Characterisation of Bacillus cereus isolated from milk silo tanks at eight different dairy plants

Raw milk was collected over I year from silo tanks at eight different dairy plants. On the average the level of spores and the percentage of psychrotrophic isolates was higher in summer milk (median 198 spores L-1; 48% psychrotrophs) than in winter milk (median 86 spores L-1; 35% psychrotrophs). Systematic differences in the average level of psychrotrophic spores were found between some dairies. There was an over all high diversity in the Bacillus cereus flora but the percentage of unique RAPD-patterns (fingerprints) among isolates differed between the dairies (34-54%). For all dairies mesophilic isolates with identical RAPD-patterns from up to 6 sampling occasions were found. An indication of a mesophilic in house flora was found at the dairy with 34% unique RAPD-patterns and the lowest percentage of psychrotrophic isolates (27%). In some dairies clusters of RAPD-patterns from psychrotrophic isolates were found. Eight of the RAPD-patterns were found among isolates from several of the investigated dairies indicating that some strains of B. cereus are widely spread and that certain strains may be selected for in the silo tank environment. (C) 2003 Elsevier Ltd. All rights reserved.