INTRODUCTION

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Bile acids are synthesized from cholesterol and conjugated to either glycine or taurine in the liver (20). They then pass into the intestine, where the amino acid may be hydrolyzed from the conjugated bile acid by bacterial enzymes. These enzymes constitute a class collectively known as conjugated bile salt hydrolases (BSHs). They are expressed by gastrointestinal bacteria of several genera, including Bacteroides, Clostridium, Enterococcus, Bifidobacterium, and Lactobacillus (15).

How the capacity to express BSHs contributes to the functions of bacteria in the human gastrointestinal tract has been speculated about for several years (6, 12, 19, 23); two major hypotheses have been advanced. One hypothesis states that bacteria of some species that are able to deconjugate bile salts may be able to use the amino acid taurine as an electron acceptor. Evidence which supports this hypothesis has been obtained for certain Clostridium species (12, 24). The second hypothesis states that BSHs decrease the toxicity of conjugated bile acids for bacteria (19). Compared with their conjugated counterparts, deconjugated bile acids have decreased solubility and diminished detergent activity and may, therefore, be less toxic to bacteria in the intestine (6). It has also been suggested that BSHs are detergent shock proteins that protect the bacteria that produce them from the toxicity of bile acids in the gastrointestinal tract (1, 8). We used strains of several species of the genus Lactobacillus to test the hypothesis that BSHs protect bacteria against the toxicity of conjugated bile acids. In this study we compared strains isolated from the human intestine and from dairy products.

	MATERIALS AND METHODS

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Media and chemicals. MRS (Becton Dickinson) broth and agar were used in all experiments. Sodium salts of taurodeoxycholic acid (TDCA) and taurocholic acid (TCA) were obtained from Sigma. [24-¹⁴C]TCA was obtained from New England Nuclear.

Bacterial strains. Forty-nine Lactobacillus strains isolated from the human intestine, dairy products, or other sources were obtained from culture collections in Belgium, Germany, Japan, and the United States (Table 1). They were grown anaerobically in MRS broth at 37°C. Stock cultures stored at 80°C were prepared from overnight cultures grown in MRS to which 15% glycerol was added just prior to freezing.

BSH assays. The strains were tested for TDCA hydrolase activity with an assay involving MRS agar plates supplemented with 0.5% TDCA (MRS-TDCA) (5, 18). Overnight MRS broth cultures were inoculated onto the agar medium, which was then incubated anaerobically for 5 days. BSH activity was present when deoxycholic acid precipitated in the agar medium below and around a colony. When necessary, a stereoscope was used to detect the precipitate. The strains were tested for TCA hydrolase activity with a radiochemical assay (7, 18). This assay was positive when [¹⁴C]cholic acid was detected by liquid scintillation counting in supernatant solutions from cultures to which [¹⁴C]TCA had been added.

Assay for toxicity of conjugated bile salt. The strains were tested for the capacity to resist the bactericidal activity of a conjugated bile salt (TDCA) with an assay modified from the assay described by De Smet et al. (6). A stationary-phase culture inoculum (1%) was added to MRS broth supplemented with TDCA at a concentration of 0, 1, 3, or 5 mM. At zero time and after 1, 5, and 10 h of anaerobic incubation at 37°C, dilutions of the bacterial suspensions were prepared. Aliquots of the dilutions were smeared onto MRS agar plates, which were then incubated anaerobically at 37°C for 48 h. Population estimates were made from viable counts.

Statistical analysis. Population estimates made in assays to determine the capacities of the strains to resist the toxicity of TDCA were analyzed by analysis of variance and linear regression analysis with programs posted at http://faculty.vassar.edu/~lowry/VassarStats.html.

	RESULTS

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BSH activities. Four of the 49 Lactobacillus strains tested failed to produce colonies on MRS-TDCA. Of the 45 strains that grew on this medium, 27 expressed TDCA hydrolase activity (Table 1); 21 of these 27 strains had been isolated from the human intestine, 4 were from dairy products, and 2 were from other sources. All 49 strains were tested for TCA hydrolase activity with the radiochemical assay (Table 1). Twenty-seven of the strains also exhibited this type of activity; 20 of these 27 strains had been isolated from human sources, 5 were from dairy products, and 2 were from other sources. One isolate from a human intestine, Lactobacillus buchneri JCM 1069, was able to express TDCA hydrolase activity but did not express TCA hydrolase activity. Conversely, a strain from the dairy product Kefir grains, Lactobacillus kefir BCCM 9480, was positive in the assay for TCA hydrolase activity but negative in the assay for TDCA hydrolase activity (Table 1).

