Seasonal occurrence of psychrotrophic Bacillus species in raw milk, and studies on the interactions with mesophilic Bacillus sp.

Mesophilic and psychrotrophic isolates of Bacillus species displayed seasonal incidences in raw and pasteurised milk. The incidence of mesophilic isolates was highest in the winter and lowest in the summer/autumn while pschrotroph incidence was conversely lowest in the winter and highest in the late summer/autumn. Spores of Bacillus sp. were isolated from raw milk taken from farm milk machines and bulk tanks, milk tankers, diary silos and pasteurised milk. A consistent seasonal fluctuation in incidence throughout these samples suggested that spores of Bacillus sp. derived from the farm environment survived as important contaminants right through the milk chain to the pasteurised product. Up to seven mesophilic Bacillus sp. were isolated from a single sample with three species commonly occurring in most samples. The predominant mesophilic species isolated were B. pumilus, B. licheniformis and B. subtilis. The dominant psychrotrophic isolate was B. cereus. Selected mesophilic isolates were examined for possible antagonistic effects on the growth of psychrotropic B. cereus and B. pumilus isolates. Bacillus subtilis and B. licheniformis were found to produce antagonistic factors. It was considered that these factors may influence the incidence and growth of psychrotrophic isolates in the farm environment or in milk but the factors are not yet fully characterised or identified.