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Screening Cultures for MRSA

Surveillance is a critical component of any program for controlling multi-drug resistant organisms (MDROs). Many institutions are using active surveillance cultures to identify patients who are colonized with a targeted MDRO. With respect to MRSA, an increasing number of hospitals are screening patients upon admission and on a periodic basis (usually weekly). The anterior nares is the primary site that is swabbed for screening.

There are several selective and/or differential media that can be used for this purpose.

Baird Parker Agar is a selective medium for the isolation of S. aureus; on this medium S. aureus produces black colonies with a clear halo.

Mannitol Salt Agar is also a selective medium; S. aureus produces yellow colonies which contrast with the red color of the medium.

Chromogenic agars have been developed for the isolation and presumptive identification of different species of bacteria and yeast. The media are formulated so that as different organisms utilize various substrates in the media, the organism of interest produce colonies with a unique color. Chromogenic agars specifically designed for the detection of MRSA are commercially available.

In addition to culture methods, there are now commercially available, FDA approved methodologies for screening for MRSA by PCR. Although equipment and cost factors may not make these a viable option for every laboratory, they may offer greater sensitivity and improved turnaround times.