Effects of porcine bile on survival of Bacillus cereus vegetative cells and Haemolysin BL enterotoxin production in reconstituted human small intestine media

摘要

To determine the effects of porcine bile (PB) on Bacillus cereus vegetative cells and Haemolysin BL (HBL) enterotoxin production in reconstituted small intestine media (IM). The effects of PB on the growth of B. cereus vegetative cells in reconstituted IM at PB concentrations ranging between 0 and 3p"0 g lp# were examined. Four gastric media (GM) named GM-J broth (JB), GM-chicken, GM-milk and GM-pea were prepared by mixing equal volumes of a gastric electrolyte solution containing pepsin with JB, chicken, semi-skimmed milk and pea soup, respectively. Bacillus cereus was inoculated at approx. 2 x 10t CFU mlp# into each GM at pH 5p"0 for 30 min at 37pC, then mixed to the same volume of double-strength JB (IM) and PB to give concentrations of between 0 and 3p"0 g of PB per litre at pH 6p"5 and incubated at 37pC. The diarrhoeal B. cereus strain F4430/73 grew in IM-JB, IM-chicken and IM-milk at PB concentrations of up to 0p"6, 1p"5 and 1p"2 g lp#, respectively. Growth was observed in IM-pea at all concentrations tested. The highest PB concentrations allowing a 3 log B. cereus increase in IM-JB, IM-chicken, IM-milk and IM-pea after a 7-10 h incubation period were 0p"3, 0p"9, 0p"9 and 3p"0 g lp#, respectively. The effect of PB on B. cereus cells was strongest in IM-JB, followed by IM-chicken, IM-milk and IM-pea. Haemolysin BL enterotoxin was detectable in IM-chicken, IM-whole milk, IM-semi-skimmed milk and IM-pea up to PB concentrations of only 0p"6, 0p"6, 0p"3 and 0p"9 g lp#, respectively. The diarrhoeal B. cereus strain F4433/73 behaved similarly to B. cereus strain F4430/73, whereas the food strain TZ415 was markedly more susceptible to bile. The tolerance of B. cereus cells to PB strongly depends on the type of food contained in the IM. Bile tolerance is also subject to strain variation. The probability that B. cereus cells will grow in the small intestine, produce toxins and cause diarrhoea is likely to depend on the food they are ingested with, on the bile tolerance of the B. cereus strain, and on bile concentration.