A 52-year-old man undergoes elective abdominal surgery for peptic ulcer disease. On postoperative day 5, the surgical wound shows signs of infection with purulent discharge. Which is the most common causative organism in surgical site infections (SSI) occurring in the early postoperative period?

Explanation

## Most Common Causative Organism in Early Surgical Site Infections **Key Point:** Staphylococcus aureus is the most common causative organism of surgical site infections (SSI) in the early postoperative period (within 5–7 days), accounting for approximately 40–50% of all SSIs. ### Epidemiology of Surgical Site Infections | Organism | Frequency | Typical Timeline | Common Site | |----------|-----------|------------------|-------------| | Staphylococcus aureus | 40–50% | Early (0–7 days) | Skin, soft tissue | | Escherichia coli | 20–25% | Early to late | GI tract, abdominal | | Pseudomonas aeruginosa | 10–15% | Late (>7 days) | Moist environments | | Clostridium perfringens | <5% | Late (3–5 days) | Gas gangrene, rare | ### Why Staphylococcus aureus Dominates Early SSI 1. **Source:** Normal skin flora; colonizes the operative field despite antiseptic preparation 2. **Pathogenesis:** Forms biofilms on surgical materials and sutures; resists initial immune response 3. **Timing:** Proliferates during the inflammatory phase (days 0–5) when local defenses are overwhelmed 4. **Virulence factors:** Alpha-toxins, protein A (evades opsonization), and coagulase production **High-Yield:** Gram-positive cocci (especially Staph aureus and Streptococcus pyogenes) account for ~60% of early SSI; gram-negative organisms and anaerobes are more common in late SSI (>7 days) or when GI tract is breached. **Clinical Pearl:** MRSA (methicillin-resistant S. aureus) is an increasing concern in hospital-acquired SSI; empiric coverage with vancomycin or linezolid may be warranted in high-risk settings. ### Prevention Strategy **Mnemonic: SCIP** — Surgical Care Improvement Project - **S**kin antisepsis (chlorhexidine or povidone-iodine) - **C**hoose appropriate prophylactic antibiotics (cefazolin for clean surgery) - **I**ntraoperative redosing if case prolonged - **P**ost-operative glucose control and normothermia [cite:Sabiston Textbook of Surgery Ch 6]