Behavior of Bacillus cereus under conditions simulating the proximal gut

Introduction: B. cereus is a foodborne pathogen that can cause diarrhea through production of pore forming enterotoxins in the ileum of the host. The mechanism of the toxico-infection is not evident, because B. cereus vegetative cells cannot survive gastric passage and the resistant spores cannot easily germinate and proliferate in the unfavorable intestinal biotic and abiotic environment. Even when growth occurs, the involved toxins are susceptible to the proteolytic enzymes secreted by the host or the competitive microbiota.Purpose: We hypothesized that adhesion of B. cereus on the ileal mucosa may protect cells from the toxic effect of the intestinal slurry and preserve or enhance the activity of enterotoxins due to the direct and close contact of B. cereus with the eukaryotic site.Methods: We have used an in vitro system to mimic the proximal gut conditions in order to investigate the behavior of B. cereus NVH 0500/00 in the stomach and small intestine. The total incubation period lasted 8 hours.Results: A stepwise pH gradient from 5 to 2 for 2 hours (1 pH unit reduction per half an hour) representing the stomach resulted in a 2.5 log decrease in B. cereus concentration due to the death of most vegetative cells. pH values lower than 3 were those that severely influenced the resistance of the strain in the simulated stomach. The upper small intestinal phase (duodenum/jejunum) that started with the addition of bile and pancreatine (intestinal juice) at neutral pH initially did not have an effect on the bacterial counts probably because the spores that survived the acid stress were also resistant to these chemicals. After 2 hours, luminal bacteria started proliferating and completely recovered reaching the initial inoculum size within the next 4 hours of incubation at pH 7 (ileal lumen). The presence of mucin beads (ileal mucus) during this incubation stage did not influence the number of suspended cells but resulted in an overall 10% increase in the total B. cereus population due to the presence of adhered cells. Surprisingly, no enterotoxins could be detected in the lumen during the last ileal phase.Significance: With this setup, it was not possible to offer any further insight in the mechanisms that rules B. cereus diarrhea. For this purpose, further optimization of the system is required involving the increase of the mucin to lumen ratio and the investigation of the possibility that enterotoxins produced by B. cereus adhered cells are actually trapped in the mucin mesh preventing potential denaturation.

Jaar van publicatie:2013