Evaluation of isolation procedures and chromogenic agar media for the detection of MRSA in nasal swabs from pigs and veal calves.

Since the emergence of MRSA in livestock, screening of animals for the detection of MRSA is widely practised. Different procedures are published for animal samples but a systematic comparison of methods has not been performed. The objective of this study was to compare three available commonly used procedures and three chromogenic agars for detecting MRSA in nasal swabs from pigs (n=70) and veal calves (n=100). Procedures 1 and 2 used a pre-enrichment comprising Mueller Hinton broth with 6.5% NaCl followed by selective enrichment with 4 microg/ml oxacillin+75 microg/ml aztreonam (procedure 1) and 5 microg/ml ceftizoxime+75 microg/ml aztreonam (procedure 2) respectively. Procedure 3 used a selective enrichment broth only, containing 4% NaCl, 5 microg/ml ceftizoxime+50 microg/ml aztreonam. After selective enrichment, media were streaked on to three different chromogenic agars. Significantly more MRSA were found for pig as well as for veal calf samples with procedures 1 and 2. No significant differences were found between procedures 1 and 2. For nasal swabs from pigs significantly more MRSA-positive samples were found when MRSA Screen (Oxoid) or MRSASelect (Bio-Rad) agars were used compared to MSRA ID (bioMérieux). For calf samples no significant differences between the different agars were found. In conclusion, the results of this study show that procedures 1 and 2, both using additional high salt pre-enrichment are superior and should be recommended for MRSA detection in nasal swabs from pigs and veal calves. The preferred choice of chromogenic agar depends on the sample matrix.