Plasmid-encoded glycosyltransferase operon is responsible for exopolysaccharide production, cell aggregation, and bile resistance in a probiotic strain, Lactobacillus brevis KB290.

We demonstrate here that exopolysaccharide (EPS) production, cell aggregation, and bile resistance in Lactobacillus brevis KB290 are conferred by three eps genes (gtf27, gtf28, and orf29) located on the 42.4-kb plasmid pKB290-1. The predicted products of gtf27 and gtf28 belong to the membrane-bound glycosyltransferase family whereas the orf29 gene product showed homology with the ABC transporter. On in silico analysis, these genes were found to be widely distributed among lactobacilli from publicly available genomes and metagenomes, and their function is not yet elucidated. RT-PCR analysis showed that the eps genes were organised in an operon and their expression was markedly lower in arabinose- and xylose-containing media than in a glucose-containing medium. The three eps genes were cloned and expressed in homologous and heterologous strains. Considerably less EPS was produced by the plasmid-cured KB1802 strain than by the parental KB290 strain, whereas a similar amount was produced by the KB1802 strain expressing the three eps genes. The KB1802 strain expressing gtf27 and gtf28 but not orf29 did not produce EPS. Cell aggregation and bile resistance were also decreased in KB1802 strains but were complemented by eps genes. Moreover, the three eps genes conferred these phenotypes to a Lactobacillus plantarum strain. In conclusion, the three eps genes in pKB290-1 were sufficient for EPS biosynthesis with glucose and N-acetylglucosamine, and were responsible for cell aggregation and bile resistance. We consider these phenotypes to be at least partly responsible for KB290-specific properties.