A 52-year-old man undergoes elective open cholecystectomy for chronic cholecystitis. On postoperative day 5, he develops fever (38.5°C), wound erythema, and purulent discharge from the surgical incision. Which is the most common causative organism of this surgical site infection?

Explanation

## Most Common Causative Organism of Surgical Site Infection **Key Point:** Staphylococcus aureus is the most common causative organism of surgical site infections (SSIs) overall, accounting for approximately 20–30% of all SSIs across clean and clean-contaminated procedures. ### Epidemiology of SSI Pathogens | Organism | Frequency | Common Procedure Types | Key Feature | |----------|-----------|------------------------|-------------| | **S. aureus** | 20–30% | Clean & clean-contaminated | Skin flora; most common overall | | E. coli | 15–20% | GI tract surgery | Gram-negative; endogenous | | P. aeruginosa | 8–12% | Contaminated; immunocompromised | Nosocomial; biofilm-forming | | C. perfringens | <2% | Traumatic/dirty wounds | Gas gangrene; rare in elective surgery | ### Why S. aureus Dominates 1. **Source:** Normal skin flora; present in ~30% of human population 2. **Inoculation:** Enters wound during surgical incision or via contaminated instruments 3. **Virulence:** Produces multiple exotoxins (alpha-toxin, Panton-Valentine leukocidin in MRSA strains) 4. **Timing:** Typically manifests on postoperative days 3–7 (as in this case) **High-Yield:** In clean elective surgery (cholecystectomy, hernia repair), S. aureus accounts for >50% of bacterial SSIs. This is why prophylactic antibiotics target gram-positive coverage (cephalosporins, vancomycin). ### Clinical Pearl The timing of infection (postoperative day 5) and presence of purulent discharge are classic for S. aureus SSI. Early infections (day 1–2) suggest intraoperative contamination; late infections (>2 weeks) may indicate foreign body or hematogenous seeding. **Mnemonic: SSSSS** — **S. aureus = Skin Source, Surgical Site, Staphylococcal Supremacy (most common), Suppurative (purulent), Sensitive to beta-lactams (unless MRSA)** ### Risk Factors for S. aureus SSI - Nasal carriage of S. aureus (NCCLS screening recommended preoperatively) - Obesity, diabetes, immunosuppression - Prolonged operative time - Inadequate skin antisepsis