MRSA colonization: Is it a quantitative or qualitative relationship?

Leonard Mermel

Few studies have assessed the importance of quantitative differences in MRSA carriage at various body sites. Len Mermel et al. from Warren Alpert Medical School (Brown) has a new study out in the Journal of Clinical Microbiology that assessed the relationship between quantitative burden of MRSA colonization at various body sites and colonization at other sites. For example, if patients are heavily colonized in the anterior nares, are they more likely to be colonized elsewhere and carriage at which sites are related to heavy nares colonization?

In a cohort of patients known to be MRSA+ during the prior year, the study team collected swabs from the each nares, each axilla, each groin site, and the perineum. Swabs were tested for presence of MRSA using CHROMagar plates and log10 colony counts. 

In this known MRSA+ cohort, 53 of 60 patients were MRSA+ at at least 1 site and 29/123 cultures were only positive after broth enrichment.  75% (40/53) were colonized extranasally. Sensitivity ranged from 91% in the nares, 63% in the groin, 47% in the perineum down to 32% in the axilla. The greatest combined sensitivity was 98% for nares+groin.

Mean log10 counts were highest in the nares (1.95) and lowest in the axilla (0.87). As the colonization count increased in the nares, the number of distant sites colonized increased.  For each 1 log increase, the OR=2.1 for distant colonization. As far as risk factors, diabetes was associated with colonization in the perineum. One interesting finding was that mean log counts were lower in patients with a current or previous infection than in patients who had never been infected (1.6 vs 2.4, p=0.3)

Mermel LA et al J Clin Microbiol 2011