What antibiotics are used to treat specific bacterial infections?

Comprehensive Guide to Antibiotics and Their Target Bacteria

Antibiotics should be selected based on their specific activity against bacterial pathogens, with consideration for resistance patterns, severity of infection, and patient factors to optimize outcomes in terms of morbidity and mortality.

Classification of Antibiotics by WHO AWaRe Framework

The World Health Organization (WHO) categorizes antibiotics into three groups to guide appropriate use 1:

1. Access Group - First-line treatments with lower resistance potential
2. Watch Group - Higher resistance potential, should be monitored and used judiciously
3. Reserve Group - Last-resort options for multidrug-resistant infections

Common Bacterial Pathogens and First-Choice Antibiotics

Respiratory Tract Infections

Community-Acquired Pneumonia

• Typical pathogens: Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis
• Atypical pathogens: Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella species
• First-choice treatment:
  • Mild (outpatient): Amoxicillin 1g PO q8h or macrolide (clarithromycin/azithromycin) 2
  • Moderate-severe (inpatient): Beta-lactam (ceftriaxone, cefotaxime) plus macrolide 2
  • Duration: 5 days if clinically stable 1

Nosocomial Pneumonia

• Common pathogens: Pseudomonas aeruginosa, MRSA, Enterobacteriaceae
• Treatment: Piperacillin-tazobactam 4.5g IV q6h plus aminoglycoside 3
• For MRSA risk: Add vancomycin or linezolid 2

Skin and Soft Tissue Infections

Impetigo

• Pathogens: Staphylococcus aureus, Streptococcus pyogenes
• Treatment: Dicloxacillin, cefalexin, or clindamycin 1

Cellulitis

• Pathogens: Streptococcus species, Staphylococcus aureus
• Treatment:
  • Non-purulent: Penicillin, clindamycin, cefazolin, or cefalexin 1
  • Purulent (MRSA suspected): Vancomycin, linezolid, clindamycin, or daptomycin 1

Necrotizing Fasciitis

• Pathogens: Group A Streptococcus, mixed anaerobes, Clostridium
• Treatment: Vancomycin or linezolid plus piperacillin-tazobactam or carbapenem 1

Urinary Tract Infections

Urethritis

• Gonococcal: Ceftriaxone 1g IM/IV single dose plus azithromycin 1g PO single dose 1
• Non-gonococcal: Doxycycline 100mg PO BID for 7 days 1
• Chlamydia trachomatis: Azithromycin 1-1.5g PO single dose or doxycycline 100mg BID for 7 days 1
• Mycoplasma genitalium: Azithromycin 500mg PO day 1, then 250mg for 4 days 1

Gastrointestinal Infections

Intra-abdominal Infections

• Pathogens: Enterobacteriaceae, anaerobes, enterococci
• Treatment: Piperacillin-tazobactam 3.375g IV q6h 3

Specific Antibiotics and Their Target Bacteria

Beta-lactams

Penicillins

• Penicillin G/V: Streptococcus pneumoniae (penicillin-susceptible), group A streptococci 1
• Amoxicillin: Streptococcus pneumoniae, Haemophilus influenzae (non-beta-lactamase producing) 1
• Amoxicillin-clavulanate: H. influenzae (beta-lactamase positive), Enterobacteriaceae, anaerobes 1
• Oxacillin/Dicloxacillin: Methicillin-susceptible S. aureus (MSSA) 1

Cephalosporins

• Ceftriaxone/Cefotaxime: Streptococcus pneumoniae, Neisseria gonorrhoeae, Enterobacteriaceae 1
• Cefazolin: MSSA, streptococci 1
• Cefuroxime: H. influenzae, Moraxella catarrhalis 1

Carbapenems

• Ertapenem: Enterobacteriaceae (including ESBL-producers) 1
• Meropenem/Imipenem: Pseudomonas aeruginosa, Enterobacteriaceae, anaerobes 1

Macrolides

• Azithromycin/Clarithromycin: Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella 1

Tetracyclines

• Doxycycline: Mycoplasma pneumoniae, Chlamydia trachomatis, Rickettsia, Borrelia, Ureaplasma urealyticum 4

Fluoroquinolones

• Levofloxacin/Moxifloxacin: Respiratory pathogens including S. pneumoniae, H. influenzae, atypicals 1
• Ciprofloxacin: Pseudomonas aeruginosa, Enterobacteriaceae 1

Glycopeptides

• Vancomycin: MRSA, resistant S. pneumoniae 1

Oxazolidinones

• Linezolid: MRSA, VRE 1

Lipopeptides

• Daptomycin: MRSA, VRE 1

Special Considerations

Antibiotic Resistance

• MRSA: Vancomycin, linezolid, daptomycin, or ceftaroline 1
• ESBL-producing Enterobacteriaceae: Carbapenems (ertapenem preferred) 1
• Pseudomonas aeruginosa: Consider dual therapy initially with beta-lactam plus aminoglycoside or fluoroquinolone 1

Duration of Therapy

• Community-acquired pneumonia: 5 days if clinically stable 1
• COPD exacerbation: 5 days if clinical signs of bacterial infection 1
• Skin and soft tissue infections: 7-14 days 1

Pitfalls and Caveats

1. Overuse of broad-spectrum antibiotics can lead to resistance development and collateral damage to the microbiome 5

2. Fluoroquinolones should be used judiciously due to resistance concerns and adverse effects 1

3. Macrolide monotherapy has limitations in areas with high drug-resistant S. pneumoniae prevalence 2

4. Delayed antibiotic administration in sepsis increases mortality risk by approximately 8% per hour 6

5. Culture results should guide de-escalation of empiric therapy whenever possible to reduce resistance development 6

6. Penicillin allergy assessment is critical as broad alternatives may have increased toxicity or reduced efficacy 7

7. Antibiotic-associated diarrhea and C. difficile infection are significant complications of antibiotic therapy, particularly with broad-spectrum agents 5

By matching the appropriate antibiotic to the suspected or confirmed pathogen, clinicians can optimize treatment outcomes while minimizing adverse effects and resistance development.