What are the guidelines for choosing an appropriate antibiotic for various infections?

How to Choose an Antibiotic for Any Case

Start with the WHO AWaRe framework: default to Access antibiotics (amoxicillin, amoxicillin-clavulanate, cefalexin) as first-line empiric therapy, escalate to Watch antibiotics (fluoroquinolones, carbapenems) only when Access agents are inadequate or local resistance demands it, and reserve Reserve antibiotics exclusively for confirmed or suspected multidrug-resistant organisms. 1

Step 1: Identify the Infection Syndrome

The first decision point is determining the anatomic site and severity of infection, as this dictates likely pathogens and appropriate empiric coverage 1, 2:

• Skin and soft tissue infections: Beta-lactams (amoxicillin-clavulanate 875/125 mg twice daily or cefalexin) are first-choice agents 1, 3
• Respiratory infections: Amoxicillin or amoxicillin-clavulanate first, with cefalexin, doxycycline, or macrolides as second choices 1
• Intra-abdominal infections (mild-moderate): Amoxicillin-clavulanate or ampicillin + gentamicin + metronidazole 3
• Intra-abdominal infections (severe): Cefotaxime or ceftriaxone + metronidazole, or piperacillin-tazobactam 3
• Necrotizing infections (mixed): Piperacillin-tazobactam 3.37 g every 6-8 hours IV plus vancomycin, or carbapenems (meropenem 1 g every 8 hours IV, imipenem 1 g every 6-8 hours IV) 3

Step 2: Assess Community vs. Healthcare-Associated Infection

This distinction fundamentally changes pathogen likelihood and resistance patterns 2, 4:

Community-acquired infections:

• Narrower spectrum agents from Access group are appropriate 1
• Lower risk of multidrug-resistant organisms 2
• Penicillin, amoxicillin-clavulanate, or first-generation cephalosporins typically adequate 1, 5

Healthcare-associated infections:

• Require broader coverage for resistant organisms including multidrug-resistant gram-negative bacilli and MRSA 2, 4
• Consider antipseudomonal coverage (cefepime, ceftazidime, piperacillin-tazobactam, or carbapenems) 1
• Add vancomycin if MRSA suspected based on local prevalence or risk factors 3

Step 3: Apply the Spectrum Narrowing Principle

Choose the narrowest spectrum that covers likely pathogens 1:

• For gram-positive infections, penicillin, cloxacillin, and erythromycin cover 90% of cases 5
• For streptococcal necrotizing fasciitis: Penicillin 2-4 million units every 4-6 hours IV plus clindamycin 600-900 mg every 8 hours IV 3
• For staphylococcal infections (methicillin-susceptible): Nafcillin or oxacillin 1-2 g every 4 hours IV, or cefazolin 1 g every 8 hours IV 3
• For MRSA: Vancomycin 30 mg/kg/day in 2 divided doses IV (target trough 15-20 µg/mL), linezolid 600 mg every 12 hours, or daptomycin 4-6 mg/kg/day 3, 6

Step 4: Consider Local Resistance Patterns

Local surveillance data should override general recommendations when resistance rates are high 1, 2, 7:

• If local MRSA prevalence is >10-15% in skin infections, add empiric vancomycin 3
• If extended-spectrum beta-lactamase (ESBL) organisms are common, escalate to carbapenems 3
• Inadequate initial empiric therapy increases mortality, hospital stay, and costs—err on the side of broader coverage initially 2, 4

Step 5: Plan for De-escalation

Initiate broad-spectrum therapy when necessary, but narrow within 2-4 days based on culture results and clinical response 2, 4:

• Obtain cultures before starting antibiotics whenever possible 4
• Reassess at 48-72 hours when susceptibility data available 2
• Switch from IV to oral when clinically stable 3
• Standard duration for most skin and soft tissue infections is 7-14 days 3

Critical Pitfalls to Avoid

Reflexive broad-spectrum prescribing: 30-50% of antibiotic prescriptions are inappropriate 1. Start with Access antibiotics unless specific risk factors demand escalation.

Fluoroquinolone overuse: Permanent disabling side effects can outweigh benefits in routine cases 1. Reserve for specific indications only.

Ignoring severity markers: Severe pain disproportionate to findings, systemic toxicity, crepitus, bullae, or skin necrosis suggest necrotizing infection requiring immediate surgical consultation and broad-spectrum coverage 3.

Delayed surgical intervention: For necrotizing fasciitis, surgical debridement is the definitive treatment—antibiotics alone are inadequate 3.

Inadequate gram-negative coverage: When polymicrobial infection suspected (perianal abscess, bowel trauma, decubitus ulcers, injection drug use), ensure anaerobic and gram-negative coverage with agents like piperacillin-tazobactam or cefotaxime + metronidazole 3.

Special Pathogen Considerations

Clostridial myonecrosis: Clindamycin 600-900 mg every 8 hours IV plus penicillin 2-4 million units every 4-6 hours IV 3

Aeromonas hydrophila or Vibrio vulnificus (water exposure): Doxycycline 100 mg every 12 hours IV plus ceftriaxone 1-2 g every 24 hours IV 3

Animal bites: Amoxicillin-clavulanate 875/125 mg twice daily orally covers Pasteurella multocida and oral anaerobes 3

Human bites: Amoxicillin-clavulanate or ampicillin-sulbactam 1.5-3.0 g every 6 hours IV to cover Eikenella corrodens and anaerobes 3