Inactivation of Bacillus cereus vegetative cells by gastric acid and bile during in vitro gastrointestinal transit

Abstract

Background: The foodborne pathogen Bacillus cereus can cause diarrhoeal food poisoning by production of enterotoxins in the small intestine. The prerequisite for diarrhoeal disease is thus survival during gastrointestinal passage.

Methods: Vegetative cells of 3 different B. cereus strains were cultivated in a real composite food matrix, lasagne verde, and their survival during subsequent simulation of gastrointestinal passage was assessed using in vitro experiments simulating transit through the human upper gastrointestinal tract (from mouth to small intestine).

Results: No survival of vegetative cells was observed, despite the high inoculum levels of 7.0 to 8.0 log CFU/g and the presence of various potentially protective food components. Significant fractions (approx. 10% of the consumed inoculum) of B. cereus vegetative cells survived gastric passage, but they were subsequently inactivated by bile exposure in weakly acidic intestinal medium (pH 5.0). In contrast, the low numbers of spores present (up to 4.0 log spores/g) showed excellent survival and remained viable spores throughout the gastrointestinal passage simulation.

Conclusion: Vegetative cells are inactivated by gastric acid and bile during gastrointestinal passage, while spores are resistant and survive. Therefore, the physiological form (vegetative cells or spores) of the B. cereus consumed determines the subsequent gastrointestinal survival and thus the infective dose, which is expected to be much lower for spores than vegetative cells. No significant differences in gastrointestinal survival ability was found among the different strains. However, considerable strain variability was observed in sporulation tendency during growth in laboratory medium and food, which has important implications for the gastrointestinal survival potential of the different B. cereus strains.

Figure 1

Inactivation of vegetative cells during the dynamic in vitro simulation of gastrointestinal passage with B. cereus ATCC 14579 (1), B. cereus NVH 1230–88 (2) and B. cereus FF 73 (3); the B. cereus total (a) and spore (b) counts were determined by plating on MYP without and with heat treatment (10 min at 80°C), respectively, in the lasagne verde (□), the mouth phase (◊), the stomach phase (Δ) and the duodenum phase (○); the average values and standard deviation of triplicate experiments on different days are presented.

Figure 2

Survival of B. cereus NVH 1230–88 in Tryptone Soya Broth (TSB) at 37°C with pH 7.2 (A) and pH 5.0 (B) with varying bile (Oxgall, Difco) concentrations: 0.0 g/L (white), 1.0 g/L (light grey), 5.0 g/L (dark grey) and 10.0 g/L (black).