Antimicrobial Agents: Beta-Lactams and Glycopeptides covers beta-lactams and glycopeptides for INI CET (AIIMS PG).
β-Lactam Antibiotics — Mechanism:
- Bind to penicillin-binding proteins (PBPs) — transpeptidases that cross-link peptidoglycan
- Inhibit peptidoglycan synthesis → cell wall weakens → osmotic lysis
- Human cells lack cell walls → selective toxicity
Penicillins:
Penicillin G (Benzylpenicillin):
- IV/IM only (acid-labile — destroyed by gastric acid)
- Narrow spectrum — mostly gram-positive cocci (streptococci, staphylococci without penicillinase), gram-negative cocci (meningococcus), spirochetes (syphilis)
- Not effective against: Staph aureus (unless penicillin-sensitive), Enterobacteriaceae, Pseudomonas, anaerobes
- Side effects: Hypersensitivity (most important — 1-10% of patients; anaphylaxis in 0.01%); drug fever; hemolytic anemia
Penicillin V (acid-stable oral):
- Oral equivalent of Penicillin G; less active; used for mild strep pharyngitis
Anti-staphylococcal penicillins (penicillinase-resistant):
- Methicillin, Nafcillin, Oxacillin, Cloxacillin, Dicloxacillin
- Used for suspected Staph aureus (penicillinase-producing) — nafcillin/oxacillin are most commonly used
- MRSA: Not effective (MRSA has altered PBP — mecA gene)
Aminopenicillins (extended spectrum):
- Ampicillin, Amoxicillin: Gram-positive + some gram-negative (H. influenzae, E. coli, Proteus)
- Amoxicillin-clavulanate (Augmentin): Clavulanate = β-lactamase inhibitor; effective against β-lactamase-producing Staph, H. influenzae, E. coli
- Ampicillin-sulbactam (Unasyn): Same principle
Anti-pseudomonal penicillins:
- Piperacillin (used with tazobactam = Zosyn): Antipseudomonal activity
Cephalosporins (β-lactams with 7-ACA core):
| Generation | Coverage | Examples | Notes |
|---|---|---|---|
| 1st | Gram-positive (esp. Staph) | Cefazolin | Surgical prophylaxis |
| 2nd | Gram-positive + some gram-neg | Cefuroxime, Cefaclor | Otitis media, respiratory |
| 3rd | Broad gram-negative | Ceftriaxone, Cefotaxime | Serious infections, meningitis |
| 4th | Broad + Pseudomonas | Cefepime | Hospital-acquired infections |
| 5th | Broadest + MRSA | Ceftaroline | MRSA pneumonia, skin infections |
⚡ Exam Tip for INI CET (AIIMS PG): Ceftriaxone (3rd gen) is a first-line agent for bacterial meningitis (covers N. meningitidis, S. pneumoniae, H. influenzae) and gonorrhea. Cefepime (4th gen) has better gram-positive coverage and Pseudomonas coverage than ceftriaxone.
Carbapenems ( broadest-spectrum β-lactams):
- Imipenem, Meropenem, Ertapenem, Doripenem
- Resistant to most β-lactamases (but NOT metallo-β-lactamases)
- Used for serious nosocomial infections, mixed aerobic-anaerobic infections
- Imipenem-cilastatin: Cilastatin inhibits renal dehydropeptidase (prevents renal metabolism of imipenem)
Monobactams:
- Aztreonam: Only gram-negative coverage (Pseudomonas, Neisseria); no cross-reactivity with other β-lactams — can use in penicillin-allergic patients
Glycopeptides:
- Vancomycin: Cell wall inhibitor; bactericidal; gram-positive ONLY
- Used for: MRSA, serious Staph infections, C. difficile (oral), penicillin-resistant S. pneumoniae
- Red man syndrome: Histamine release from rapid IV infusion (not allergy — slow infusion prevents it)
- Nephrotoxicity (especially with aminoglycosides — synergistic)
- Ototoxicity (especially in renal failure)
- Monitor: Trough levels (keep >10 μg/mL for serious infections; <20 to minimize toxicity)
- Teicoplanin: Similar to vancomycin; longer half-life; can be given IM
- Daptomycin: Lipopeptide — depolarizes bacterial membrane (not cell wall); used for MRSA skin/soft tissue infections and S. aureus bacteremia; NOT for pneumonia (sequestered by lung surfactant)