Performance of MRSA ID chromogenic medium for detection of methicillin-resistant Staphylococcus aureus directly from blood cultures and clinical specimens

Abstract number: 1733_1099

Colakoglu S., Uncu H., Senger S., Turunc T., Demiroglu Y., Arslan H.

Objectives: Rapid identification of methicillin-resistant Staphylococcus aureus (MRSA) in clinical samples are an important part of the infection control measures taken to control the spread of MRSA. The purpose of this study is to evaluate the performance of MRSA ID chromogenic medium compared with nonspecific media routinely used for the detection of MRSA directly from blood cultures and clinical specimens.

Methods: We analysed 774 blood cultures from 471 patients and 94 clinical samples (wounds or abscesses) from 71 patients. Each positive blood culture bottle (Bactec Plus/F;BD) and clinical sample were directly inoculated to primary culture media and MRSA ID (bioMérieux, France). All cultures were incubated in aerobic condition at 370C for 24–48 h. Suspect colonies were identified as MRSA based on positive reaction on the tube coagulase test with rabbit plasma, the detection of DNase and growth on Mueller-Hinton (MH) oxacillin agar (6 mg of oxacillin/mL, according to CLSI). Inoculated MRSA ID plates were interperated in accordance with the manufacturer' s instructions. Growth of colonies showing distinctive green coloration was considered to be positive. No growth or colonies with colours other than green were considered negative. Discordant results were confirmed by mecA gene PCR.

Results: The results obtained with MRSA ID are summarised in Tables 1 and 2. Three methicillin-susceptible S. aureus (MSSA) isolates gave false-positive results on MRSA ID and these strains gave negative result with the mecA PCR.

Table 1. Results for MRSA ID medium after 24 h and 48 h of incubation

Table 2. Strains producing green colonies which have a different appearance from MRSA colonies

Conclusions: (1) MRSA ID is highly effective for the isolation and presumptive identification of MRSA directly from wound samples and blood cultures. (2) The use of MRSA ID with primary culture media should decrease the time (18–24 h) to reporting positive results compared with conventional methods.