Toxin production by Bacillus cereus dairy isolates in milk at low temperatures

Abstract

A total of 136 strains of Bacillus cereus isolated from milk and cream were evaluated for toxin production based on HeLa S3, Vero, and human embryonic lung (HEL) cell cytotoxicity in vitro. HEL cell monolayers were more susceptible than the other two cell lines. The percentage of isolates exhibiting HEL cytotoxicity was similar (43.0 and 48.4%) when the strains were grown in brain heart infusion broth containing 0.1% glucose (BHIG) at 7 and 24 h, respectively, at 30 degrees C. In milk, only 21.8% of isolates showed HEL cytotoxicity at 7 h, and the number increased significantly to 73.2% at 24 h at 30 degrees C. Further, 102 toxin-positive isolates were acclimatized to grow at 8 degrees C in milk. Ninety-four (92.2%) of the strains produced HEL cytotoxicity of various degrees with no strict correlation to bacterial cell numbers and also elicited vascular permeability reaction in rabbit skin. Under aerated growth conditions (agitation, 200 rpm) B. cereus elicited cytotoxicity in BHIG and in milk at temperatures of 30, 15, and 8 degrees C. However, in nonaerated (stagnant) cultures toxin production was diminished (BHIG) or completely lost (milk) at all temperatures. Toxin production at 8 degrees C was evaluated in two different types of commercial cardboard milk packages by inoculation with a potent toxigenic dairy isolate. No detectable HEL cytotoxicity was observed in milk in any of the packages either at stagnant conditions or during mechanical shaking. However, the same strain produced cytotoxin in whipped cream at 8 degrees C.