PH8.3 | PH8.3 | Antibacterial Drug Classes — SDL Guide — Summary & Reflection

KEY TAKEAWAYS

Cell-wall inhibitors: β-Lactams (penicillins, cephalosporins [1st–5th gen], carbapenems, monobactams) inhibit PBPs; bactericidal, time-dependent (T>MIC). Vancomycin (glycopeptide) binds D-Ala-D-Ala; for MRSA, C. diff colitis (oral); Red Man Syndrome with rapid infusion.

30S inhibitors: Aminoglycosides (gentamicin, tobramycin, amikacin) — bactericidal, concentration-dependent (AUC/MIC; once-daily dosing); nephrotoxic + ototoxic; inactive vs anaerobes. Tetracyclines (doxycycline, minocycline) — bacteriostatic; broad-spectrum incl. Rickettsia, Chlamydia, Brucella; avoid <8 yrs and pregnancy.

50S inhibitors: Macrolides (erythromycin, azithromycin, clarithromycin) — bacteriostatic; gram-positives + atypicals; QT prolongation + CYP3A4 inhibition (erythromycin/clarithromycin).

DNA/RNA inhibitors: Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) — bactericidal, concentration-dependent; generations 1–4; QT prolongation, tendinopathy; avoid in pregnancy/<18 yrs. Moxifloxacin NOT for UTI (no renal excretion).

Folate inhibitors: TMP-SMX — sequential blockade → bactericidal combination; UTI, PCP prophylaxis/treatment; avoid in pregnancy (kernicterus), G6PD deficiency.

Newer agents: Linezolid (50S, bacteriostatic, MRSA/VRE; myelosuppression, serotonin syndrome). Daptomycin (membrane depolarisation, gram-positive only — NOT for pneumonia/inactivated by surfactant). Colistin (outer-membrane LPS disruption, last-resort gram-negative; nephrotoxic). Ceftazidime-avibactam (KPC/OXA-48 CRE — NOT for NDM or Acinetobacter).

Key prescribing reminders: Ceftriaxone fails ESBL organisms in vivo even if 'sensitive' in vitro (inoculum effect). Nitrofurantoin not for pyelonephritis (poor tissue penetration). Daptomycin not for pneumonia (surfactant inactivation). Aminoglycosides not for anaerobes (need O2 for uptake).

REFLECT

The six classes in this SDL — sulfonamides, quinolones, beta-lactams, macrolides, tetracyclines, and aminoglycosides — each emerged in response to a public health need: tuberculosis, gonorrhoea, pneumonia, gram-negative sepsis, rickettsial fever. Now, in 2026, each faces resistance that threatens its utility. As a prescribing clinician, which single prescribing habit — if adopted by every doctor in India — would have the greatest impact on preserving these drugs? Think of one patient interaction in your clinical training where an antibiotic was prescribed without clear indication. What would you do differently now?