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

Common Organisms in Post-Appendectomy Wound Infections

The most common organisms in post-appendectomy wound infections presenting weeks after surgery are polymicrobial, dominated by Escherichia coli (85% of cases), followed by Klebsiella pneumoniae (26%), Streptococcus species (25%), Enterococcus species (18%), and anaerobes, particularly Bacteroides fragilis. 1

Primary Pathogens

Aerobic Bacteria

• E. coli is the predominant aerobic pathogen, isolated in approximately 85% of appendicitis-related infections 2
• Klebsiella pneumoniae accounts for 26% of cases 2
• Streptococcus species (including viridans group streptococci) are present in 25% of infections 1, 2
• Enterococcus species are isolated in 18% of cases and are associated with worse outcomes in intra-abdominal infections 1, 2
• Pseudomonas aeruginosa appears in 15% of cases and is significantly correlated with surgical site infections (p=0.002) 2

Anaerobic Bacteria

• Bacteroides fragilis is the dominant anaerobic pathogen, found in 78% of appendiceal infections and over 90% of wound infections following appendectomy 1, 3
• Anaerobes are present in approximately 30% of post-operative peritonitis cases 1
• The infection is typically polymicrobial with synergistic aerobic-anaerobic combinations 4, 3

Critical Clinical Context

Healthcare-Associated vs. Community-Acquired Pattern

This patient presenting weeks post-operatively represents a healthcare-associated infection (HA-IAI), which has different microbiology than community-acquired infections. 1

• Post-operative infections show lower susceptibility rates and different resistance patterns compared to community-acquired cases 1
• Enterococci are isolated in 47.1% of post-operative peritonitis cases, compared to lower rates in community-acquired infections 1
• Candida species appear in 28.9% of post-operative cases 1

Resistance Patterns to Consider

• Extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae are increasingly common, particularly with recent antibiotic exposure (within 90 days) 1
• Seven of eight P. aeruginosa isolates in one study were resistant to cefuroxime, though all remained sensitive to ceftazidime, cefepime, and amikacin 2
• The proportion of multidrug-resistant organisms (MDRO) increases with time from initial surgery 1

Recommended Empiric Coverage

For post-appendectomy wound infections with fever, empiric antibiotics must cover both aerobic gram-negative rods (especially E. coli) and anaerobes (especially Bacteroides fragilis). 1, 5, 6

First-Line Regimens

• Cephalosporin + metronidazole (e.g., cefazolin or cefoxitin plus metronidazole) 5, 7, 6
• Fluoroquinolone + metronidazole (e.g., levofloxacin plus metronidazole) 5, 6
• Carbapenem monotherapy (e.g., ertapenem, meropenem) 5, 6

When to Add MRSA Coverage

Add vancomycin if the patient has risk factors including: 7

• Nasal MRSA colonization
• Prior MRSA infection
• Recent hospitalization
• Recent antibiotic use

Essential Diagnostic Steps

Wound Management

Immediately remove sutures and open the incision to allow drainage if purulent material is present or if erythema extends >5 cm from the incision with induration. 5, 7, 6

• Obtain Gram stain and culture of any purulent drainage before starting antibiotics 5, 7, 6
• Implement daily dressing changes with wound inspection 5, 7
• Source control (drainage) is the primary treatment; antibiotics are adjunctive 7

Culture Collection

• Collect 1-2 mL of fluid or tissue in an anaerobic transport system 1
• Inoculate directly into aerobic and anaerobic blood culture bottles when possible 1
• Peritoneal swabs and fluid from drain tubes are not recommended 1

Common Pitfalls to Avoid

• Do not delay surgical drainage waiting for culture results - source control is paramount 7
• Do not assume superficial appearance means no deep infection - post-appendectomy infections can rapidly progress to involve fascia and deeper structures 7
• Do not use antibiotics without adequate source control - antibiotics alone are ineffective without drainage in established wound infections 7, 8
• Do not forget anaerobic coverage - Bacteroides is present in >90% of post-appendectomy wound infections and requires specific antimicrobial therapy 3

Monitoring and Follow-Up

• Reassess the wound daily for improvement 7
• Monitor temperature curve and vital signs 7
• Adjust antibiotics based on culture results when available 1
• Consider imaging (CT) to evaluate for deeper abscess if no improvement within 48-72 hours 7