Influence of incubation temperature, ph and food substrate on production of Bacillus cereus extracellular proteins and toxins

Abstract

A total of 422 isolates of Bacillus cereus, isolated from fresh vegetables were characterized alongside a reference strain (NRRL-B14726) and conditions influencing thier extracellular protein and toxin production were evaluated. Optimal temperature and pH conditions for growth of the isolates were 28 to 37°C and pH 6 to 7, respectively. Under these conditions, permissive for growth, the isolates produced extracellular proteins, with molecular weights ranging from 14 to 150 kDa, including those consistent with the 39, 45 and 105 kDa proteins of the Non-Haemolytic Enterotoxin (NHE) complex. There was no growth or extracellular protein detected at 4°C and pH 2 and 4. All isolates grew on rice medium, but only B24 produced an 8 kDa protein band which is presumed to represent the emetic toxin. Results from this study re-establish the great diversity in morphological and metabolic characteristics of B. cereus strains and the importance of food-storage conditions for prevention of B. cereus foodborne illnesses. Furthermore, it shows that food substrate may be an important determinant of toxin production.   Key words: Bacillus cereus, food poisoning, diarrhoeal toxin, emetic toxin, vegetables.