Production of diarrheal toxin by selected strains of Bacillus cereus

The production of diarrheal toxin by six selected strains of Bacillus cereus was monitored during growth at 32 °C, a temperature described as near-optimal for growth and toxin production. Toxic activity was measured in culture filtrates and cellular extracts sampled at three different times during growth. Two alternative methods, a cytotoxicity test on Chinese hamster ovary (CHO) cells and a commercial immunological test (BCET-RPLA, Oxoid) were used. Toxin titres were in agreement with epidemiological characteristics and toxicity demonstrated by using other systems in other examinations. A comparison of intra- and extracellular toxicities measured at the exponential and stationary growth phases showed that the toxin was essentially secreted during the exponential phase. For several strains, secretion peaked during the period from the middle exponential phase to the beginning of the stationary phase. There was no important overall increase of the toxicity during full and late stationary phase. The level was stable or even lower, thus indicating that diarrheal toxin production during stationary phase was small, if any, and that the toxin was unstable under these conditions. Statistical analysis of toxicities showed that the cytotoxicity test was correlated with the immunological test (significant at a 1% level). For routine determinations, a toxicologic laboratory may use any of the two methods, depending oft its facilities, the immunological test being relatively expensive.