenase that dissolves collagen in connective tissue. The connective tissue between cells, hyaluronic acid, is degraded by many bacteria (e.g., streptococci, clostridia, and staphylococci) that produce hyaluronidase. Streptokinase and staphylokinase are examples of enzymes that break down blood clots. Other enzymes include phospholipase C, proteases, DNAase, lipases, and lysins.
Bacterial toxins
Many bacteria produce toxins that may cause damage to the host. Diseases such as diphtheria, tetanus, staphylococcal scalded skin syndrome, and cholera are caused by the production of a toxin at the site of the infection. Despite efforts to classify toxins, many are labeled by the site on which they act; for example, neurotoxins act on the nervous system, hemotoxins bring about the lysis of erythrocytes, hepatotoxins affect the liver, and enterotoxins act on the intestine. These differences in cell site are related to receptor specificity and ability of the toxin to bind to a host cell membrane receptor.
Exotoxins and endotoxins
Toxins that are released from the bacterial cell are termed exotoxins and can be released from both gram-positive and gram-negative bacteria. They can be single proteins or polymeric toxins composed of A and B subunits. The B component of subunit toxins bind to specific receptors on the host cell membrane, causing the release of the A or active subunit. Examples of A–B toxins include tetanus toxin, Pseudomonas exotoxin A, Shiga toxin, botulinum toxin, cholera toxin, and diphtheria toxin. The genes for exotoxin production may be located on the bacterial genome or encoded on a plasmid or lysogenic bacteriophage. In the gram-negative organism, part of the cell envelope is an endotoxin (lipopolysaccharide). Importantly, the toxic moiety of endotoxin is lipid A, which is released when the organism lyses. It binds to receptors on the cell membrane of B cells and macrophages, causing the release of interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α), prostaglandins, and IL-6, which causes fever and hypoglycemia. Further, lipopolysaccharide activates the alternate complement pathway and causes the release of mediators from mast cells that increase vascular permeability, leading to hypertension and shock; lipopolysaccharide also causes platelets to be sticky and causes disseminated intravascular coagulation (DIC). In addition to the endotoxin, other cell wall components, including peptidoglycan, teichoic acids, and lipoteichoic acids, cause the induction of fever and are therefore pyrogenic.
Superantigens
In some instances, organisms produce superantigens that are capable of activating T cells by specific binding to the T cell and linkage to a class II major histocompatibility complex moiety on another cell type. Such linkage causes T-cell activation and the release of IL-1 and IL-2; the effect on T cells also can result in the loss of a T-cell response. Superantigens are produced by Staphylococcus aureus that results in toxic shock syndrome, Streptococcus pyogenes (erythrogenic toxins), and staphylococcal enterotoxins.
Resistance to Antimicrobial Agent
Bacterial resistance to an antimicrobial agent may be intrinsic or acquired (Fig. 28.2). Acquired resistance can occur due to spontaneous mutations or by the transfer of drug-resistant genes.
Fig. 28.2 Bacterial resistance.
Some bacteria are naturally insensitive to antibacterial drugs and can grow and multiply in their presence. Other bacteria that are normally sensitive to antibacterial agents may develop mutant strains such that when an antibacterial agent is given, the sensitive bacteria are killed, but the mutant bacteria are able to multiply unimpeded.
Spontaneous mutations. Spontaneous DNA mutations are rare, occurring in one in 106 to one in 108 base pairs. The chance that a given mutation will lead to antibiotic resistance is even rarer. On the other hand, the fast replication rate of bacteria, as well as the large numbers of cells attained, increases the chance that a spontaneous mutation will lead to antibiotic resistance. The antibiotic provides selective pressure on the organisms, killing the nonmutant cells while the resistant mutants proliferate.
