The bactericidal effect of bile on vegetative B. cereus cells is pH dependent [54]. At a neutral pH, the survival of B. cereus was dependent on the bile concentration in Tryptone Soya Broth. A
growth of these microorganisms was observed with a bile concentration of 1.0 g/L. However, in conditions with the same bile concentration but at pH 5.0, approximately 99% of the B. cereus
vegetative cells were instantly inactivated by the exposure to bile.The effect of bile salts on microorganisms can be compared to the antimicrobial activity of organic acids: both are more effective at a low pH. The acid dissociation constant (pK a ) of unconjugated bile acids is approximately 5.1, whereas conjugation with taurine leads to a pK a of approximately 1.5 and conjugation with glycine leads to a pK a of approximately 3.7 [55,56]. Consequently, the majority of conjugated bile acids are dissociated at most physiological pH values in the intestine (pH~6.5), i. e., 95% at pH 5.0 and 99% at pH 7.0. Dissociated forms of bile acids require active transport systems to cross the cell membrane of bacteria [57]. As a result, more conjugated bile acids can enterthe cells at pH ~ 5.0 than at a neutral pH.Furthermore, once the undissociated bile acids have entered the cells, the majority of them undergo dissociation, since the internal pH of vegetative B. cereus cells ranges between 6.0 and 7.5. This causes a rapid decrease of the internal pH of the cells, which eventually leads to their death; in other words, lethal internal acidification of the cells takes place