## Microbiology of Surgical Site Infections in Clean Surgery **Key Point:** In clean, elective general surgical procedures, SSI is most commonly caused by skin flora, of which *Staphylococcus aureus* (both methicillin-sensitive and methicillin-resistant strains) is the most frequent pathogen. ### Source of Infection in Clean Surgery Clean surgical procedures have minimal contamination risk. SSIs in this setting arise primarily from: 1. Patient's own skin flora (endogenous source) 2. Surgeon's hands or gloves (exogenous source) 3. Operating room environment (rare in modern facilities) **Staphylococcus aureus** is a normal skin colonizer and the leading cause of SSI in clean surgery, accounting for ~20–30% of all SSIs in this category. ### Comparison of Organisms in SSI | Organism | Setting | Notes | |----------|---------|-------| | *S. aureus* (MSSA/MRSA) | Clean surgery (most common) | Skin flora; endogenous source | | *Coagulase-negative Staphylococcus* | Clean surgery (2nd most common) | Skin flora; especially prosthetic implants | | *E. coli*, *Klebsiella*, *Proteus* | Clean-contaminated (GI, biliary, gynae) | Gut flora; exogenous/endogenous | | *Clostridium difficile* | Post-antibiotic, not typical SSI | Causes antibiotic-associated colitis, not wound infection | | *Pseudomonas aeruginosa* | Contaminated/dirty surgery; immunocompromised | Environmental organism; rare in clean elective surgery | | *Enterococcus* | Clean-contaminated GI surgery | Lower virulence; often polymicrobial | **High-Yield:** Remember that in clean surgery, the infection source is almost always the **patient's own skin flora**. *S. aureus* is the most virulent and most common skin pathogen causing SSI. **Clinical Pearl:** Surgical antibiotic prophylaxis in clean surgery is typically directed against gram-positive cocci (e.g., cephalosporins, cloxacillin) to cover *S. aureus*, not gram-negative or anaerobic organisms.
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