Toxicity of conjugated bile salt. The 49 strains were also assayed to determine their capacities to resist the toxicity of the conjugated bile salt TDCA (Table 1). Sample data from the assays and the methods used for statistical analysis are shown in Table 2. Of the 49 isolates, 25 were resistant, 17 were susceptible, and 7 gave inconclusive (statistically ambiguous) results (Table 1). Fourteen of the 30 human isolates were resistant, compared to nine of the dairy strains. Two of the strains from sources other than human specimens and dairy products also resisted the toxicity of TDCA. Of the 26 strains that were able to express both TDCA hydrolase and TCA hydrolase activities, 15 resisted TDCA toxicity, 6 were sensitive to it, and 5 gave inconclusive results. Of the 17 strains that gave negative results in both enzymatic assays, 7 were resistant to the bactericidal activity, 9 were sensitive to it, and 1 gave a statistically ambiguous result (Table 1).

	DISCUSSION

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Strains of certain species of the genus Lactobacillus are well known to be members of the indigenous gastrointestinal microbiota of humans and other animals (21). Numerous factors are thought to be involved in the capacities of these bacteria to colonize the intestinal tract. Some of these factors are the capacity to adhere to intestinal epithelium or to colonize the mucous gel overlaying this epithelium (2, 4, 11, 14, 16), the capacity to withstand low pH (6, 9), and the capacity to express certain proteins (16, 17). Some of these proteins may have BSH activities. Conjugated bile salts are periodically released into the intestinal environment (22). These salts are known to be toxic to bacteria (6, 10, 23). Therefore, bacteria in the intestine may express BSHs to protect themselves from this toxicity (1, 8). As noted above, we examined this hypothesis in this study. Our findings do not support the hypothesis. The capacities of lactobacilli to resist the toxicity of TDCA and the capacities of lactobacilli to express TDCA hydrolase and TCA hydrolase activities appear to be independent properties. Several years ago, using technologies different from our technology, Gilliland and Speck (10) and Tannock et al. (23) reached a similar conclusion.

The conclusion described above has been disputed, however, by findings obtained in experiments conducted by De Smet et al. (6). In their experiments, these investigators used some genetically engineered, isogenic strains derived from a wild-type isolate of Lactobacillus plantarum. Using assays similar to some of the assays employed in our study, they obtained data which indicated that the capacity of L. plantarum to resist the toxicity of TDCA is related to its ability to express TDCA hydrolase activity (6). These data are consistent with our findings obtained with two isolates of L. plantarum (BCCM 18201 and BCCM 18207) but are inconsistent with our data obtained with another strain of this species (BCCM 18035) (Table 1). Moreover, they are inconsistent with our overall conclusion in this study and the conclusions of Gilliland and Speck (10) and Tannock et al. (23). Only further research may resolve the discrepancies.

A side benefit of our study was an observation made with two strains concerning specificity in the catalytic activities of BSHs (6, 13). One of the strains, L. buchneri JCM 1069, expressed TDCA hydrolase activity but not TCA hydrolase activity. Conversely, the other strain, L. kefir BCCM 9480, expressed TCA hydrolase activity but not TDCA hydrolase activity. The specificity of BSHs may be influenced either by the amino acid in the conjugate or by other side chains on the steroid moiety (10, 13). The TDCA and TCA used in our assays for BSH activities both have taurine as their amino acid moiety. They differ, however, at the 7 position of their steroid moieties. TDCA lacks a hydroxyl group at the 7 position that is present in TCA. Presumably, therefore, the hydrolase expressed by BCCM 9480 recognizes this 7 hydroxyl group, while the hydrolase expressed by JCM 1069 recognizes the 7 position lacking the group. Whatever the underlying mechanism, our observations confirm the findings of other investigators that BSHs can exhibit substrate specificity dictated by the structure of the steroid moiety of the bile salt conjugate (6).

Lactobacilli have long been used as probiotics in humans (17). A problem facing probiotic use is survival and colonization of the intestine by the bacteria after oral consumption. Probiotics must survive the low pH of the stomach and the conjugated bile acids in the duodenum (17). The introduced bacteria need to overcome major obstacles, therefore, in order to travel through the stomach and small intestine to the large bowel. In the latter region, moreover, they must compete for the resources available with the intestinal microbiota. Such obstacles may explain the finding that introduced lactobacilli disappear from human feces after oral administration has ended (16).

Producers of probiotic bacteria may be able to improve the survivability of probiotic lactobacilli by manipulating genes that encode properties which enable the bacteria to survive the obstacles in the gastrointestinal tract (3). One such property may be the capacity to express a BSH. Our findings do not support the hypothesis that the capacity to express such an enzyme is related to the capacity of lactobacilli to resist the toxicity of conjugated bile salts. They do support a hypothesis, however, that the capacity to express a BSH is on some level important for the bacteria living in the human intestine (17). A high proportion of strains isolated from the human intestine express both TDCA hydrolase and TCA hydrolase activities. Therefore, BSH activity may be important in some way for the bacteria to survive in and colonize the intestine. At this time, however, the function remains obscure. Given the potential importance of the enzymes, however, genes encoding them may be important targets for genetic manipulation (17).