Comparison of selective and nonselective primary enrichments for the detection of Listeria monocytogenes in cheese

A completely selective enrichment procedure was compared with two partially nonselective ones for the detection of Listeria monocytogenes in cheeses. After enrichment for approximately 48 h, the enrichment media were streaked on selective agars and presumptive Listeria colonies were confirmed using PCR. In some cases, PCR was also performed directly on the enrichment broth. The conventional, completely selective enrichment procedure was not always the best choice for the detection of stressed L. monocytogenes in cheeses. Especially in the case of semi-hard cheeses from pasteurized milk and soft cheeses of the blue veined and the red smear types, the methods that incorporated a nonselective enrichment step gave better results than the completely selective method. For mold ripened, soft cheeses, the results were highly dependent on the brand of cheese and time of sampling, but the best results were obtained using the completely selective enrichment procedure.