Incidence and genetic variability of Listeria species from three milk processing plants

The presence of Listeria in three milk processing environments as a potential source of milk contamination was assessed. Swab samples (n = 210) taken from milk processing plants were examined. Sample sites included the milk processing equipment, besides areas handling raw and pasteurized milk. The USDA Listeria-selective enrichment procedure was used to process the samples. Forty one (19.52%) Listeria isolates were recovered. The isolates were further subjected to biochemical and genotypic characterization. Out of 41 isolates, 16 (7.62%) were confirmed as Listeria monocytogenes, 2 (0.95%) as L. ivanovii, 19 (9.05%) as L innocua. 1 (0.48%) as L. seeligeri and 3 (1.43%) as L grayi. All the L monocytogenes isolates were positive for the hlyA gene. PCR based serotyping revealed all L monocytogenes to be of 1/2a, 1/2c, 3a and 3c serovar group. Ascl and Apal restriction analysis yielded four PFGE clusters for 16 L monocytogenes isolates obtained from raw milk collector, milk silos, buttermilk mixer, cheese and other milk product processor. No predominant PFGE cluster was observed among these L monocytogenes isolates. The main sources of L monocytogenes were found to be raw milk collector and milk silos. In the present study L. monocytogenes was isolated from milk and milk products processing plants which could cross-contaminate the processed products and may possess a potential threat to public health. (C) 2011 Elsevier Ltd. All rights reserved.