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?

Question

Accepted Answer

B. Staphylococcus aureus. ## 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

Answer

A. Clostridium perfringens

Answer

C. Escherichia coli

Answer

D. Pseudomonas aeruginosa

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

Epidemiology of SSI Pathogens

Why S. aureus Dominates

Clinical Pearl

Risk Factors for S. aureus SSI

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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