Listeria isolated from traditional cheeses of Tabriz area: Occurrence, diversity and phenotypic characteristics

Contamination of milk and its products with pathogenic organisms such as Listeria could be considered as a health threat for the consumers. The aim of this study was to investigate the occurrence and diversity of Listeia spp. in traditional cheeses of Tabriz area and to assess antibiotic resistance, biofilm formation and thermal-resistance of the isolates. A total number of 100 traditional cheese was collected randomly. Listeria spp. were isolated and identified using conventional culture methods. According to the results, 12 out of 100 cheese samples were found positive for Listeria spp. Amongst, 5 and 7 isolates were identified as monocytogens and ivanovii, respectively. The highest rate of resistance was observed for oxacillin (100%) and nalidixic acid (75%), and the highest rate (100%) of sensitivity was related to cotrimoxazole, tetracycline, gentamicin, erythromycin and vancomycin. Results of biofilm assay reveled that all 5 strains of L. monocytogens produced low amount of biofilm, meanwhile strains of L. ivanovii produced a range (weak, moderate and high) of biofilm level. Based on thermal-resistance outcomes, thermalisation process had insignificant impact on Listeria strains; whereas application of “rapid pasteurization” process for 5 sec, reduced the populations of both Listeia specis to below the detectable limit (P