Antibiotic Selection for Common Bacterial Infections
Core Principle
Antibiotic selection must be guided by infection site, local resistance patterns, and patient-specific factors including penicillin allergy status, with narrow-spectrum agents preferred when appropriate to minimize resistance development and adverse effects. 1, 2
Urinary Tract Infections (UTIs)
Uncomplicated Cystitis (First-Line Options)
- Nitrofurantoin 100 mg twice daily for 5 days is a primary recommendation for uncomplicated lower UTI 1, 2
- Sulfamethoxazole-trimethoprim demonstrates equivalent efficacy to fluoroquinolones for uncomplicated UTI (no difference in short-term or long-term symptomatic cure) 1
- Fosfomycin offers minimal resistance and good safety profile for lower UTI 1
- Amoxicillin-clavulanate serves as an alternative, particularly in young children 1
Acute Pyelonephritis
- Ceftriaxone or ciprofloxacin are recommended for acute pyelonephritis in adults 1
- Levofloxacin 750 mg daily for 5 days is effective for uncomplicated pyelonephritis, but should NOT be first-line due to serious adverse effects that outweigh benefits 2
- Fluoroquinolones should only be used when local E. coli resistance rates are <10% 2
Complicated UTIs and Pediatric Considerations
- Parenteral ampicillin plus aminoglycoside or third-generation cephalosporin for newborns and infants 1
- Third-generation cephalosporin for uncomplicated pyelonephritis in children >6 months 1
- Ciprofloxacin 100-250 mg orally twice daily for 5 days shows 87-94% cure rates even in complicated UTI with complicating factors 3
Pneumonia
Community-Acquired Pneumonia (CAP)
- Amoxicillin remains the reference first-line agent for bacterial bronchitis in COPD 2
- Amoxicillin-clavulanate for patients with frequent exacerbations (≥4/year) or FEV1 <35% 2
- Ciprofloxacin demonstrates equivalent efficacy to amoxycillin (81% vs 82% successful outcomes) with higher bacterial eradication rates (87% vs 64%) 4
- Oral ciprofloxacin 500-750 mg twice daily achieves 100% treatment success in community-acquired bacterial pneumonia 5
Hospital-Acquired and Nursing Home-Acquired Pneumonia
- Sequential IV/oral ciprofloxacin (200-400 mg IV q12h, then 750 mg PO q12h) shows equivalent efficacy to IV/IM ceftriaxone (50% vs 54% successful outcomes) 6
- Ceftriaxone 2 g IV every 24 hours during acute phase, then 1 g IM every 24 hours for convalescence 6
- Treatment duration: 14 days for nursing home-acquired infections 6
Skin and Soft Tissue Infections
Uncomplicated Skin Infections
- Ceftriaxone is safe and effective for skin and soft tissue infections caused by staphylococci and streptococci 7
- Dosing advantage: every 12 hours in children, every 24 hours in adults due to long half-life 7
- Can be administered IV or IM in both adults and children 7
Necrotizing Fasciitis in Penicillin-Allergic Patients
- Clindamycin 600-900 mg IV every 8 hours PLUS ciprofloxacin 400 mg IV every 12 hours is the primary recommendation for mixed necrotizing infections 8
- Alternative: Metronidazole 500 mg IV every 6 hours PLUS aminoglycoside or fluoroquinolone 8
- For streptococcal necrotizing fasciitis: vancomycin, linezolid, quinupristin/dalfopristin, or daptomycin for severe penicillin hypersensitivity 8
- Vancomycin 30 mg/kg/day IV in 2 divided doses for MRSA involvement in penicillin-allergic patients 8
Penicillin Allergy Management
Type I Hypersensitivity (Anaphylaxis, Stevens-Johnson Syndrome)
- Avoid ALL β-lactams entirely including cephalosporins 8, 9
- Use clindamycin/fluoroquinolone combination for necrotizing infections 8
- Fluoroquinolones or vancomycin-based regimens for other infections 8
Non-Type I Hypersensitivity
- Cephalosporins may be considered, but NOT as monotherapy for necrotizing infections 8
- Amoxicillin is contraindicated in patients with history of serious hypersensitivity reactions to penicillins or other β-lactams 9
Critical Dosing Information
Amoxicillin (FDA-Approved Dosing)
- Adults: 750-1750 mg/day in divided doses every 8-12 hours 9
- Pediatric patients >3 months: 20-45 mg/kg/day in divided doses every 8-12 hours 9
- Neonates and infants ≤3 months: maximum 30 mg/kg/day divided every 12 hours 9
- Reduce dose in severe renal impairment (GFR <30 mL/min) 9
H. pylori Eradication (Adults Only)
- Triple therapy: Amoxicillin 1 gram + clarithromycin 500 mg + lansoprazole 30 mg, all twice daily for 14 days 9
- Dual therapy: Amoxicillin 1 gram + lansoprazole 30 mg, three times daily for 14 days 9
Critical Pitfalls to Avoid
Monotherapy Errors
- Never use clindamycin or metronidazole as monotherapy for mixed necrotizing infections—gram-negative coverage is essential 8
- Do not forget staphylococcal coverage if suspected, as metronidazole lacks activity against this pathogen 8
Surgical Delays
- Do not delay surgical debridement while optimizing antibiotics in necrotizing fasciitis—mortality increases dramatically with delayed intervention 8
Fluoroquinolone Overuse
- Avoid fluoroquinolones for uncomplicated UTIs—serious adverse effects outweigh benefits per FDA advisory and current guidelines 2
- Recent fluoroquinolone use is a contraindication for repeat fluoroquinolone therapy 2
- Local resistance >10% should prompt alternative selection 2
Resistance Considerations
- Guidelines recommend local resistance rates <10% for pyelonephritis and <20% for lower UTI when selecting empiric antibiotics 1
- Broad-spectrum fluoroquinolone use creates selection pressure for extended-spectrum β-lactamases and C. difficile infection 2
Drug Interactions with Amoxicillin
- Coadministration with probenecid is not recommended 9
- Concomitant oral anticoagulants may increase prothrombin time prolongation 9
- Coadministration with allopurinol increases rash risk 9
- May reduce efficacy of oral contraceptives 9