The microbiology of necrotizing soft tissue infections

OBJECTIVE: A large number of necrotizing soft tissue infections (NSTI) treated at a single institution over an 8-year period were analyzed with respect to microbial pathogens recovered, treatment administered, and outcome. Based on this analysis, optimal empiric antibiotic coverage is proposed. METHODS: A retrospective chart review of all patients with documented NSTI was conducted. Microbiologic variables were tested for impact on outcome using Fisher's exact test and multivariate analysis by logistic regression. RESULTS: Review of the charts of 198 patients with documented NSTI revealed 182 patients with sufficient microbiologic information for analysis. These 182 patients grew an average of 4.4 microbes from original wound cultures, although a single pathogen was responsible in 28 patients. Eighty-five patients had combined aerobic and anaerobic growth, the most common organisms being, in order, Bacteroides species, aerobic streptococci, staphylococci, enterococci, Escherichia coli, and other gram-negative rods. Clostridial growth was common but did not affect mortality unless associated with pure clostridial myonecrosis. Mortality was affected by the presence of bacteremia, delayed or inadequate surgery, and degree of organ system dysfunction on admission. CONCLUSIONS: NSTI are frequently polymicrobial and initial antibiotic coverage with a broad-spectrum regimen is warranted. The initial regimen should include agents effective against aerobic gram-positive cocci, gram-negative rods, and a variety of anaerobes. The most common organisms not covered by initial therapy were enterococci. All wounds should be cultured at initial debridement, as changes in antibiotic coverage are frequent once isolates are recovered.