Transfer of drug-resistant genes. Bacteria are able to transfer genes that confer resistance to each other. This usually occurs via plasmids, which are small, circular, extrachromosomal pieces of DNA; or via transposons, which are small pieces of DNA that can hop from DNA molecule to DNA molecule. Once in a chromosome or plasmid, the transposons can be integrated stably. They then act by the four main mechanisms described in Table 28.2 to achieve resistance.
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Table 28.2 |
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Mechanism of Resistance |
Example |
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Inactivation of the drug |
Bacterial β-lactamases (penicillinases) inactivate penicillins and cephalosporins by cleaving the β-lactam ring of the drug. |
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Mutation of the target |
Bacteria synthesize modified targets against which the drug has no effect (e.g., group B Streptococcus, which is frequently responsible for peripartum maternal and neonatal infections, can develop resistance to erythromycin via genes that modify the ribosomal target of the drug). |
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Prevention of the drug from entering the cell by decreasing permeability of the cell |
Changes in porins in the outer cell membrane can reduce the amount of antibiotic that can enter the bacterium. |
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Actively transporting the drug out of the cell |
The multidrug resistance pump exports a variety of foreign molecules, including some antibiotics, and imports protons in an exchange reaction. |
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Resistance is more likely in cases of hospital-acquired infections because widespread antibiotic use in hospitals selects for resistant organisms. Furthermore, hospital strains are often resistant to multiple antibiotics. This resistance is usually due to the acquisition of plasmids carrying several genes that encode the enzymes that mediate resistance. Multidrug resistance (MDR) occurs when microorganisms develop resistance to multiple classes of antibiotics, either by use of the MDR pump or by acquiring various resistance genes.
Patient Factors
When selecting an antimicrobial agent, the mode of administration, dosing regimen, and patient's acute health status, as well as his or her overall health, need to be considered with regard to the factors listed in Table 28.3.
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Table 28.3 |
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Factor |
Explanation |
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Renal disease |
Drugs that are eliminated by the kidneys may accumulate in renal disease, causing toxicity. This may necessitate a dose reduction of any antibiotic given. |
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Hepatic disease |
A dose reduction may also be necessary for antibiotics that are extensively metabolized and excreted by the liver. Some antibiotics are contraindicated in liver disease. |
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Pregnancy |
All antibiotics are able to cross the placenta, so the risk of teratogenesis must be considered. |
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Lactation |
The potential for a toxic accumulation of drug in the infant via breast milk must be considered. |
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Immune status |
Patients with compromised immune systems (e.g., those undergoing cancer chemo-therapy or with HIV) will generally require higher doses and longer courses of treatment. |
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Age |
Older patients tend to have decreased renal function; infants have poorly developed drug detoxification mechanisms. |
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Abbreviation:HIV, human immunodeficiency virus. |
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Drug Factors
The pharmacokinetics of drugs has a bearing on antimicrobial selection. Table 28.4 lists the factors that should be considered.
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Table 28.4 |
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Factor |
Explanation |
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Site of infection |
Access of the antimicrobial agent to the site of infection determines whether or not an adequate drug concentration can be achieved. – Drugs that are extensively bound to plasma proteins may not penetrate the site of infection to the same extent as those that show less protein binding. – If the infection involves the central nervous system, the drug must penetrate the blood–brain barrier (lipid-soluble and low-molecular-weight drugs). |
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Mode of administration |
– Many agents are rapidly and completely absorbed after oral administration and can be given by mouth. Sometimes an initial injection will be followed by a course of oral therapy. – In patients with severe acute infections, drugs may be given intravenously or intramuscularly, so that effective therapeutic levels of antibiotics can reach the site of infection more rapidly. |
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Antibiotics in abscesses
Antibiotics affect the growth and replication of bacteria; as such, they are most effective against actively growing bacterial cultures. When infection becomes more stagnant (e.g., in abscesses), antibiotics alone are often not sufficient, as they are unable to penetrate the capsule that forms around the abscess, and they tend to be less effective in low pH environments. In these cases, the abscess should be incised and drained to allow most of the pus to be evacuated and to promote better penetration of the antibiotic to any residual bacteria.
