What antibiotics are used to treat common bacterial infections based on the suspected causative pathogen?

Antibiotics for Common Bacterial Infections Based on Causative Pathogens

The appropriate antibiotic selection for bacterial infections should be based on the suspected or confirmed causative pathogen, with treatment tailored to provide effective coverage while minimizing resistance development. Empiric therapy should be guided by knowledge of likely pathogens and their susceptibility patterns, with adjustments made once culture results are available.

Common Respiratory Tract Pathogens and Treatments

Streptococcus pneumoniae

• First-line: Amoxicillin (high-dose) 1
• Alternatives:
  • Doxycycline (for mild cases in previously healthy patients) 2, 3
  • Respiratory fluoroquinolones (levofloxacin) for drug-resistant strains 2
  • Macrolides (limited effectiveness due to increasing resistance) 2

Haemophilus influenzae

• First-line: Amoxicillin-clavulanate (addresses β-lactamase production) 1
• Alternatives:
  • Cephalosporins (ceftriaxone, cefuroxime) 1
  • Doxycycline 3
  • Fluoroquinolones (ciprofloxacin) 1

Moraxella catarrhalis

• First-line: Amoxicillin-clavulanate 1
• Alternatives: Doxycycline, macrolides, or tetracyclines 1

Mycoplasma pneumoniae

• First-line: Macrolides 2
• Alternative: Doxycycline 2, 3

Skin and Soft Tissue Infection Pathogens

Staphylococcus aureus (MSSA)

• First-line: Oxacillin/nafcillin or cefazolin 1
• Alternatives: Cephalexin (oral), dicloxacillin 1

Methicillin-resistant S. aureus (MRSA)

• First-line: Vancomycin 1, 4
• Alternatives: Linezolid, clindamycin, daptomycin, or sulfamethoxazole-trimethoprim 1

Group A Streptococcus (Necrotizing fasciitis)

• First-line: Penicillin plus clindamycin 1

Mixed infections (polymicrobial)

• First-line: Vancomycin plus piperacillin-tazobactam or a carbapenem 1
• Alternative: Vancomycin plus ceftriaxone and metronidazole 1

Gastrointestinal Infection Pathogens

Salmonella species

• First-line: Ciprofloxacin 1

Shigella species

• First-line: Ciprofloxacin 1, 3

Clostridioides difficile

• First-line: Oral vancomycin 4
• Alternative: Metronidazole (for mild cases)

Urinary Tract Infection Pathogens

Escherichia coli

• First-line: Ciprofloxacin (if local resistance <10%) 1, 3
• Alternatives: Nitrofurantoin, fosfomycin, sulfamethoxazole-trimethoprim (if susceptible)

Pseudomonas aeruginosa

• First-line: Ciprofloxacin 1, 3
• Alternative: Aminoglycoside plus anti-pseudomonal β-lactam 5

Klebsiella species

• First-line: Cephalosporins or fluoroquinolones 3
• Alternative: Carbapenems (for ESBL-producing strains)

Bloodstream Infections

Gram-positive cocci

• First-line: Vancomycin (for suspected MRSA or pending susceptibilities) 4
• Alternative: Daptomycin or linezolid

Gram-negative bacilli

• First-line: Aminoglycoside plus anti-pseudomonal β-lactam 5
• Alternative: Carbapenem or piperacillin-tazobactam

Duration of Therapy

• Community-acquired pneumonia: Minimum 5 days, extending therapy based on clinical stability (resolution of vital sign abnormalities, ability to eat, normal mentation) 1, 2
• COPD exacerbation/acute bronchitis: 5 days 1
• Skin and soft tissue infections: 7-14 days depending on severity and response 1

Important Considerations

1. Empiric therapy selection factors:

   • Local resistance patterns
   • Patient factors (allergies, comorbidities)
   • Previous antibiotic exposure
   • Severity of illness

2. Diagnostic approach:

   • Obtain appropriate cultures before starting antibiotics when possible
   • Consider rapid diagnostic tests when available 6, 7
   • Gram stain can guide initial therapy

3. Therapy adjustment:

   • Narrow spectrum once pathogen and susceptibilities are identified
   • De-escalate from broad-spectrum agents when appropriate
   • Consider IV to oral switch when clinically improved

4. Pitfalls to avoid:

   • Using unnecessarily broad-spectrum antibiotics
   • Failing to adjust therapy based on culture results
   • Not considering local resistance patterns
   • Prolonging antibiotic duration beyond recommended guidelines
   • Overlooking potential drug interactions or adverse effects

5. Resistance considerations:

   • Fluoroquinolone resistance in E. coli varies geographically (3-10%) 1
   • MRSA prevalence affects empiric therapy choices
   • Pneumococcal resistance to macrolides is increasing 2

The AWaRe classification from WHO provides a framework for antibiotic selection, categorizing antibiotics as Access (first-line options), Watch (higher resistance potential), and Reserve (last-resort options) 1. This helps guide appropriate antibiotic use while minimizing resistance development.