What are the most likely causative organisms in a post-appendectomy patient presenting with an infected wound and fever?

Post-Appendectomy Wound Infection: Causative Organisms

The correct answer is A) Gram-negative aerobes and anaerobes. Post-appendectomy wound infections are polymicrobial, dominated by gram-negative aerobic organisms (particularly Escherichia coli) combined with anaerobic bacteria (especially Bacteroides fragilis), reflecting the intestinal flora encountered during appendectomy 1, 2.

Microbiology of Post-Appendectomy Wound Infections

The bacterial profile is predictably polymicrobial because appendectomy involves entry into the gastrointestinal tract:

• Escherichia coli is present in 85% of post-appendectomy infections, making it the dominant pathogen 2, 3
• Bacteroides fragilis is the primary anaerobic organism, found in 78% of appendiceal infections and over 90% of wound infections following appendectomy 2
• Klebsiella pneumoniae appears in 26% of cases 2, 3
• Anaerobes are present in approximately 30% of post-operative peritonitis cases 2

Additional organisms frequently isolated include:

• Streptococcus species (25% of infections) 2, 3
• Enterococcus species (18% of cases, associated with worse outcomes) 2, 3
• Pseudomonas aeruginosa (15% of cases, significantly correlated with surgical site infections) 3

Why This Matters Clinically

The polymicrobial nature directly determines antibiotic selection. The Infectious Diseases Society of America guidelines explicitly state that surgical site infections occurring after operations on the intestinal tract have "a high probability of having a mixed gram-positive and gram-negative flora with both facultative and anaerobic organisms" 1.

Empiric coverage must address both components:

• Gram-negative aerobes (primarily E. coli) require coverage with cephalosporins, fluoroquinolones, or carbapenems 1, 2
• Anaerobes (primarily B. fragilis) require metronidazole or carbapenem coverage 1, 2

Recommended Empiric Antibiotic Regimens

For post-appendectomy wound infections with fever and purulent discharge, the following regimens provide appropriate coverage 1, 2:

• Cephalosporin + metronidazole (e.g., cefazolin or cefoxitin plus metronidazole)
• Fluoroquinolone + metronidazole (e.g., levofloxacin plus metronidazole)
• Carbapenem monotherapy (e.g., ertapenem, meropenem)

Critical Clinical Pitfalls

Avoid these common errors:

• Do not use antibiotics targeting only gram-positive organisms (like cefazolin alone), as this misses the dominant gram-negative and anaerobic pathogens 1
• Do not use antibiotics targeting only gram-negative aerobes without anaerobic coverage, as anaerobes are critical in the pathogenesis of post-appendectomy wound infections 4
• Do not assume clean wound flora (which would be predominantly S. aureus and streptococci); appendectomy is a clean-contaminated procedure with predictable intestinal flora 1

Historical evidence confirms the importance of anaerobic coverage: A 1979 randomized trial demonstrated that clindamycin (with anaerobic activity) reduced wound infection rates from 33% to 17%, while cefazolin (without anaerobic coverage) failed to significantly reduce infection rates, leading investigators to conclude that "anaerobic organisms are more important than faecal aerobic organisms in the pathogenesis of wound infection after appendicectomy" 4.

Management Beyond Antibiotics

Primary therapy requires surgical drainage 1:

• Open the infected wound and evacuate purulent material
• Continue dressing changes until healing by secondary intention
• Obtain Gram stain and culture before starting antibiotics 1, 2

Antibiotics are adjunctive when systemic signs are present 1:

• Temperature ≥38.5°C or pulse rate ≥100 beats/min warrant a short course (24-48 hours) of antibiotics
• If minimal systemic signs, antibiotics may be unnecessary after adequate drainage