Comparison of BBL chromagar MRSA to conventional media for the detection of methicillin resistant staphylococcus aureus in surveillance nasal swabs

ABSTRACT

Objectives This study aims to detect MRSA nasal carriers among medical staff and patients in Dermatology ward Hospital Kuala Lumpur by using two methods, the conventional blood sheep agar (BSA) and the novel BBL CHROMagar MRSA (C-MRSA). It also aims to compare the BSA medium with the C-MRSA medium in terms of specificity, sensitivity and time to detection to MRSA. Method A single centre, prospective study where 100 nasal swab samples were taken from medical staff and inpatients, then plated on to both BSA and C-MRSA. After 24 hours incubation, the plates were examined for presence of bacterial colonies, then incubated for another 24 hours if no colonies were present. All colonies on C-MRSA and BSA were subjected to coagulase and susceptibility testing for confirmation of MRSA. MRSA strains produce mauve colonies on CMRSA from hydrolysis of the chromogenic substance, thus C-MRSA uses colour as a diagnostic tool. Results Mauve colonies were present on nine C-MRSA plates in the first 24 hours which were all confirmed to be MRSA. Another nine CMRSA plates isolated bluish colonies which were not MRSA. There were colonies on 96 BSA plates, nine of which were MRSA. C-MRSA medium has 100% sensitivity and specificity in detecting MRSA. Both culture media had similar detection rates of MRSA from nasal swabs, however C-MRSA allows for earlier detection of MRSA within 24 hours compared to BSA which takes 48 hours. 2.2% of ward staff and 15.7% of inpatients were found to be MRSA carriers. Conclusion CHROMagar MRSA allows for more rapid identification of MRSA carriers within 24 hours compared to the conventional BSA which takes 48 hours. This allows earlier action to be taken to reduce the spread of MRSA infection.