Chapter 20: Antimicrobial Drugs

Antimicrobial Drugs distinguishes between broad-spectrum and narrow-spectrum antibiotics, and explains the implications of disrupting normal microbiota and causing superinfections. The five major mechanisms of antimicrobial action are examined in detail: (1) inhibition of cell wall synthesis, as seen in β-lactam antibiotics like penicillins and cephalosporins; (2) inhibition of protein synthesis, targeting 70S ribosomes with drugs like chloramphenicol, streptomycin, and tetracycline; (3) injury to the plasma membrane, affecting membrane integrity with polymyxin B and daptomycin; (4) inhibition of nucleic acid synthesis, which interferes with DNA or RNA replication via rifamycins and quinolones; and (5) inhibition of essential metabolite synthesis, particularly via competitive inhibition using sulfonamides. The chapter also explores antiviral, antifungal, antiprotozoan, and antihelminthic drugs, explaining their challenges due to the similarity of eukaryotic pathogens to human cells. It discusses agents like acyclovir for herpes viruses, azoles for fungal infections, chloroquine for malaria, and ivermectin for parasitic worms. The role of synergism and antagonism in drug combinations is also introduced. A major focus is placed on antibiotic resistance—how bacteria evade drugs through mechanisms such as enzymatic degradation (e.g., β-lactamase), alteration of drug targets, efflux pumps, and reduced permeability. The misuse and overuse of antibiotics in medicine and agriculture are highlighted as key contributors to resistance. The chapter concludes with methods for evaluating antimicrobial effectiveness, such as the disk-diffusion (Kirby-Bauer) method, E test, and broth dilution tests to determine the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). Overall, this chapter equips students with foundational knowledge to understand how antimicrobial agents work, how resistance emerges, and how this information guides clinical decision-making and public health policy.