Methicillin-Susceptible Staphyloccocus aureus Carriers Unlikely to Acquire MRSA

MRSA microscopic
MRSA microscopic
Study results can be used to support an antimicrobial stewardship policy discouraging empiric coverage for MRSA infections in a population of MSSA carriers.
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.

Related Articles

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.”

Visit Infectious Disease Advisor’s conference section for continuous coverage live from IDWeek 2017.


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.