Functional characterization of a mucus-specific LPXTG surface adhesin from probiotic Lactobacillus rhamnosus GG

In spite of the wealth of clinical evidence supporting the health benefits of Lactobacillus rhamnosus GG in humans, there is still a lack of understanding of the molecular mechanisms behind its probiosis. Current knowledge suggests that the health-promoting effects of this probiotic strain might be partly dependent on its persistence in the intestine and adhesion to mucosal surfaces. Moreover, L. rhamnosus GG contains mucus-binding pili that might also explain the occupation of its ecological niche as a comparatively less stringent allochthonous intestine-dwelling bacterium. To uncover additional surface proteins involved in mucosal adhesion, we investigated the adherence properties of the only predicted protein (LGG_02337) in L. rhamnosus GG that exhibits homology with a known mucus-binding domain. We cloned a recombinant form of the gene for this putative mucus adhesin and established that the purified protein readily adheres to human intestinal mucus. We also showed that this mucus adhesin is visibly distributed throughout the cell surface and participates in the adhesive interaction between L. rhamnosus GG and mucus, although less prominently than the mucus-binding pili in this strain. Based on primary structural comparisons, we concluded that the current annotation of the LGG_02337 protein likely does not accurately reflect its predicted properties, and we propose that this mucus-specific adhesin be called the mucus-binding factor (MBF). Finally, we interpret our results to mean that L. rhamnosus GG MBF, as an active mucus-specific surface adhesin with a presumed ancillary involvement in pilus-mediated mucosal adhesion, plays a part in the adherent mechanisms during intestinal colonization by this probiotic.