RESUMEN
The purpose of this preliminary study was to determine the prevalence of raw milk contamination with Listeria monocytogenes. In this study, 100 bulk tank milk samples were collected randomly and delivered to Pegah Pasteurization Factory in Mashhad. For isolation and identification of L. monocytogenes, the samples were first enriched using cold enrichment method in Listeria enrichment broth, followed by plating onto supplemented Oxford agar. For final identification of suspected colonies a multiplex-PCR assay, using two pair of primers was employed. The prs primers are specific for putative phophoribosyl pyrophosphate synthetase [prs] gene of Listeria spp. and the LM lip1 primers are specific for prf A gene of its monocytogenes serovar. Using this method, the contamination of raw milk with L. monocytogenes was determined to be 4% and the sensitivity of the primers was 3.5 x 10[3] cfu ml[-1], and the specificity was determined to be 100%. Considering the high specificity and sensitivity of the employed multiplex-PCR assay, it is recommended to use this method for the identification of suspected colonies of Listeria spp. And L. monocytogenes