Genes Involved in Galactooligosaccharide Metabolism in Lactobacillus reuteri and Their Ecological Role in the Gastrointestinal Tract.

Strains of Lactobacillus reuteri are commonly used as probiotics due to their demonstrated therapeutic properties. Many strains of L. reuteri also utilize the prebiotic galactooligosaccharide (GOS), providing a basis for formulating synergistic synbiotics that could enhance growth or persistence of this organism in vivo In this study, in-frame deletion mutants were constructed to characterize the molecular basis of GOS utilization in L. reuteri ATCC PTA-6475. Results suggested that GOS transport relies on a permease encoded by lacS, while a second unidentified protein may function as a galactoside transporter. Two β-galactosidases, encoded by lacA and lacLM, sequentially degrade GOS oligosaccharides and GOS disaccharides, respectively. Inactivation of lacL and lacM resulted in impaired growth in the presence of GOS and lactose. In vitro competition experiments between the wild-type and ΔlacS ΔlacM strains revealed that the GOS-utilizing genes conferred a selective advantage in media with GOS but not glucose. GOS also provided an advantage to the wild-type strain in experiments in gnotobiotic mice but only on a purified, no sucrose diet. Differences in cell numbers between GOS-fed mice and mice that did not receive GOS were small, suggesting that carbohydrates other than GOS were sufficient to support growth. On a complex diet, the ΔlacS ΔlacM strain was outcompeted by the wild-type strain in gnotobiotic mice, suggesting that lacL and lacM are involved in the utilization of alternative dietary carbohydrates. Indeed, the growth of the mutants was impaired in raffinose and stachyose, which are common in plants, demonstrating that α-galactosides may constitute alternate substrates of the GOS pathway.IMPORTANCE This study shows that lac genes in Lactobacillus reuteri encode hydrolases and transporters that are necessary for the metabolism of GOS, as well as α-galactoside substrates. Coculture experiments with the wild-type strain and a gos mutant clearly demonstrated that GOS utilization confers a growth advantage in medium containing GOS as the sole carbohydrate source. However, the wild-type strain also outcompeted the mutant in germfree mice, suggesting that GOS genes in L. reuteri also provide a basis for utilization of other carbohydrates, including α-galactosides, ordinarily present in the diets of humans and other animals. Collectively, our work provides information on the metabolism of L. reuteri in its natural niche in the gut and may provide a basis for the development of synbiotic strategies.