A 52-year-old man undergoes elective open cholecystectomy for symptomatic cholelithiasis. On postoperative day 5, he develops fever (38.5°C), purulent drainage from the surgical wound, and erythema at the incision site. Culture of the wound fluid is sent. Which is the most common causative organism in surgical site infections following clean-contaminated procedures?

Explanation

## Most Common Organism in Surgical Site Infections **Key Point:** *Staphylococcus aureus* is the single most common causative organism in surgical site infections (SSIs) overall, including clean-contaminated procedures such as open cholecystectomy. ### Epidemiology of SSI by Surgery Type | Surgery Type | Most Common Organism | Source | |---|---|---| | All SSIs (overall) | *S. aureus* | Skin/endogenous flora | | Clean (orthopedic, cardiac, vascular) | *S. aureus* | Skin flora | | Clean-contaminated (GI, biliary, gynec, urologic) | *S. aureus* (overall); gram-negatives (e.g., *E. coli*) also significant | Skin + endogenous GI flora | | Contaminated/Dirty | Polymicrobial | Mixed aerobic and anaerobic | **High-Yield:** According to CDC/NHSN surveillance data and standard surgical references (Sabiston Textbook of Surgery; Schwartz's Principles of Surgery), *S. aureus* — including MRSA — consistently ranks as the **#1 pathogen** in SSIs across all wound classes. It accounts for approximately 20–30% of all SSIs. While gram-negative enteric organisms such as *E. coli* are important contributors in GI/biliary procedures, they do not surpass *S. aureus* as the single most common isolate even in clean-contaminated cases. **Clinical Pearl:** *E. coli* and other gram-negative enteric bacteria are the predominant organisms when considering **intra-abdominal infections** (e.g., bile peritonitis, intra-abdominal abscess) following biliary surgery. However, for **superficial and deep incisional SSIs** — which is what this vignette describes (purulent wound drainage, erythema at incision) — *S. aureus* remains the leading pathogen because the skin is the primary inoculum source at the time of incision. ### Why S. aureus Leads in SSIs 1. Ubiquitous skin colonizer — present at every incision site regardless of surgery type 2. Produces virulence factors (coagulase, toxins) that facilitate wound invasion 3. Biofilm formation on sutures and devitalized tissue 4. MRSA strains add to the burden in healthcare settings **Mnemonic:** **"S. aureus = Surgical Adversary #1"** — regardless of the organ system operated upon, skin flora (dominated by *S. aureus*) is the primary source of incisional SSIs. [cite: Sabiston Textbook of Surgery, 21st ed., Ch. 12; CDC/NHSN SSI Surveillance Report; Schwartz's Principles of Surgery, 11th ed.]