The impact of resistance to methicillin in Staphylococcus aureus bacteremia on mortality

Background: It is unclear whether methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) differ in virulence. We therefore carried out a prospective study of cases of S. aureus bacteremia over a period of 2 years at our university hospital. We report herein the results of a comparative analysis of the clinical characteristics and mortality rates associated with cases of bacteremia caused by MRSA and MSSA. Methods: Over a 2-year period, we reviewed the medical records of hospitalized patients with blood cultures positive for S. aureus. Demographic characteristics, underlying diseases, diagnosis, clinical features, severity, laboratory findings, antimicrobial treatment, and resistance to methicillin were analyzed as possible risk factors for death attributed to bacteremia. Results: The rate of mortality attributed to bacteremia was 27.3% (27 patients): 13 (43.3%) deaths were recorded for patients infected with MRSA and 14 (20.3%) for patients infected with MSSA (P=0.0339, OR=3.00 (1.08--8.46)). Two variables were significantly associated with death within 14 days of the onset of bacteremia in univariate and multivariate analyses after adjusting for antimicrobial treatment: malignant hematologic disease (protective factor; OR=0.184 (0.038--0.882)) and resistance to methicillin (risk factor; OR=2.97 (1.12--7.88)). Conclusion: This study shows that S. aureus bacteremia has a high mortality rate, especially if the strain involved is methicillin-resistant, regardless of patient age and the efficacy of antimicrobial treatment.