Post-Appendectomy Wound Infection: Common Organisms
The correct answer is A) Gram-negative aerobes and anaerobes. Post-appendectomy wound infections are polymicrobial, involving both gram-negative facultative/aerobic organisms (predominantly E. coli) and obligate anaerobes (particularly Bacteroides fragilis group), reflecting the colonic flora encountered during appendiceal surgery 1.
Microbiological Profile
The microbiology of post-appendectomy wound infections mirrors the flora of complicated intra-abdominal infections:
- E. coli is the dominant pathogen, isolated in 71% of complicated intra-abdominal infections including appendiceal sources 1
- Anaerobic organisms are consistently present, with Bacteroides fragilis found in 35% and other Bacteroides species in 71% of cases 1
- Additional gram-negative aerobes include Klebsiella species (14%), Proteus mirabilis (5%), and Enterobacter species (5%) 1
- Gram-positive organisms (Streptococcus species in 38%, enterococci in 12-23%) are also present but are secondary contributors 1
Why Both Aerobes AND Anaerobes Matter
Appendiceal and colon-derived infections harbor both facultative and obligate anaerobic organisms by definition 1. The Infectious Diseases Society of America explicitly states that coverage for obligate anaerobic bacilli must be provided for appendiceal-derived infections 1. This is why:
- Surgical site infections after intestinal tract operations have a high probability of mixed gram-positive and gram-negative flora with both facultative and anaerobic organisms 1
- Any antibiotic considered appropriate for intra-abdominal infection should cover this polymicrobial spectrum 1
- Failure to cover anaerobes leads to treatment failure, as demonstrated by the increasing resistance of B. fragilis group to agents like clindamycin 2
Clinical Implications for Antibiotic Selection
Empiric therapy must target both gram-negative aerobic/facultative bacilli AND obligate anaerobes 1:
- Recommended single-agent regimens include ticarcillin-clavulanate, ertapenem, moxifloxacin, or piperacillin-tazobactam 1, 3
- Combination therapy options include metronidazole with ceftriaxone, cefotaxime, levofloxacin, or ciprofloxacin 1
- Avoid ampicillin-sulbactam due to high resistance rates among community-acquired E. coli 1
- Avoid clindamycin monotherapy due to increasing B. fragilis group resistance 1, 2
Common Pitfalls
The major error is selecting antibiotics that cover only gram-negative aerobes (option B) or only anaerobes (option D), as this provides inadequate coverage for the polymicrobial nature of these infections 1, 2. While gram-positive organisms are present, they are not the predominant pathogens (making option C incorrect), and empiric enterococcal coverage is not necessary for community-acquired infections 1.
For patients with fever and systemic signs (temperature ≥38.5°C or pulse ≥100 beats/min), a short course of antibiotics for 24-48 hours is indicated after wound opening and drainage 1.