Determining the source of Bacillus cereus and Bacillus licheniformis isolated from raw milk, pasteurized milk and yoghurt

Abstract

Aims: Strain-specific detection of Bacillus cereus and Bacillus licheniformis in raw and pasteurized milk, and yoghurt during processing.

Methods and results: Randomly selected isolates of Bacillus spp. were subjected to PCR analysis, where single primer targeting to the repetitive sequence Box elements was used to fingerprint the species. The isolates were separated into six different fingerprint patterns. The results show that isolates clustered together at about the 57% similarity level with two main groups at the 82% and 83% similarity levels, respectively. Contamination with identical strains both of B. cereus and B. licheniformis in raw and pasteurized milk was found as well as contaminated with different strains (in the case of raw milk and yoghurt/pasteurized milk and yoghurt). Several BOX types traced in processed milk samples were not discovered in the original raw milk.

Conclusions: BOX-PCR fingerprinting is useful for characterizing Bacillus populations in a dairy environment. It can be used to confirm environmental contamination, eventually clonal transfer of Bacillus strains during the technological processing of milk.

Significance and impact of the study: Despite the limited number of strains analysed, the two Bacillus species yielded adequately detectable banding profiles, permitting differentiation of bacteria at the strain level and showing their diversity throughout dairy processing.