Bacillus cereus emetic toxin production in relation to dissolved oxygen tension and sporulation

Abstract

Seven emetic toxin-producing strains of Bacillus cereus were examined for growth, sporulation and toxin production in skim milk medium in shaken and unshaken batch cultures under aerobic, microaerobic and anaerobic atmospheres. High levels of toxin production were detected in aerobic and microaerobic cultures. Static cultures yielded 90% less toxin than equivalent shaken cultures. Emetic toxin production was detected in aerobic and microaerobic cultures when bacterial counts reached > 6·0 log 10 cfu ml −1, which also coincided with spore production. However, no correlation was found between spore count and toxin production (R 2 = 0·032). In anaerobic culture, production of emetic toxin was undetectable and the dissolved oxygen tension in the growth medium fell below 125±46 ppm within 6 h of inoculation. In aerobic culture, most toxin was produced between 16 and 22 h of incubation, when the dissolved oxygen tension had decreased to 60−70 ppm. Therefore, while O 2-dependent respiration is fundamentally required for production of the emetic toxin, most toxin production occurs when little free oxygen is available.