Bile salt deconjugation by Lactobacillus plantarum 80 and its implication for bacterial toxicity

The effects of bile salts on the survival of lactobacilli were investigated using glycocholic acid, cholic acid and deoxycholic acid as model compound s and the bile salt hydrolase active Lactobacillus plantarum 80 (BSH+) and its BSH negative mutant. The detrimental effects of cholic acid, i.e. growt h inhibition and cytotoxicity at a concentration of 1 and 5 mmol l(-1), res pectively, were considered to be due to the hydrophobic protonated form of the molecule, which brings about membrane damage. The conversion of glycoch olic acid to cholic acid by the BSH active L. plantarum 80 caused a growth inhibition which was comparable with the inhibition observed in the broth s upplemented with 1 mmol l(-1) cholic acid. Deoxycholic acid caused toxicity through membrane damage when the compound was in solution. Its toxicity di sappeared in the culture broth as the molecule precipitated. In case of cho lic acid, the toxicity could be removed by buffering the solution at pH 7.0 . It was calculated that at this pH most of the cholic acid molecules were ionized. The results led to the formulation of an extended hypothesis about the ecological significance of bile salt transformations. Primary deconjug ation is carried out to counteract intracellular acidification. Yet, the de conjugated molecule can be harmful at moderately acidic pi-I-values. In thi s case, the BSH+ strains could effectively profit from their activity in ca se they are associated with 7 alpha-dehydroxylating bacteria which dehydrox ylate the deconjugated bile salts. The dehydroxylated molecule has a low so lubility and precipitates at moderately acidic pH.