STUDIES ON THE BACILLUS FLORA OF MILK AND MILK-PRODUCTS

Bacillus licheniformis and B. cereus were the most commonly isolated s pecies of Bacillus found in milk at all stages of processing. Bacillus licheniformis was ubiquitous in the farm environment and counts in ra w milks heat-treated in the laboratory were higher during the winter m onths, whilst B. cercus was associated with cattle feed throughout the year, and tended to be more common in raw milks during the summer mon ths. Although B. licheniformis was usually isolated in larger numbers than B. cereus, this pattern changed after raw and pasteurized milks a nd reconstituted milk powders were pre-incubated at ambient temperatur es, and B. cereus came to dominate the Bacillus population, reaching l evels associated with enterotoxin production. Investigation of the gro wth kinetics of strains of both species showed that B. cereus grew fas ter than B. licheniformis at ambient temperatures. It is suggested tha t post-pasteurization contamination, which is commonly blamed for spoi lage of milk and milk products by B. cereus, is not necessarily the mo st important source of this organism.