## Microbiology of Surgical Site Infections **Key Point:** The causative organisms of SSI vary by surgical type and contamination level. In clean surgeries, skin flora predominate. ### Organisms by Surgical Category | Surgical Type | Most Common Organism | Source | |---------------|----------------------|--------| | Clean (non-contaminated) | *Staphylococcus aureus* | Skin flora; patient or surgical team | | Clean-contaminated (GI, biliary) | *E. coli*, anaerobes | Endogenous flora | | Contaminated | Mixed flora | Environmental/procedural | | Dirty/infected | Polymicrobial | Pre-existing infection | **High-Yield:** *Staphylococcus aureus* (including MRSA) accounts for approximately 20–30% of all SSIs and is the leading pathogen in clean surgeries. Both methicillin-sensitive (MSSA) and methicillin-resistant (MRSA) strains are common. **Clinical Pearl:** Coagulase-negative staphylococci and *S. aureus* together account for >40% of SSIs in clean surgeries, reflecting their abundance on skin and mucous membranes. **Mnemonic:** **SKIN FLORA RULE** — In clean surgeries, expect skin commensals (*Staphylococcus aureus*, coagulase-negative staph, *Corynebacterium*) as the primary culprits. ### Why *S. aureus* Dominates Clean Surgery SSI 1. Abundant on skin and anterior nares of patients and surgical personnel 2. Ability to adhere to surgical implants and form biofilms 3. Produces virulence factors (α-toxin, protein A) that evade immune clearance 4. Frequently colonizes hospitalized patients
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