Methicillin resistance in Staphylococcus aureus infections among patients colonized with methicillin-susceptible Staphylococcus aureus.

We have noticed that patients colonized with methicillin-susceptible Staphylococcus aureus (MSSA) rarely get methicillin-resistant S. aureus (MRSA) infections. The purpose of this study was to compare the odds of a Staphylococcus aureus (SA) infection being an MRSA infection in MSSA carriers, MRSA carriers and non-carriers of SA. Hospitalizations of adult patients at the Cleveland Clinic Health System from 2008 to 2015 were screened to identify those where the patient was tested for SA colonization. The first such hospitalization was identified. Among these 90 891 patients, those who had an SA infection during the hospitalization were included. SA carrier status (MRSA, MSSA, or non-carrier), was defined based on the first nasal SA test result. The association of carrier status and MRSA infection was examined. The mean (±standard deviation (SD)) age of the 1999 included patients was 61 (17) years, and 1160 (58%) were male. Thirty percent, 26%, and 44%, were MRSA carriers, MSSA carriers and non-carriers, respectively. Of the 601 SA infections in MRSA carriers (reference group), 552 (92%) were MRSA infections compared with 42 (8%) of 516 in MSSA carriers (odds ratio (OR) 0.008, 95% confidence interval (CI) 0.005-0.012, p <0.0001) and 430 (49%) of 882 in non-carriers (OR 0.072, 95% CI 0.051-0.100, p <0.0001), after controlling for age, sex, hospital length of stay and calendar year. Among patients with SA infection, the odds of the infection being an MRSA infection are 125-times lower in an MSSA carrier than in an MRSA carrier.

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