Antimicrobial Pharmacology — AI Study Guide

Mastering Antimicrobial Pharmacology

Antimicrobials are drugs that kill or inhibit microorganisms. They are classified by their target organism (antibacterial, antifungal, antiviral, antiparasitic) and by their mechanism. For antibiotics, five major mechanisms: (1) cell wall synthesis inhibition (beta-lactams, vancomycin, fosfomycin), (2) protein synthesis inhibition (aminoglycosides, macrolides, tetracyclines, chloramphenicol, linezolid, clindamycin), (3) DNA/RNA synthesis inhibition (fluoroquinolones, rifampin), (4) cell membrane disruption (polymyxins, daptomycin), (5) folate synthesis inhibition (sulfonamides, trimethoprim).

Beta-lactam antibiotics are the largest and most clinically important antibiotic class. They inhibit penicillin-binding proteins (PBPs), which catalyze cross-linking of peptidoglycan in bacterial cell walls. Subclasses: penicillins (ampicillin, amoxicillin, piperacillin), cephalosporins (first through fifth generation with increasing gram-negative coverage), carbapenems (broadest spectrum, including Pseudomonas and ESBL producers), and monobactams (aztreonam, gram-negative only). Beta-lactamase inhibitors (clavulanate, sulbactam, tazobactam) restore activity against beta-lactamase-producing organisms.

Antibiotic resistance arises through multiple mechanisms: (1) enzymatic inactivation (beta-lactamases destroy the beta-lactam ring), (2) altered target (PBP2a in MRSA, altered ribosome in macrolide resistance), (3) efflux pumps (actively export antibiotic from the cell), (4) decreased permeability (outer membrane changes reduce drug entry), (5) enzymatic modification (aminoglycoside-modifying enzymes). Resistance can be intrinsic (natural) or acquired (through mutation or horizontal gene transfer via plasmids).

Antifungal drugs target ergosterol — the fungal equivalent of mammalian cholesterol. Azoles (fluconazole, voriconazole, posaconazole) inhibit ergosterol synthesis via CYP51 (lanosterol demethylase); they interact with many CYP450-metabolized drugs. Amphotericin B binds ergosterol and disrupts membrane integrity — the most potent antifungal but nephrotoxic. Echinocandins (caspofungin, micafungin) inhibit beta-glucan synthesis in the fungal cell wall — excellent safety profile, fungicidal against Candida.

Frequently Asked Questions: Antimicrobial Pharmacology

How do beta-lactam antibiotics work?

Beta-lactam antibiotics (penicillins, cephalosporins, carbapenems) inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), which are enzymes that catalyze the final cross-linking step of peptidoglycan synthesis. Without cross-linking, the cell wall is weakened. Bacteria continue to grow and lyse under osmotic pressure. Beta-lactams are bactericidal and work best against rapidly dividing bacteria. Beta-lactamases (produced by resistant bacteria) inactivate these drugs by cleaving the beta-lactam ring.

What is the difference between bactericidal and bacteriostatic antibiotics?

Bactericidal antibiotics kill bacteria directly: beta-lactams, vancomycin, aminoglycosides, fluoroquinolones, metronidazole. Bacteriostatic antibiotics inhibit bacterial growth without killing, relying on the host immune system to eliminate bacteria: tetracyclines, macrolides, clindamycin, sulfonamides, chloramphenicol, linezolid. Bactericidal drugs are generally preferred for serious infections (meningitis, endocarditis, immunocompromised patients) where immune function alone is insufficient to clear infection.

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