[Newly developed MRSA medium].

Abstract

We have developed a new selective medium, tentatively named MR(SA)2, for rapid identification of methicillin-resistant Staphylococcus aureus (MRSA) from clinical specimens. MR(SA)2 medium contained modified Müller-Hinton agar supplemented with 75 g of NaCl, 10 g of mannit, 20 mg of bromcresol purple, 20 g of egg-yolk, 4 mg of oxacillin, and 12.5 mg of ceftisoxime per 1,000 ml. There were no differences between the growth of MRSA strains on this medium at 30 C and that of 37 C. This medium can detect the egg-yolk reaction instead of the coagulase reaction. By single streaking of a test material on the surfaces of MR(SA)2 agar, MRSAs can easily be distinguished from methicillin-resistant coagulase-negative staphylococci (MR-CNS), other bacteria and fungi from their colony morphologies in a quantitative manner. A few MRSA strains would not form colonies on this medium because of their susceptibilities to ceftisoxime, but this may not inpede its use, since most MRSA strains isolated from clinical materials showed resistance to ceftisoxime. From the above results, the MR(SA)2 medium may be suitable for rapid detection of MRSA and MR-CNS.