Frequency of Listeria Species in Raw Milk and Traditional Dairy Products in Isfahan, Iran

Listeria monocytogenes is a psychrophilic bacterium with drought-resistant characteristics and also stable in osmotic stress; these abilities of Listeria make it grow in food stuff, leading to contamination. The aim of this study was to assess the frequency of Listeria species in raw milk and traditional unpasteurized dairy products in Isfahan, Iran. A total of 292 samples of raw milk and traditional, unpasteurized dairy products was examined for presence of Listeria spp. using a two-step selective enrichment recommended by USDA. All isolates were subjected to standard biochemical test. L. monocytogenes strains were further confirmed by PCR amplification. Of 292 samples, 21 (7.19%) and 4 (1.36%) were positive for Listeria spp. and pathogenic L. monocytogenes, respectively. The prevalence of Listeria spp. in raw milk, ice cream, cream, and freni was 5/91 (5.49%), 12/63 (19.04%), 3/27 (11.11%) and 1/25 (4%), respectively. Listeria was not detected from yogurt, butter, kashk, and cheese. Listeria innocua at 16/21 (5.44%) was the most prevalent species isolated, followed by L. monocytogenes at 4/21 (19%) and L. seeligeri at 1/21 (4.7%). All strains of L. monocytogenes identified by biochemical tests were also confirmed by PCR. The study shows the prevalence of L. monocytogenes in raw milk and traditional dairy products sold in the market. Consumption of raw milk with mild heat treatment or its usage in traditional dishes is a common practice in Iran. Therefore, the lack of appropriate control measures could pose serious health problems. The lack of knowledge of the risks of Listeriosis transmission indicates the need for implementation of a food safety education program. In addition, the Iranian food safety authorities should urgently set up an effective standard to screen all susceptible food products for presence of Listeria.