Comparison of chromogenic agar in the detection of MRSA

Abstract number: 1733_1098

Curry A., Walsh M., Matheson M., O'Reilly A., Knowles S.

To compare two different chromogenic agars for the isolation of MRSA in order to find the most appropriate agar for use in our laboratory.

Nasal, umbilical and groin screening swabs from each neonate in the NICU were pooled together. Each screen was put in Brain Heart Infusion enrichment broth, which was incubated for 24 hours aerobically at 37°C and subsequently subcultured onto both MRSA ID agar and MRSA Select agar. Both agars were incubated for 48 hours aerobically at 37°C and checked for the presence of green (MRSA ID) or pink (MRSA Select) colonies at 24 and 48 hours. All green or pink colonies had confirmatory tests for MRSA.

In total 300 screens were tested on both agars. The same 10 screens were identified MRSA positive on both agars after 24 hours. Incubation for 48 hours did not yield any additional MRSA isolates. Therefore both agars were comparable in sensitivity and specificity for MRSA identification. Other factors influenced our decision to choose a chromogenic agar for our laboratory. MRSA ID agar grew as large green colonies on a clear agar and was considered easy to read. MRSA Select grew as small pink colonies on an opaque agar, which were easy to read in large numbers. However they could easily be missed when a scanty growth was present under our laboratory lighting conditions. Also on both agars, Enterococci grew as a green or pink fine growth after 24 hours and it distorted white colonies that grew coagulase negative Staphylococci (CoNS) giving them a green or pink hue. On MRSA ID agar it was easy to distinguish between true MRSA and CoNS that appeared green due to the presence of Enterococci, however on MRSA Select, as CoNS were also small colonies, the pink from the Enterococci distorted the CoNS, giving them the appearance of MRSA and resulted in them having to be confirmed as CoNS. Cefoxitin resistant Gram-negative bacilli also grew on both agars after 48 hours incubation and in some cases a wet prep was required to exclude them as possible MRSA. One strain of methicillin sensitive Staphylococcus aureus also grew on both agars after 48 hours incubation.

In conclusion, both agars were comparable in sensitivity and specificity for identifying MRSA. MRSA ID agar was chosen as the preferred agar in our laboratory due to the ease in distinguishing true MRSA from CoNS affected by the presence of Enterococci and also the ease of reading the agar under our laboratory conditions.