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

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Abstract
Staphylococcus aureus virulence factors may determine infection presentation. Whether SCCmec 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 SCCmec typing of methicillin-resistant S. aureus (MRSA) isolates, and stratified the results according to SCCmec 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 SCCmec type revealed significant differences. SCCmec type II caused highest mortality (33.3%) versus type IVa (13.5%), other MRSA (12.5%), and MSSA (8.9%). SCCmec IVa produced the highest metastatic infection (26.9% versus 9.1% [SCCmec II], 8.3% [other MRSA], and 15.6% [MSSA]). Persistent bacteremia (>or= 7 days) was similar in all SCCmec types (16.7 to 20.7%); each exceeded MSSA (6.7%; P=0.05). In multivariate analysis, SCCmec II was a predictor of mortality (odds ratio [OR]=3.73; 95% confidence interval [CI] = 1.81 to 7.66; P=0.009), SCCmec IVa was a predictor of metastatic infection (OR=3.52; CI=1.50 to 8.23; P=0.004), and MRSA (independent of SCCmec type) was a predictor of persistent bacteremia (OR=4.16; CI=1.47 to 11.73; P=0.007). These findings suggest that SCCmec-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 SCCmec type II and metastatic infection with SCCmec type IVa.

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