ABSTRACTDairy farms are a reservoir forListeria monocytogenes, and the reduction of this pathogen at the farm level is important for reducing human exposure. The objectives of this research were to study the diversity ofL. monocytogenesstrains on a single dairy farm, assess strain dynamics within the farm, identify potential sources ofL. monocytogenesin bulk tank milk and milk filters, and assess the adherence abilities of representative strains. A total of 248L. monocytogenesisolates were analyzed by pulsed-field gel electrophoresis (PFGE). Combined AscI and ApaI restriction analysis yielded 40 PFGE types (strains). The most predominant strains were T (28.6%), D (22.6%), and F (14.9%). A high level of heterogeneity of strains among isolates from fecal (Simpson's index of diversity [SID] = 0.96) and environmental (SID = 0.96) samples was observed. A higher homogeneity of strains was observed among isolates from milk filters (SID = 0.71) and bulk tank milk (SID = 0.65). Six of 17L. monocytogenesisolates (35.3%) were classified in anin vitroassay as having a “low adherence ability,” 9 (52.9%) were classified as having a “medium adherence ability,” and 2 (11.8%) were classified as having a “high adherence ability.” TheL. monocytogenesstrains that were predominant and persistent showed significantly better adherence than did strains that were only sporadic, predominant, or persistent (P= 0.0006). Our results suggest that the milking system was exposed to severalL. monocytogenesstrains from different sources. Only 3 strains, however, were successful in persisting within the milking system, suggesting that some strains are more suitable to that particular ecological environment than others.
Identification of dairy farm management practices associated with the presence of psychrotolerant sporeformers in bulk tank milk
