Abstract
Probiotic bacteria must remain viable in stomach content containing HC1 and in small bowel content containing conjugated bile acids (CBA) at toxic concentrations. Lactobacillus strains vary in their capacity to survive in HCl at concentrations approximating those in the stomach. More study is needed, however, of the mechanisms in lactobacilli and bifidobacteria of intrinsic resistance to hydrogen ion. Likewise, more study is needed of the mechanisms by which such bacteria survive in small bowel content containing CBA. One mechanism may be a capacity to express conjugated bile salt hydrolase (BSH) which catalyzes hydrolysis of the conjugating amino acid from the steroid moiety of the CBA. The steroid product of such deconjugation has physical properties rendering them less toxic to bacteria than the conjugated form. We are developing methods for testing the hypothesis that BSH activity enhances resistance in lactobacilli to the toxicity of CBA. Two peptides with BSH activity have been purified from the intracellular content of Lactobacillus johnsonii 100-100. The peptides form in the organism four trimeric isozymes. The activity of the isozymes is enhanced by an extracellular factor called BSH-EF induced in the bacterium by CBA. The gene encoding one BSH peptide and two adjacent genes encoding CBA transporters have been cloned into Escherichia coli. The transport activity of those genes is enhanced by BSH-EF. Sequences of the cloned BSH gene have been used as primers and probes in PCR and Southern blotting, respectively, for detecting BSH genes in various Lactobacillus species. Also assessed was the capacity of those strains to express BSH activity and to resist the toxicity of CBA. High BSH activity is a predictor of resistance to CBA toxicity. However, BSH activity appears to be coded by several genes of unrelated sequence. Our goal in characterizing these genes is to develop methods for enhancing resistance of lactobacilli to CBA toxicity.
