Methicillin-Susceptible Staphyloccocus aureus Carriers Unlikely to Acquire MRSA

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MRSA infections were less common in carriers of methicillin-susceptible <i>Staphylococcus aureus.</i>
MRSA infections were less common in carriers of methicillin-susceptible Staphylococcus aureus.
This article is part of Infectious Disease Advisor's coverage of IDWeek 2017™, taking place in San Diego, CA. Our on-site staff will be reporting on the latest breaking research and clinical advances in infectious diseases. Check back regularly for highlights from IDWeek 2017.

SAN DIEGO — Carriers of methicillin-susceptible Staphylococcus aureus (MSSA) were less likely to acquire methicillin-resistant S aureus (MRSA) infections while hospitalized compared with MRSA carriers, according to a study presented at IDWeek 2017.

A total of 90,981 adults hospitalized at the Cleveland Clinic between 2008 and 2015 who were screened for S aureus were classified as carrying MSSA (n=12,510; 14%), MRSA (n=5774; 6%), or neither (n=72,697; 80%). Patients were then evaluated for S aureus bacteremia or non-bacteremic S aureus infections during their hospital stay, and the association between carrier status and subsequent S aureus infections was evaluated.

A higher percentage of acquired S aureus bacteremia was methicillin-resistant in MRSA carriers (93%) compared with MSSA carriers (7%; odds ratio [OR]: 0.006; 99% CI, 0.003-0.012) and non-carriers (53%; OR: 0.089; 99% CI, 0.052-0.144)). In a similar fashion, more non-bacteremic S aureus infections were MRSA in MRSA carriers (91%) compared with MSSA carriers (9%; OR: 0.10; 99% CI, 0.006-0.017) and non-carriers (45%; OR: 0.086; 99% CI, 0.056-0.127).

When participants were divided into subgroups by age, hospital, length of stay, and year, MSSA carrier status was still associated with a lower risk of MRSA infection.

Comparable results were obtained in a Monte Carlo simulation that ran 1000 trials correcting for false positive and false negative nasal MRSA tests.

The researchers concluded that "[S aureus] infections in MSSA carriers are 100 times less likely to be MRSA infections than are [S aureus] infections in MRSA carriers… This fact can be used to support an antimicrobial stewardship policy of actively discouraging empiric coverage for MRSA infection in MSSA carriers."

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Reference

Shrestha N, Fraser TG, Gordon S. Patients colonized with methicillin-susceptible Staphylococcus aureus rarely get methicillin-resistant Staphylococcus aureus infections. Presented at: IDWeek 2017; October 4-8, 2017; San Diego, CA. Poster 2166.

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