Overview of Antibacterial Drugs
Antibacterial drugs are derived from bacteria or molds or are synthesized de novo. Technically, “antibiotic” refers only to antimicrobials derived from bacteria or molds but is often (including in The Manual) used synonymously with “antibacterial drug.”
Antibiotics have many mechanisms of action, including inhibiting cell wall synthesis, l, increasing cell membrane permeability, and interfering with protein synthesis, nucleic acid metabolism, and other metabolic processes (eg, folic acid synthesis).
Antibiotics sometimes interact with other drugs, raising or lowering serum levels of other drugs by increasing or decreasing their metabolism or by various other mechanisms (see Table: Common Effects of Antibiotics on Other Drugs). The most clinically important interactions involve drugs with a low therapeutic ratio (ie, toxic levels are close to therapeutic levels). Also, other drugs can increase or decrease levels of antibiotics.
Many antibiotics are chemically related and are thus grouped into classes. Although drugs within each class share structural and functional similarities, they often have different pharmacology and spectra of activity.
Common Effects of Antibiotics on Other Drugs
Antibiotics should be used only if clinical or laboratory evidence suggests bacterial infection. Use for viral illness or undifferentiated fever is inappropriate in most cases; it exposes patients to drug complications without any benefit and contributes to bacterial resistance.
Certain bacterial infections (eg, abscesses, infections with foreign bodies) require surgical intervention and do not respond to antibiotics alone.
Cultures and antibiotic sensitivity testing are essential for selecting a drug for serious infections. However, treatment must often begin before culture results are available, necessitating selection according to the most likely pathogens (empiric antibiotic selection).
Whether chosen according to culture results or not, drugs with the narrowest spectrum of activity that can control the infection should be used. For empiric treatment of serious infections that may involve any one of several pathogens (eg, fever in neutropenic patients) or that may be due to multiple pathogens (eg, polymicrobial anaerobic infection), a broad spectrum of activity is desirable. The most likely pathogens and their susceptibility to antibiotics vary according to geographic location (within cities or even within a hospital) and can change from month to month.
For serious infections, combinations of antibiotics are often necessary because multiple species of bacteria may be present or because combinations act synergistically against a single species of bacteria. Synergism is usually defined as a more rapid and complete bactericidal action from a combination of antibiotics than occurs with either antibiotic alone. A common example is a cell wall–active antibiotic (eg, a β-lactam, vancomycin) plus an aminoglycoside.
In vivo antibiotic effectiveness involves many factors, including
Pharmacology (eg, absorption, distribution, concentration in fluids and tissues, protein binding, rate of metabolism or excretion)
Pharmacodynamics (ie, the time course of antibacterial effects exerted by drug levels in blood and at the site of infection)
Drug interactions or inhibiting substances
Host defense mechanisms
Bactericidal drugs kill bacteria. Bacteriostatic drugs slow or stop in vitro bacterial growth. These definitions are not absolute; bacteriostatic drugs may kill some susceptible bacterial species, and bactericidal drugs may only inhibit growth of some susceptible bacterial species. Bactericidal antibiotics may be preferred for patients who have infections that impair host defenses locally (eg, meningitis, endocarditis) or who are immunocompromised (eg, neutropenic). More precise quantitative methods identify the minimum in vitro concentration at which an antibiotic can inhibit growth (minimum inhibitory concentration, or MIC) or kill (minimum bactericidal concentration, or MBC). An antibiotic with bactericidal activity is important if host defenses are impaired locally at the site of infection (eg, in meningitis or endocarditis) or systemically (eg, in patients who are neutropenic or immunocompromised in other ways).
The predominant determinant of bacteriologic response to antibiotics is either the
β-Lactams and vancomycin exhibit time-dependent bactericidal activity. Increasing their concentration above the MIC does not increase their bactericidal activity, and their in vivo killing is generally slow. In addition, because there is no or very brief residual inhibition of bacterial growth after concentrations fall below the MIC (postantibiotic effect, or PAE), β-lactams are most often effective when serum levels of free drug (drug not bound to serum protein) exceed the MIC for ≥ 50% of the time. Because ceftriaxone has a long serum half-life, free serum levels exceed the MIC of very susceptible pathogens for the entire 24-h dosing interval. However, for β-lactams that have serum half-lives of ≤ 2 h, frequent dosing or continuous infusion is required. For vancomycin, trough levels should be maintained at least at 15 to 20 μg/mL.
Aminoglycosides, fluoroquinolones, and daptomycin exhibit concentration-dependent bactericidal activity. Increasing their concentrations from levels slightly above the MIC to levels far above the MIC increases their rate of bactericidal activity and decreases the bacterial load. In addition, if concentrations exceed the MIC even briefly, aminoglycosides and fluoroquinolones have a PAE on residual bacteria; duration of PAE is also concentration-dependent. If PAEs are long, drug levels can be below the MIC for extended periods without loss of efficacy, allowing less frequent dosing. Consequently, aminoglycosides and fluoroquinolones are usually most effective as intermittent boluses that reach peak free serum levels ≥ 10 times the MIC of the bacteria; usually, trough levels are not important.
For many antibiotics, oral administration results in therapeutic blood levels nearly as rapidly as IV administration. However, IV administration is preferred in the following circumstances:
Oral antibiotics cannot be tolerated (eg, because of vomiting).
Oral antibiotics cannot be absorbed (eg, because of malabsorption after intestinal surgery).
Intestinal motility is impaired (eg, because of opioid use).
No oral formulation is available (eg, for aminoglycosides).
Patients are critically ill, possibly impairing GI tract perfusion or making even the brief delay with oral administration detrimental.
Doses and scheduling of antibiotics may need to be adjusted for the following:
Patients with renal failure (see Table: Usual Doses of Commonly Prescribed Antibiotics)
Patients with hepatic insufficiency (most commonly for cefoperazone, chloramphenicol, metronidazole, rifabutin, and rifampin)
Pregnancy and breastfeeding affect choice of antibiotic. Penicillins, cephalosporins, and erythromycin are among the safest antibiotics during pregnancy; tetracyclines are contraindicated. Most antibiotics reach sufficient concentrations in breast milk to affect a breastfed baby, sometimes contraindicating their use in women who are breastfeeding.
Usual Doses of Commonly Prescribed Antibiotics
Complications of antibiotic therapy include superinfection by nonsusceptible bacteria or fungi and cutaneous, renal, hematologic, and GI adverse effects.
Adverse effects frequently require stopping the causative drug and substituting another antibiotic to which the bacteria are susceptible; sometimes, no alternatives exist.
Resistance to an antibiotic may be inherent in a particular bacterial species or may be acquired through mutations or acquisition of genes for antibiotic resistance that are obtained from another organism. Different mechanisms for resistance are encoded by these genes (see Table: Common Mechanisms of Antibiotic Resistance). Resistance genes can be transmitted between 2 bacterial cells by the following mechanisms:
Transformation (uptake of naked DNA from another organism)
Transduction (infection by a bacteriophage)
Conjugation (exchange of genetic material in the form of either plasmids, which are pieces of independently replicating extrachromosomal DNA, or transposons, which are movable pieces of chromosomal DNA)
Plasmids and transposons can rapidly disseminate resistance genes.
Antibiotic use preferentially eliminates nonresistant bacteria, increasing the proportion of resistant bacteria that remain. Antibiotic use has this effect not only on pathogenic bacteria but also on normal flora; resistant normal flora can become a reservoir for resistance genes that can spread to pathogens.