Pathogenic significance of methicillin resistance for patients with Staphylococcus aureus bacteremia

To assess whether methicillin resistance is a microbial characteristic asso ciated with deleterious clinical outcome, we performed a cohort study on 90 8 consecutive episodes of Staphylococcus aureus bacteremia and a case-contr ol study involving 163 pairs of patients matched for preexisting comorbidit ies, prognosis of the underlying disease, length of hospitalization, and ag e. Of 908 bacteremic episodes, 225 (24.8%) were due to methicillin-resistan t S. aureus (MRSA). Multivariate analysis did not reveal that methicillin r esistance was an independent predictor for mortality when shock, source of bacteremia, presence of an ultimately or rapidly fatal underlying disease, acquisition of the infection in an intensive care unit (ICU), inappropriate empirical therapy, female sex, and age were taken into account. Nonetheles s, methicillin resistance was an independent predictor for shock. The case- control study could not confirm that shock was Linked to MRSA when prior an timicrobial therapy, inappropriate treatment, ICU residence, and female sex were considered. Our data suggest that cohort studies tend to magnify the relationship of MRSA with clinical markers of microbial pathogenicity and t hat this effect is a shortcoming of these kind of studies that is caused by inadequate control for underlying diseases.