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.