What antibiotics are used to treat bacterial infections?

Antibiotics: Target Diseases and Bacteria

Antibiotics should be selected based on the specific bacterial infection being treated, with consideration for the most likely pathogens and their susceptibility patterns to maximize clinical outcomes while minimizing resistance development. 1

Classification of Bacterial Infections and First-Line Antibiotics

Skin and Soft Tissue Infections

• Mild infections:

  • First choice: Amoxicillin-clavulanic acid or cloxacillin 1
  • Alternative: Cefalexin

• Necrotizing fasciitis:

  • Combination therapy: Clindamycin plus piperacillin-tazobactam or ceftriaxone plus metronidazole (with or without vancomycin) 1

• MRSA infections:

  • First choice: Vancomycin, linezolid, clindamycin, daptomycin, or sulfamethoxazole-trimethoprim 1
  • For diabetic wound infections with MRSA: Linezolid, daptomycin, or vancomycin 1

Respiratory Tract Infections

• Community-acquired pneumonia:

  • For adults without comorbidities: Amoxicillin, doxycycline, or a macrolide 1
  • For adults with comorbidities: β-lactam with a macrolide or a respiratory fluoroquinolone 1
  • Minimum treatment duration: 5 days 1

• Acute bronchiolitis:

  • Antibiotics generally not indicated (viral etiology) 1
  • If bacterial superinfection suspected: Amoxicillin-clavulanic acid, cefuroxime-axetil, or cefpodoxime-proxetil 1

• COPD exacerbations:

  • When bacterial infection suspected: 5-day course of aminopenicillin with clavulanic acid, a macrolide, or a tetracycline 1

Intra-abdominal Infections

• Mild to moderate infections:

  • First choice: Amoxicillin-clavulanic acid or ampicillin + gentamicin + metronidazole 1
  • Second choice: Ciprofloxacin + metronidazole or ceftriaxone + metronidazole 1

• Severe infections:

  • First choice: Ceftriaxone + metronidazole or piperacillin-tazobactam 1
  • Second choice: Meropenem or ampicillin + gentamicin + metronidazole 1

Mechanism of Action and Target Bacteria

Beta-lactams (Penicillins, Cephalosporins, Carbapenems)

• Mechanism: Inhibit cell wall synthesis
• Target bacteria:
  • Penicillins: Streptococci, pneumococci, non-beta-lactamase producing staphylococci
  • Amoxicillin-clavulanic acid: Extended coverage including beta-lactamase producers
  • Cephalosporins: Broader gram-negative coverage with advancing generations
  • Carbapenems: Broadest spectrum, including ESBL-producing Enterobacteriaceae 2

Glycopeptides (Vancomycin)

• Mechanism: Inhibit cell wall synthesis
• Target bacteria: Gram-positive organisms including MRSA and Enterococcus 3
• Important note: Should be used judiciously to prevent resistance development 3

Macrolides (Erythromycin, Azithromycin, Clarithromycin)

• Mechanism: Inhibit protein synthesis
• Target bacteria: Atypical pathogens (Mycoplasma, Chlamydia), some streptococci 1

Fluoroquinolones (Ciprofloxacin, Levofloxacin, Moxifloxacin)

• Mechanism: Inhibit DNA gyrase
• Target bacteria: Gram-negative bacteria including Pseudomonas (ciprofloxacin), respiratory pathogens (newer generations) 2
• Caution: Reserve for specific indications due to resistance concerns 1

Lincosamides (Clindamycin)

• Mechanism: Inhibit protein synthesis
• Target bacteria: Anaerobes, streptococci, staphylococci 4
• Special use: Particularly valuable in necrotizing fasciitis due to toxin suppression 1

Special Considerations

Antibiotic Duration

• Most bacterial infections can be treated with shorter courses than traditionally prescribed:
  • Community-acquired pneumonia: 5 days 1
  • COPD exacerbations: 5 days 1
  • Intra-abdominal infections: 5-7 days after source control 1

Antibiotic Resistance

• Patients should be counseled that antibiotics only treat bacterial infections, not viral infections 3, 4
• Completing the full prescribed course (which is now often shorter than traditional courses) is essential to prevent resistance development 3
• Recent antibiotic use is a major risk factor for resistant infections 1

Common Pitfalls to Avoid

1. Unnecessary antibiotic use for viral infections

   • Most upper respiratory infections are viral and do not require antibiotics

2. Inappropriate broad-spectrum coverage

   • Reserve broad-spectrum agents for severe infections or when narrow-spectrum agents are likely to be ineffective

3. Excessive treatment duration

   • Shorter courses (5-7 days) are as effective as longer courses for many infections 5

4. Failure to adjust therapy based on culture results

   • Always obtain cultures in severe or healthcare-associated infections 1
   • De-escalate to narrower spectrum agents when possible

5. Ignoring local resistance patterns

   • Empiric therapy should be guided by local antibiogram data

By selecting the appropriate antibiotic for the specific bacterial infection and using the optimal duration of therapy, clinicians can maximize treatment efficacy while minimizing the development of antibiotic resistance.