Seasonal variation in spore levels of Bacillus cereus and its psychrotrophic strains in raw milk in Hokkaido, Japan, and evaluation of strain diversity

Abstract

Bacillus cereus is a spore-forming bacterium that adversely affects the quality of high-temperature short-time (HTST) pasteurised milk. Using a membrane filtration method, we investigated B. cereus spore levels in raw milk from dairy plants in two different regions of Hokkaido, Japan, over 1 year. B. cereus spore levels were significantly higher in autumn (median, 25 spores L−1) in the Tokachi region and in summer (median, 38 spores L−1) in the Kushiro region than in the other seasons, whereas the bacterium's psychrotrophic spore levels showed no seasonal variation. Multiple-locus variable-number tandem repeat analysis (MLVA) showed that B. cereus in raw milk had high strain diversity; MLVA groups consisting of a single strain accounted for 76.6% of the total number of MLVA groups in Tokachi and 68.5% in Kushiro. Three MLVA groups were detected in both regions in three seasons or more; these MLVA groups are therefore widely distributed in Hokkaido.