What are the typical first-line antibiotic treatments for bacterial infections?

First-Line Antibiotic Treatments for Bacterial Infections

The first-line antibiotic treatment for bacterial infections depends on the specific infection site, with amoxicillin being the most common first-choice for many community-acquired infections including respiratory tract infections, while trimethoprim-sulfamethoxazole or nitrofurantoin are preferred for uncomplicated urinary tract infections. 1

General Principles for Antibiotic Selection

When selecting first-line antibiotics, several factors should be considered:

• Infection site and likely pathogens
• Local resistance patterns
• Patient factors (allergies, comorbidities, pregnancy status)
• Potential for adverse effects
• Cost and accessibility

First-Line Antibiotics by Infection Type

Respiratory Tract Infections

Community-Acquired Pneumonia (CAP)

• Mild to moderate CAP (outpatient):

  • First choice: Amoxicillin (Access group) 1
  • Second choice: Doxycycline or amoxicillin-clavulanic acid 1

• CAP with comorbidities:

  • Combination of a β-lactam (amoxicillin or amoxicillin-clavulanic acid) and a macrolide (clarithromycin) 1

• Severe CAP (hospitalized):

  • Cefotaxime or ceftriaxone plus clarithromycin 1

Important note: While some guidelines recommend macrolides as first-line for uncomplicated CAP, the WHO guidelines prioritize amoxicillin due to increasing macrolide resistance and cardiovascular risk concerns with azithromycin 1.

Skin and Soft Tissue Infections

• Uncomplicated skin infections:

  • First choice: Cephalexin or dicloxacillin 2
  • Second choice: Clindamycin (if penicillin allergic) 2

• Complicated skin infections:

  • Amoxicillin-clavulanic acid 3

Urinary Tract Infections (UTIs)

• Uncomplicated cystitis in women:

  • First choice: Nitrofurantoin, fosfomycin, or trimethoprim-sulfamethoxazole (if local resistance <20%) 4
  • Second choice: Cephalexin or amoxicillin-clavulanic acid 4

• Complicated UTIs:

  • Ciprofloxacin with metronidazole or ceftriaxone with metronidazole 1

Intra-abdominal Infections

• Mild to moderate:

  • First choice: Amoxicillin-clavulanic acid 1
  • Second choice: Ciprofloxacin + metronidazole 1

• Severe:

  • First choice: Cefotaxime or ceftriaxone + metronidazole 1
  • Second choice: Piperacillin-tazobactam 1

Neonatal Sepsis

• First choice: Ampicillin + gentamicin 5
• Second choice: Amikacin + cloxacillin or cefotaxime 5

Antibiotic Resistance Considerations

Antibiotic resistance is a growing concern that affects first-line treatment choices:

• Penicillin-resistant Streptococcus pneumoniae: Higher doses of amoxicillin (90 mg/kg/day) may overcome resistance 6
• ESBL-producing organisms: Carbapenems are often required 4
• MRSA: Vancomycin, linezolid, or trimethoprim-sulfamethoxazole may be needed 1

Important Warnings and Precautions

• Fluoroquinolones (e.g., levofloxacin): Associated with tendon rupture, peripheral neuropathy, and CNS effects; should be reserved for situations where other antibiotics cannot be used 7
• Macrolides (e.g., azithromycin): May cause QT prolongation and exacerbate myasthenia gravis 8
• Prolonged empiric therapy: Should be avoided beyond 48 hours without evidence of infection to prevent antimicrobial resistance 5

Best Practices for Antibiotic Prescribing

1. Start narrow, go broad only when necessary: Begin with the narrowest spectrum antibiotic likely to be effective
2. Reassess at 48-72 hours: Consider de-escalation if cultures are negative or show susceptibility to narrower agents
3. Appropriate duration: Shortest effective course (often 5-7 days for many infections)
4. Consider local antibiograms: Base empiric therapy on local resistance patterns
5. Combination therapy: Only indicated for specific scenarios such as severe infections, polymicrobial infections, or to prevent resistance 9

Common Pitfalls to Avoid

• Using broad-spectrum antibiotics when narrow-spectrum would suffice
• Continuing antibiotics despite negative cultures and low clinical suspicion
• Failing to adjust therapy based on culture results
• Not considering patient-specific factors (allergies, renal/hepatic function)
• Using fluoroquinolones as first-line when safer alternatives exist

By following these evidence-based recommendations for first-line antibiotic therapy, clinicians can effectively treat bacterial infections while minimizing the risk of adverse effects and antimicrobial resistance.