Which of the following organisms is MOST commonly associated with surgical site infections in clean, elective general surgical procedures?

Question

Accepted Answer

A. Staphylococcus aureus. ## 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.

Answer

B. Clostridium difficile

Answer

C. Pseudomonas aeruginosa

Answer

D. Enterococcus faecalis

Which of the following organisms is MOST commonly associated with surgical site infections in clean, elective general surgical procedures?

Explanation

Microbiology of Surgical Site Infections in Clean Surgery

Source of Infection in Clean Surgery

Comparison of Organisms in SSI

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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