Negative Correlation between MRSA and MSSA Prevalence in Nursing Homes

Background: It is unknown whether MSSA carriage protects against MRSA acquisition by competing for colonization of the anterior nares. Competition may be relevant to decolonization strategies that may eliminate MSSA and predispose to MRSA acquisition in endemic settings such as nursing homes. 

Objective: To evaluate whether an inverse association exists between MRSA and MSSA prevalence in nursing homes in Orange County, CA.

Methods: We performed a prospective cross-sectional study of admission prevalence and point prevalence of MRSA and MSSA in 23 nursing homes in Orange County, CA from January 1, 2009 to September 30, 2010.  To determine admission prevalence, we collected bilateral nares swabs from all new residents within 3 days of admission until we obtained 100 swabs.  To determine point prevalence, we collected bilateral nares swabs from a random sample of 100 residents per facility.  For nursing homes with <100 beds, we returned after a period of at least twice the mean length-of-stay until we obtained 100 swabs.  All swabs were plated on sheep blood and Spectraä MRSA chromogenic agar (Remel, KA).  If MRSA was detected, no further tests were performed.  If MRSA was not detected, blood agar was used to identify MSSA. We obtained descriptive data for facilities and evaluated the association between MRSA and MSSA prevalence using tests of correlation and linear regression. In order to account for the possibility that MSSA co-colonization was missed on a MRSA+ plate, we also assessed the correlation between MRSA prevalence and the fraction of non-MRSA+ swabs that grew MSSA.

Results: In 23 participating nursing homes, we swabbed 3,317 total residents over 21 months.  A median of 32% of residents were non-white (range, 2-94%), and a median of 75% were Medicaid insured.  Most residents were 65-84 years old (median 41%; range, 20-80%)  

Figure 1 shows admission and point prevalence of MRSA versus MSSA across all nursing homes.   Admission MRSA and MSSA prevalence were not correlated.  However, for point prevalence, there was an inverse correlation when comparing MRSA to MSSA point prevalence across facilities regardless of whether MSSA prevalence was measured among all residents sampled (r=-0.70; p<0.001), or among those who did not harbor MRSA (r=-0.42; p<0.05). Similarly, in linear regression models, MRSA point prevalence decreased significantly as MSSA point prevalence increased among all residents sampled (β =-1.34; 95% CI: -0.72,-1.97; p<0.001) and among those who did not harbor MRSA  (β =-0.79; 95% CI: -0.02,-1.56; p<0.05). 

Conclusions:   The inverse association between MRSA and MSSA point prevalence suggests MSSA carriage may protect against MRSA acquisition in nursing homes.  The minimal association on admission prevalence further suggests competition may occur during the nursing home stay.  Nevertheless, at least half the variation in MRSA prevalence was not explained by MSSA prevalence and suggests a large role for other factors.