What is the antibiotic of choice for treating intra-abdominal infections?

Antibiotic Selection for Intra-Abdominal Infections

For intra-abdominal infections, piperacillin-tazobactam is the antibiotic of choice for severe infections, while ertapenem is preferred for mild-to-moderate community-acquired infections due to its appropriate spectrum and once-daily dosing. 1

Classification of Intra-Abdominal Infections

Antibiotic selection depends on:

1. Severity of infection (mild-to-moderate vs. severe)
2. Source of infection (community-acquired vs. healthcare-associated)
3. Anatomical location of perforation

Community-Acquired Infections

Mild-to-Moderate Severity

For mild-to-moderate community-acquired intra-abdominal infections:

First-line options (single agents):

• Ertapenem 1g IV every 24 hours 1
• Ticarcillin-clavulanate 3.1g IV every 6 hours 1

Alternative regimens (combinations):

• Cefazolin or cefuroxime plus metronidazole 1
• Ceftriaxone 1g every 24h + metronidazole 500mg every 8h IV 1
• Ciprofloxacin 400mg IV every 12h + metronidazole 500mg every 8h IV 1

High Severity

For severe community-acquired infections:

First-line options (single agents):

• Piperacillin-tazobactam 3.375g every 6h or 4.5g every 8h IV 1, 2
• Meropenem 1g every 8h IV 1, 3
• Imipenem-cilastatin 500mg every 6h IV 1

Alternative regimens (combinations):

• Third/fourth-generation cephalosporin (cefotaxime, ceftriaxone, ceftazidime, cefepime) plus metronidazole 1
• Ciprofloxacin or levofloxacin plus metronidazole 1

Healthcare-Associated Infections

For healthcare-associated intra-abdominal infections:

• Piperacillin-tazobactam 3.375g every 6h or 4.5g every 8h IV 1, 2
• Meropenem 1g every 8h IV 1, 3
• Imipenem-cilastatin 500mg every 6h IV 1
• Doripenem 500mg every 8h IV 1

Microbiology Considerations

Intra-abdominal infections typically involve:

1. Gram-negative aerobic and facultative bacilli (especially E. coli)
2. Gram-positive streptococci
3. Obligate anaerobic bacilli (especially B. fragilis group)

The location of perforation determines the likely pathogens:

• Stomach/duodenum/proximal small bowel: primarily gram-positive and gram-negative aerobes
• Distal small bowel: gram-negative facultative and aerobic organisms plus anaerobes
• Colon: facultative and obligate anaerobic organisms, including B. fragilis 1

Important Caveats and Pitfalls

1. Avoid ampicillin-sulbactam due to high rates of resistance among community-acquired E. coli 1

2. Avoid cefotetan and clindamycin as sole agents due to increasing resistance among B. fragilis group 1

3. Avoid aminoglycosides for routine use in adults with community-acquired intra-abdominal infection due to toxicity concerns and availability of equally effective alternatives 1

4. Empiric coverage of Enterococcus is not necessary for community-acquired infections 1

5. Empiric antifungal therapy is not recommended for community-acquired infections 1

6. Consider local resistance patterns when selecting fluoroquinolones, as resistance rates of E. coli to these agents have increased in many regions 1

7. For healthcare-associated infections, obtain cultures to guide therapy as these infections often involve resistant organisms 1

8. Duration of therapy is typically 7-10 days for most intra-abdominal infections, but can be extended to 14 days for more severe cases 1, 2

Special Considerations

For biliary tract infections:

• Mild-to-moderate community-acquired acute cholecystitis: Cefazolin, cefuroxime, or ceftriaxone
• Severe community-acquired acute cholecystitis: Same as for severe intra-abdominal infections
• Acute cholangitis following bilio-enteric anastomosis: Same as for healthcare-associated infections 1

Remember that appropriate source control (drainage of infected fluid collections, control of ongoing contamination) is essential alongside antibiotic therapy for successful treatment of intra-abdominal infections 1.