Significantly larger numbers of methicillin-resistant Staphylococcus aureus bacteria are recovered from polymicrobial respiratory and wound sites by use of chromogenic primary media than by use of conventional culture.

Previous studies have validated the properties and documented the utility of chromogenic agar for surveillance of methicillin-resistant Staphylococcus aureus (MRSA). In this study, we used one of the chromagars, MRSASelect (Bio-Rad), as one of the primary isolation media for selected wound and respiratory clinical specimens which, in our institution, were typically polymicrobial. We examined a total of 638 specimens; 142 (22%) MRSA isolates were recovered. Twenty-six of these isolates were recovered only on the MRSASelect plate, representing a 28% (15/54) increase for endotracheal aspirates/sputa and a 15% increase for superficial wounds/ulcers (11/73) compared to the results with conventional culture. One isolate (1 CFU) was recovered by conventional medium alone. MRSASelect has generally been used for surveillance cultures; however, we document that an additional 21% of MRSA isolates would have gone unreported in these selected clinical specimens using only standard culture media. For 40% (6/15) of inpatients, MRSA isolated from the MRSASelect plate was the sole indicator of MRSA. Although these isolates can represent either colonization or infection, they are a potential reservoir of infection and nosocomial transmission. Our data support the focused use of chromogenic selective media for the increased detection of MRSA in polymicrobial wound and respiratory specimens, which could have an impact on both clinical treatment and infection control.