28.3 Empiric Treatment of Infectious Diseases and Combination Therapy
Empiric Treatment
The selection of an antimicrobial agent for a patient who is diagnosed with an infectious disease can be empirical, that is, initiated with a drug that is most likely to treat the case at hand. The choice of an antibiotic with which to initiate empiric therapy is based on the most likely pathogen for a given infection and the susceptibility profile of the suspected pathogen. The site and severity of the infection, as well as patient factors also have an important bearing on the choice of agent. With empiric therapy, an otherwise healthy outpatient with a mild infection caused by a pathogen with known antibiotic susceptibility can be treated immediately, successfully, and without further testing. In more severe or prolonged infections, in patients who are hospitalized or have other illnesses, or when the causative pathogen exhibits antibiotic resistance, empiric factors may be used to initiate therapy without a delay. Once the infectious microorganism is identified by laboratory testing and its susceptibility to antibiotics determined, definitive therapy can be continued with a different agent if the empiric choice was not optimal.
Combination Therapy
In cases of superinfection or resistance, combinations of antibiotics may be warranted. The resultant antiinfective activity of two drugs may be
– Indifferent (the addition of the second drug makes no difference)
– Additive (the total effect of the two drugs is equal to the sum of the effect of each drug given individually)
– Synergistic (the effect of the two drugs given together is greater than the sum of the two drugs given individually). These interactions are the most important clinically, and several types can be exploited to achieve better therapeutic results. For example, two drugs may sequentially block a microbial metabolic pathway, one drug may enhance the entry of a second drug into bacteria or fungi, or one drug may prevent the inactivation of a second drug by microbial enzymes.
– Antagonistic (the effect of the drugs given together is less than the sum of the drugs given individually). Antagonism is rarely observed clinically, but it could occur if a bacteriostatic drug, which inhibits protein synthesis, is given with a bactericidal drug that depends on cell growth to be effective.
28.4 Prophylaxis of Infection with Antimicrobial Agents
Antibiotics may be used prophylactically to prevent infection in individuals exposed to contagious pathogens or to prevent recurrent infections. Because of the potential for the development of antibiotic resistance and the potential to cause superinfections, specific guidelines have been developed for the prophylactic uses of antibiotics. Prophylaxis is recommended for patients undergoing procedures that will cause bacteremia (e.g., dental, upper gastrointestinal, or respiratory tract procedures) for whom the complications of infection would be catastrophic.
This includes
– Patients with a history of bacterial endocarditis (see page 289)
– Patients with prosthetic heart valves
– Cardiac transplantation patients who have developed valve problems
Specific guidelines are also in place for surgical patients and for the treatment of wounds.
28.5 Reference Tables
Tables 28.5 and 28.6 have been included for reference when discussing the spectra of agents in the chapters that follow.
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Table 28.5 |
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Gram-positive |
Gram-negative |
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Cocci |
Bacilli |
Cocci |
Bacilli |
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Staphylococcus S. aureus S. epidermidis Streptococcus S. pyogenes S. viridans S. pneumoniae S. sanguinis S. mitis S. bovis Enterococcus E. faecalis E. mutans |
Bacillus B. cereus B. anthracis Listeria Actinomyces Clostridium C. difficile C. perfringens C. tetani C. botulinum |
Neisseria N. gonorrhoeae N. meningitides |
Enterobacteriaceae Escherichia Yersinia Proteus Serratia Salmonella Shigella Morganella Enterobacter Citrobacter Klebsiella Aeromonas Plesiomonas Campylobacter Legionella Vibrio Pseudomonas Helicobacter H. pylori Bacteroides |
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Table 28.6 |
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Mycobacterium M. tuberculosis M. leprae Spirochetes Treponema Leptospira Borrelia |
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Mycoplasma |
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Chlamydia |
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Rickettsia |
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Mechanism of Action of Antimicrobial Agents