Most Common Bacteria in Surgical Site Infections (SSI)
Staphylococcus aureus is the most common pathogen causing surgical site infections, accounting for approximately 40% of all SSIs, with methicillin-resistant S. aureus (MRSA) representing a significant and increasing proportion of these infections. 1
Bacterial Pathogens by Frequency
The distribution of pathogens in SSIs varies based on surgical procedure type but generally follows this pattern:
Gram-positive organisms:
- Staphylococcus aureus (39.9-45.9%)
- Coagulase-negative staphylococci (3.4-4.2%)
- Enterococcus species (7.7-8.2%)
- Streptococcus species (2.6-2.7%)
Gram-negative organisms:
- Pseudomonas aeruginosa (10.8-12.1%)
- Escherichia coli (7-9.7%)
- Klebsiella species (5.1-5.8%)
- Enterobacter species (5.6-5.8%)
- Proteus species (3.2-3.7%)
- Acinetobacter species (1.6-2.2%)
- Serratia species (2%)
Pathogen Distribution Based on Surgical Procedure Type
The microbial etiology of SSIs is strongly influenced by the type of surgical procedure:
Clean Procedures
In clean procedures (where gastrointestinal, gynecologic, and respiratory tracts are not entered):
- S. aureus (including MRSA) and streptococcal species predominate 1
Clean-Contaminated or Contaminated Procedures
In procedures involving the gastrointestinal tract, female genital tract, or respiratory tract:
- Mixed flora including aerobic and anaerobic organisms
- Enterobacteriaceae (E. coli, Klebsiella)
- Enterococci
- Bacteroides fragilis and other anaerobes 1
Anatomic Site-Specific Considerations
- Axilla: Higher recovery of gram-negative organisms 1
- Perineum: Higher incidence of gram-negative organisms and anaerobes 1
- Groin: Potential for enteric flora colonization even in clean procedures 1
Emerging Resistance Patterns
The increasing prevalence of resistant organisms in SSIs is a significant concern:
- MRSA: Represents 27.2-29% of S. aureus isolates overall, with community-associated MRSA becoming increasingly common 1, 2
- ESBL-producing organisms: Increasing numbers of Enterobacteriaceae (particularly E. coli and K. pneumoniae) producing extended-spectrum β-lactamases 1
Risk Factors for MRSA SSIs
Major risk factors for MRSA SSIs include:
- Prior MRSA colonization
- Prolonged hospitalization (>2 weeks)
- Advanced age (>75 years)
- Diabetes mellitus
- Sepsis 2
Clinical Implications for Treatment
When treating SSIs empirically, antibiotic selection should be guided by:
For clean procedures (trunk, head and neck, extremities):
- First-generation cephalosporin or antistaphylococcal penicillin for MSSA
- Vancomycin, linezolid, daptomycin, telavancin, or ceftaroline when MRSA risk factors are present 1
For procedures involving GI tract, perineum, female genital tract, or axilla:
- Agents active against gram-negative bacteria and anaerobes
- Cephalosporin or fluoroquinolone plus metronidazole 1
Prevention Strategies
Effective prevention of SSIs requires:
- Appropriate preoperative antibiotic prophylaxis
- Proper skin antisepsis (chlorhexidine-alcohol solutions showing higher efficacy)
- Laparoscopic technique when feasible
- Infection committee surveillance 2
Remember that SSIs rarely occur during the first 48 hours after surgery. Those that do occur in this timeframe are almost always due to S. pyogenes or Clostridium species, which require immediate attention 1.