Role of SCC mec Type in Outcome of Staphylococcus aureus Bacteremia in a Single Medical Center

Abstract

Staphylococcus aureus virulence factors may determine infection presentation. Whether SCC mec type-associated factors play a role in S. aureus bacteremia is unclear. We conducted a prospective observation of adult inpatients with S. aureus bacteremia (1 November 2005 to 31 December 2006), performed SCC mec typing of methicillin-resistant S. aureus (MRSA) isolates, and stratified the results according to SCC mec type. We studied 253 patients. MRSA accounted for 163 (64.4%) cases. The illness severity index was similar in MRSA and methicillin-sensitive S. aureus (MSSA) cases. MRSA caused higher in-hospital mortality (23.9% versus 8.9%; P = 0.003), longer bacteremia (4.7 ± 6.5 days versus 2.7 ± 2.9 days; P = 0.01), but similar metastatic infection (14.7% versus 15.6%). Stratifying the results according to SCC mec type revealed significant differences. SCC mec type II caused highest mortality (33.3%) versus type IVa (13.5%), other MRSA (12.5%), and MSSA (8.9%). SCC mec IVa produced the highest metastatic infection (26.9% versus 9.1% [SCC mec II], 8.3% [other MRSA], and 15.6% [MSSA]). Persistent bacteremia (≥7 days) was similar in all SCC mec types (16.7 to 20.7%); each exceeded MSSA (6.7%; P = 0.05). In multivariate analysis, SCC mec II was a predictor of mortality (odds ratio [OR] = 3.73; 95% confidence interval [CI] = 1.81 to 7.66; P = 0.009), SCC mec IVa was a predictor of metastatic infection (OR = 3.52; CI = 1.50 to 8.23; P = 0.004), and MRSA (independent of SCC mec type) was a predictor of persistent bacteremia (OR = 4.16; CI = 1.47 to 11.73; P = 0.007). These findings suggest that SCC mec -associated virulence factors play a role in the outcome of S. aureus bacteremia. Additional studies are needed to identify which virulence factors are the determinants of increased mortality with SCC mec type II and metastatic infection with SCC mec type IVa.