Better environmental survival of outbreak vs. sporadic MRSA isolates.

Environmental sources have been associated with prolonged epidemics of methicillin-resistant Staphylococcus aureus (MRSA). Our objective was to evaluate whether outbreak MRSA strains differ in their environmental survival from sporadic MRSA strains. The strains were detected in five adult patients unknowingly colonized by MRSA. The MRSA strains from patients No. 1 (phage pattern; III-29) and No. 2 (III-215) caused extensive outbreaks in our hospital. Contact tracing after detection of the different MRSA strains in the three other patients did not reveal further spread. Suspensions of MRSA with and without added hospital dust were made in sterile PBS. Surviving counts of MRSA were made by culture at two-weekly or longer intervals up to approximately one year. For each MRSA strain the ultimate day of viability was ascertained. The environmental survival patterns of the five MRSA strains showed qualitative and quantitative differences between the two outbreak and three sporadic strains. A gradual decline was noted for all strains. All survived longer than 6 months, but the two outbreak strains survived significantly better (P<0.01), i.e. in higher quantities (circa 1000-fold) and for a 1-3 months longer period. The survival patterns of the MRSA strains with and without added dust were similar.