What are the recommended antibiotics, their spectrum of activity, and typical doses for common bacterial infections?

Antibiotics: Mechanisms of Action, Spectrum, and Dosing

The recommended antibiotics for common bacterial infections include beta-lactams, fluoroquinolones, aminoglycosides, and others, with specific dosing regimens based on infection site, severity, and pathogen susceptibility. 1

Beta-Lactam Antibiotics

Penicillins

• Mechanism of Action: Inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs) 1
• Spectrum:
  • Amoxicillin: Gram-positive cocci, some Gram-negative rods 1
  • Amoxicillin-clavulanate: Extended spectrum including beta-lactamase producers 1
  • Dosing:
    • Amoxicillin: 500 mg PO three times daily 1
    • Amoxicillin-clavulanate: 875/125 mg PO twice daily 1
• Penicillinase-resistant penicillins (dicloxacillin, oxacillin, nafcillin):
  • Spectrum: Staphylococcus aureus (not MRSA) 1
  • Dosing:
    • Oxacillin/nafcillin: 1-2 g IV every 4 hours (adults); 50 mg/kg/dose every 6 hours (pediatrics) 1
    • Dicloxacillin: 500 mg PO four times daily 1

Cephalosporins

• Mechanism of Action: Similar to penicillins, inhibit cell wall synthesis 1
• First Generation (cefazolin, cephalexin):
  • Spectrum: Gram-positive cocci, limited Gram-negative coverage 1
  • Dosing:
    • Cefazolin: 1 g IV every 8 hours 1
    • Cephalexin: 500 mg PO every 6 hours 1
• Second Generation (cefuroxime, cefoxitin):
  • Spectrum: Improved Gram-negative coverage, some anaerobic activity (cefoxitin) 1
  • Dosing:
    • Cefuroxime: 500 mg PO twice daily or 1 g IV every 12 hours 1
    • Cefoxitin: 1-2 g IV every 6-8 hours 1
• Third Generation (ceftriaxone, cefotaxime, ceftazidime):
  • Spectrum: Broad Gram-negative coverage, variable Gram-positive activity 2
  • Dosing:
    • Ceftriaxone: 1-2 g IV daily or every 12 hours 1, 2
    • Cefotaxime: 1-2 g IV every 6-8 hours 1
    • Ceftazidime: 1-2 g IV every 8 hours (enhanced Pseudomonas coverage) 1

Carbapenems

• Mechanism of Action: Broadest spectrum beta-lactams, highly resistant to beta-lactamases 1
• Spectrum: Gram-positive, Gram-negative (including many resistant strains), and anaerobes 1
• Dosing:
  • Imipenem-cilastatin: 500 mg IV every 6 hours or 1 g every 8 hours 1
  • Meropenem: 1 g IV every 8 hours 1
  • Ertapenem: 1 g IV daily 1

Fluoroquinolones

• Mechanism of Action: Inhibit bacterial DNA gyrase and topoisomerase IV 3
• Spectrum: Broad coverage of Gram-negative bacteria, variable Gram-positive activity 3
• Agents and Dosing:
  • Ciprofloxacin: 500-750 mg PO twice daily or 400 mg IV every 12 hours 1, 3
  • Levofloxacin: 500-750 mg PO/IV daily 3, 4
  • Moxifloxacin: 400 mg PO/IV daily 1
• Clinical applications: Respiratory, urinary, skin/soft tissue infections 3
• Caution: Increasing resistance rates, especially among Gram-negative pathogens 4

Aminoglycosides

• Mechanism of Action: Bind to 30S ribosomal subunit, inhibit protein synthesis 1
• Spectrum: Primarily Gram-negative bacteria including Pseudomonas 1
• Dosing:
  • Gentamicin: 5-7 mg/kg IV daily (once-daily dosing) or 1.5 mg/kg every 8 hours 1
  • Tobramycin: 5 mg/kg IV daily 1
  • Amikacin: 15-20 mg/kg IV daily 1
• Monitoring: Serum drug concentrations required due to narrow therapeutic window 1
• Toxicity: Nephrotoxicity and ototoxicity 5

Glycopeptides and Oxazolidinones

Vancomycin

• Mechanism of Action: Inhibits cell wall synthesis by binding to D-Ala-D-Ala terminus of peptidoglycan precursors 1
• Spectrum: Gram-positive bacteria, including MRSA and Enterococci 1
• Dosing: 15-20 mg/kg IV every 8-12 hours, not to exceed 2 g daily 1
• Monitoring: Trough levels recommended for serious infections 1

Linezolid

• Mechanism of Action: Inhibits protein synthesis by binding to 23S rRNA of 50S ribosomal subunit 6
• Spectrum: Gram-positive bacteria, including MRSA and VRE 6
• Dosing:
  • Adults: 600 mg IV/PO every 12 hours 6
  • Pediatrics: 10 mg/kg IV/PO every 8 hours 6
• Duration: 10-14 days for complicated skin infections 6

Other Important Antibiotics

Clindamycin

• Mechanism of Action: Inhibits protein synthesis by binding to 50S ribosomal subunit 1
• Spectrum: Gram-positive cocci, anaerobes 1
• Dosing: 300-450 mg PO three times daily or 600-900 mg IV every 8 hours 1
• Clinical applications: Skin/soft tissue infections, anaerobic infections 1

Metronidazole

• Mechanism of Action: Disrupts DNA of anaerobic organisms 1
• Spectrum: Anaerobic bacteria and protozoa 1
• Dosing: 500 mg PO/IV every 8-12 hours 1
• Clinical applications: Anaerobic infections, often combined with other antibiotics 1

Antibiotic Selection by Infection Type

Skin and Soft Tissue Infections

• Impetigo: Dicloxacillin, cephalexin, clindamycin, or amoxicillin-clavulanate 1
• MRSA infections: Vancomycin, linezolid, clindamycin, or trimethoprim-sulfamethoxazole 1
• Necrotizing fasciitis: Vancomycin or linezolid plus piperacillin-tazobactam or carbapenem 1
• Animal bites: Amoxicillin-clavulanate (oral) or ampicillin-sulbactam (IV) 1

Respiratory Infections

• Community-acquired pneumonia: Levofloxacin 750 mg daily for 5 days or amoxicillin plus macrolide 3
• Nosocomial pneumonia: Piperacillin-tazobactam, cefepime, or carbapenem, plus vancomycin if MRSA risk 3

Intra-abdominal Infections

• Mild-moderate: Ceftriaxone plus metronidazole or fluoroquinolone plus metronidazole 1
• Severe: Piperacillin-tazobactam 3.375 g IV every 6 hours or carbapenem 1

Special Considerations

• Pediatric dosing is weight-based and often differs from adult dosing 1
• Renal function affects dosing of many antibiotics, particularly aminoglycosides and vancomycin 1
• Antibiotic resistance should guide empiric therapy choices, with adjustment based on culture results 4
• Duration of therapy varies by infection type, typically 7-14 days for most infections, but can be shorter for certain conditions 1, 3