What are the guidelines for choosing antibiotics for common bacterial infections?

Antibiotic Selection for Common Bacterial Infections

Choose the narrowest-spectrum antibiotic effective against the most likely pathogen based on infection site, local resistance patterns, and use the shortest evidence-based duration to minimize resistance and adverse effects. 1, 2

General Principles

• Prioritize narrow-spectrum agents over broad-spectrum antibiotics when the causative organism is known or highly suspected to reduce selection pressure for resistant organisms 1, 2
• Default to shorter durations (5-7 days for most infections) rather than reflexive 10-day courses, as shorter courses show equivalent clinical outcomes with fewer adverse effects 1
• Consider local resistance patterns when selecting empiric therapy, as resistance rates vary significantly by region and can render standard recommendations ineffective 1, 3

Critical pitfall: Avoid basing empiric therapy solely on clinical trial data without considering time, location, and local pathogen distribution, as antibiotic resistance patterns change rapidly 1

Respiratory Tract Infections

COPD Exacerbations and Acute Bronchitis

• Limit treatment to 5 days when bacterial infection is clinically evident (increased sputum purulence plus increased dyspnea and/or sputum volume) 1, 2
• First-line options: Amoxicillin-clavulanate or doxycycline 2
• Doxycycline dosing: 200 mg on day 1 (100 mg every 12 hours), then 100 mg daily 4

Community-Acquired Pneumonia (CAP)

• Minimum 5-day course for CAP, extending therapy only if clinical stability criteria are not met (resolution of vital sign abnormalities, ability to eat, normal mentation) 1, 2
• Outpatient treatment (healthy adults): Amoxicillin, doxycycline, or a macrolide 2
• Hospitalized patients: Second or third-generation cephalosporin (cefuroxime or ceftriaxone) 2
• Amoxicillin should be taken on an empty stomach (1 hour before or 2 hours after meals) for optimal absorption 5

Critical pitfall: Penicillin G remains optimal for severe community-acquired pneumonia when Streptococcus pneumoniae is suspected, not third-generation cephalosporins 6

Skin and Soft Tissue Infections

Non-Purulent Cellulitis

• Use 5-6 day courses of antibiotics active against streptococci in patients who can self-monitor with close follow-up 1
• Treatment options: Benzylpenicillin, phenoxymethylpenicillin, clindamycin, nafcillin, cefazolin, or cefalexin 2

Purulent Skin Infections

• Treatment options: Dicloxacillin, cefazolin, clindamycin, cefalexin, doxycycline, or trimethoprim-sulfamethoxazole 2
• Dicloxacillin should be taken 1 hour before or 2 hours after meals for best absorption 7
• Linezolid shows superior treatment success compared to vancomycin for skin and soft tissue infections 2

Impetigo

• Oral options: Dicloxacillin, cefalexin, erythromycin, clindamycin, or amoxicillin-clavulanate 2

Critical pitfall: Most skin-structure infections are due to S. aureus and are best treated with anti-staphylococcal penicillin or older cephalosporins, not third-generation agents 6

Urinary Tract Infections

Uncomplicated Bacterial Cystitis (Women)

• First-line options:

  • Nitrofurantoin for 5 days 1, 2, 8
  • Trimethoprim-sulfamethoxazole (TMP-SMZ) for 3 days 1
  • Fosfomycin as single 3-g dose 1, 8
  • Pivmecillinam for 5 days 8

• Second-line options: Oral cephalosporins (cephalexin, cefixime), fluoroquinolones, or amoxicillin-clavulanate 8

Critical pitfall: High resistance rates to TMP-SMZ and ciprofloxacin in many communities preclude their empiric use, particularly in patients recently exposed to these agents or at risk for ESBL-producing organisms 8

Uncomplicated Pyelonephritis

• Fluoroquinolones for 5-7 days OR TMP-SMZ for 14 days based on antibiotic susceptibility 1

Recurrent UTIs

• Avoid trimethoprim (21.4% resistance) and cotrimoxazole (19.3% resistance) in recurrent UTIs 9
• All other first-line agents maintain <15% resistance rates in recurrent infections 9

Intra-Abdominal Infections

Community-Acquired

• Treatment options: Cefazolin or cefuroxime plus metronidazole, OR ertapenem 2

Healthcare-Associated/Nosocomial

• Treatment options: Meropenem, imipenem-cilastatin, or piperacillin-tazobactam 2

Neonatal Sepsis

• First-choice combination: Benzylpenicillin plus gentamicin 2
• Second-choice options: Cefotaxime or ceftriaxone 2

Duration of Therapy

Standard duration for most bacterial SSTIs is 7-14 days 1, but shorter courses (5-6 days) are appropriate for non-purulent cellulitis with close follow-up 1

Critical pitfall: Physicians frequently default to 10-day courses regardless of condition; follow evidence-based shorter durations when appropriate to reduce antibiotic exposure and resistance 1, 2

Special Considerations

• Neutropenic patients: Use broad-spectrum agents with antipseudomonal activity (carbapenems, cephalosporins with antipseudomonal activity, or piperacillin-tazobactam) as monotherapy 1
• Penicillin allergy: Screen carefully for true allergy history; consider alternative beta-lactams or non-beta-lactam options based on infection type 7
• Renal impairment: Adjust dosages for renally-cleared antibiotics and monitor blood levels to avoid neurotoxicity 7
• Take entire prescribed course even if symptoms resolve early, as incomplete courses decrease effectiveness and promote resistance 7, 5