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Penicillins and cephalosporins are the major antibiotics that inhibit bacterial cell wall synthesis. They are called beta-lactams because of the unusual 4-member ring that is common to all their members. The beta-lactams include some of the most effective, widely used, and well-tolerated agents available for the treatment of microbial infections. Vancomycin, fosfomycin, and bacitracin also inhibit cell wall synthesis but are not nearly as important as the beta-lactam drugs. The selective toxicity of the drugs discussed in this chapter is mainly due to specific actions on the synthesis of a cellular structure that is unique to the microorganism. More than 50 antibiotics that act as cell wall synthesis inhibitors are currently available, with individual spectra of activity that afford a wide range of clinical applications.

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A. Classification

All penicillins are derivatives of 6-aminopenicillanic acid and contain a beta-lactam ring structure that is essential for antibacterial activity. Penicillin subclasses have additional chemical substituents that confer differences in antimicrobial activity, susceptibility to acid and enzymatic hydrolysis, and biodisposition.

B. Pharmacokinetics

Penicillins vary in their resistance to gastric acid and therefore vary in their oral bioavailability. Parenteral formulations of ampicillin, piperacillin, and ticarcillin are available for injection. Penicillins are polar compounds and are not metabolized extensively. They are usually excreted unchanged in the urine via glomerular filtration and tubular secretion; the latter process is inhibited by probenecid. Nafcillin is excreted mainly in the bile and ampicillin undergoes enterohepatic cycling. The plasma half-lives of most penicillins vary from 30 min to 1 h. Procaine and benzathine forms of penicillin G are administered intramuscularly and have long plasma half-lives because the active drug is released very slowly into the bloodstream. Most penicillins cross the blood-brain barrier only when the meninges are inflamed.


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High-Yield Terms to Learn
Bactericidal An antimicrobial drug that can eradicate an infection in the absence of host defense mechanisms; kills bacteria
Bacteriostatic An antimicrobial drug that inhibits antimicrobial growth but requires host defense mechanisms to eradicate the infection; does not kill bacteria
Beta-lactam antibiotics Drugs with structures containing a beta-lactam ring: includes the penicillins, cephalosporins and carbapenems. This ring must be intact for antimicrobial action
Beta-lactamases Bacterial enzymes (penicillinases, cephalosporinases) that hydrolyze the beta-lactam ring of certain penicillins and cephalosporins; confer resistance
Beta-lactam inhibitors Potent inhibitors of some bacterial beta-lactamases used in combinations to protect hydrolyzable penicillins from inactivation
Minimal inhibitory concentration (MIC) Lowest concentration of antimicrobial drug capable of inhibiting growth of an organism in a defined growth medium
Penicillin-binding proteins (PBPs) Bacterial cytoplasmic membrane proteins that act as the initial receptors for penicillins and other beta-lactam antibiotics
Peptidoglycan Chains of polysaccharides and polypeptides that are cross-linked to form the bacterial cell wall
Selective toxicity More toxic to the invader than to the host; a property of useful antimicrobial drugs

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