Bacterial Infections and Antimicrobial Therapy
Core concepts
Know the mechanism-of-action families cold: beta-lactams (cell wall, bactericidal, time-dependent killing), glycopeptides (cell wall, only gram-positives), aminoglycosides (30S, concentration-dependent, nephro/ototoxic, synergy for enterococcal endocarditis), fluoroquinolones (DNA gyrase/topoisomerase, aortopathy and QT cautions), macrolides (50S, QT), tetracyclines (30S, avoid in pregnancy and children under 8), and oxazolidinones (50S initiation, myelosuppression, serotonin syndrome with MAOIs/SSRIs).
Must-know organisms and syndromes
Staph aureus (MSSA vs MRSA; vancomycin for MRSA, cefazolin/nafcillin for MSSA), streptococci (penicillin for GAS; ceftriaxone for pneumococcus meningitis at high dose), enterococci (ampicillin + ceftriaxone or gentamicin for endocarditis), gram-negative rods (ESBL = carbapenem; AmpC = cefepime preferred), Pseudomonas (piperacillin-tazobactam, cefepime, or meropenem; avoid ertapenem), anaerobes (metronidazole, clindamycin, or beta-lactam/beta-lactamase inhibitor), and atypical pathogens (doxycycline or azithromycin for Mycoplasma, Chlamydia, Legionella).
Traps
Beta-lactams are time-dependent, so extended or continuous infusion matters for high-MIC organisms. Ertapenem does not cover Pseudomonas or Enterococcus. Vancomycin AUC/MIC monitoring is preferred over trough-only. Daptomycin is inactivated by pulmonary surfactant-never for pneumonia. Clindamycin does not cover MRSA reliably. Linezolid is bacteriostatic for most organisms but bactericidal for streptococci.
Worked rule of thumb
For a patient with suspected MSSA bacteremia and endocarditis, cefazolin or nafcillin is preferred over vancomycin because vancomycin has inferior outcomes for MSSA. Switch to vancomycin only if MRSA is confirmed or the patient has a severe beta-lactam allergy.