Environmental Sources of Listeria and Yersinia in Vermont Dairy Plants.

This survey was conducted to identify specific environmental sources of Listeria and Yersinia in Vermont dairy plants, and to further determine whether the type of plant and specific conditions existing within plants influenced the incidence of positive microbiological results. A total of 361 environmental samples, focusing on floors and other nonproduct contact surfaces, was taken from all of Vermont's 34 dairy processing plants. The incidence of Listeria monocytogenes (1.4%) was low compared to the incidence of Listeria innocua (16.1%). While only 2.5% yielded other Yersinia species, 10.5 % of the sites were positive for Yersinia enterocolitica . Sites positive for either Listeria or Yersinia were statistically more likely to produce a positive result for both (P<.05). Fluid plants had the highest incidence of both Listeria and Yersinia when compared to cheese plants or other types of dairy manufacturing plants. Areas associated with case washers in fluid plants had the highest incidence of microbial contamination. An additional area of concern for all types of plants was sanitizing floor mats and foot baths from which positive microbiological results were obtained. Contamination in wet areas was significantly greater than in dry areas of the plants (P<.05). Identification of the sources and conditions associated with these problematic bacterial pathogens is an important step in learning to control their incidence in dairy processing environments.