Chapter 11: Antimicrobial Drugs

Antimicrobial agents achieve their effects through five primary mechanistic pathways: disrupting bacterial cell wall synthesis, inhibiting ribosomal protein production, interfering with nucleic acid replication and transcription, compromising plasma membrane integrity, and blocking essential metabolic processes such as the folate synthesis pathway targeted by sulfonamide compounds. The historical foundation of antimicrobial therapy emerged from landmark discoveries including Alexander Fleming's identification of penicillin and Gerhard Domagk's development of Prontosil, establishing the framework for modern chemotherapy. Effective drug selection requires evaluating selective toxicity, the ability to achieve adequate concentrations at infection sites while navigating physiological barriers such as the blood-brain barrier, and spectrum of activity ranging from narrow to broad spectrum designations. Laboratory assessment of drug efficacy employs standardized techniques including the Kirby-Bauer disk diffusion assay for measuring inhibition zones and dilution methodologies for determining minimal inhibitory concentration and minimal bactericidal concentration values. Clinical application demands careful consideration of the therapeutic index and potential adverse effects including hepatotoxicity, nephrotoxicity, and antibiotic-associated colitis. A critical contemporary concern involves antimicrobial resistance development, driven by selective pressure and horizontal gene transfer via resistance plasmids, manifesting through mechanisms including enzymatic inactivation via beta-lactamase production, membrane permeability alterations, enhanced drug efflux through multidrug transporters, and target site modification. The emergence of multiantibiotic-resistant organisms necessitates prudent antimicrobial stewardship through narrow-spectrum agent selection and prevention of superinfections caused by disruption of normal microflora. The chapter concludes by characterizing distinct drug classes including beta-lactam antibiotics, tetracyclines, fluoroquinolones, polyene and azole antifungals, antivirals such as nucleoside analogues, antiprotozoal compounds, and anthelmintic medications, each exhibiting unique mechanisms and clinical applications across diverse microbial pathogens.