The presence of Listeria monocytogenesin raw milk samples in Mashhad, Iran

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 Listeriaenrichment 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 prsprimers are specific for putative phophoribosyl pyrophosphate synthetase (prs) gene of  Listeriaspp. and the LM lip1primers are specific for prfA gene of its monocytogenesserovar. Using this method, the contamination of raw milk with L. monocytogeneswas determined to be 4% and the sensitivity of the primers was 3.5 × 103 cfu ml -1, and the specificity was determined to be 100%. Considering the high specificity and sensitivityof the employed multiplex-PCR assay, it is recommended to use this method for the identification of suspected colonies of Listeriaspp. and L. monocytogenes.