Competition between methicillin-sensitive and -resistant Staphylococcus aureus in the anterior nares.

Colonization of the anterior nares with Staphylococcus aureus has been shown to be a risk factor for infection. The purpose of this study was to test the hypothesis that methicillin-resistant strains of S. aureus (MRSA) compete with methicillin-sensitive (MSSA) strains for colonization of the anterior nares. As part of the local National Health Service trust MRSA infection control strategy, patients who have been in a healthcare institution in the last year are routinely sampled and tested for MRSA colonization at the time of hospital admission. The sampling and testing methods were modified for the six-month period of this study to allow the detection of both MSSA and MRSA/MSSA co-colonization. MRSA alone was carried by 56 (8%) of 680 patients, MSSA alone by 115 patients (17%), 505 patients (74.3%) carried neither, and four patients (0.6%) carried both MRSA and MSSA. The deviance between the observed number of co-colonized swabs and that expected under the null hypothesis of no competition between MSSA and MRSA was significant (P=0.02, Fisher's exact test). The statistical approach is unaffected by the confounding effect of factors that affect the relative frequencies of MRSA or MSSA colonization. When logistic regression was used to estimate the extent of competition, controlling for effects of age and sex, we estimated a protective efficacy of MSSA colonization in the prevention of MRSA colonization of 78% (95% CI 29-99%). Results from this cross-sectional study support the hypothesis that MRSA and MSSA compete for colonization space, and provides an estimate of the extent to which MSSA interferes with MRSA colonization.