Impact of traditional hospital strain of methicillin-resistant Staphylococcus aureus (MRSA) and community strain of MRSA on mortality in patients with community-onset S aureus bacteremia

Community-associated methicillin-resistant Staphylococcus aureus (MRSA) has become an important pathogen in community and nosocomial infections. The impact of these emerging MRSA strains on mortality in adult patients with community-onset S aureus bacteremia remains uncertain. We defined community strain MRSA (CoSt-MRSA) and hospital strain MRSA (HoSt-MRSA) according to the results of staphylococcus cassette chromosome mec (SCCmec) molecular typing: CoSt-MRSA isolates had SCCmec type IV or V genes, and HoSt-MRSA isolates had SCCmec type I, II, or III genes. We quantitatively evaluated the impact of the MRSA strain on mortality in patients with CoSt-MRSA or HoSt-MRSA bacteremia by comparison with mortality in patients with methicillin-susceptible S aureus (MSSA) bacteremia.We studied an observational cohort of 500 patients with MSSA bacteremia, 111 patients with CoSt-MRSA, and 133 patients with HoSt-MRSA bacteremia from January 1, 2001, through December 31, 2007. The 90-day cumulative probability of survival in patients with MSSA, CoSt-MRSA, and HoSt-MRSA bacteremia was 71%, 70%, and 55%, respectively (p ≤ 0.014, by Wilcoxon rank-sum test).Compared to patients with MSSA bacteremia, patients with HoSt-MRSA bacteremia were associated with an increased risk of mortality in the first multivariate analysis model adjusting for all potential confounders (hazard ratio [HR], 1.525; 95% confidence interval [CI], 1.091-2.131), in the second model adjusting for all confounders except acute severity of bacteremia (HR, 1.489; 95% CI, 1.071-2.070), and in stratified analysis in patients with low Charlson comorbidity scores (score 0-2) (HR, 3.093; 95% CI, 1.507-6.350).Compared to patients with MSSA bacteremia, patients with CoSt-MRSA bacteremia did not show significant differences in mortality rate in the 2 multivariate analysis models (first model: HR, 1.106; 95% CI, 0.748-1.637; second model: HR, 1.028; 95% CI, 0.697-1.516) or in stratified analysis (HR, 1.092; 95% CI, 0.539-2.214).In conclusion, using MSSA as reference, traditional hospital strain MRSA had a higher impact on bacteremia mortality than community strain MRSA.