Mucosal adhesion properties of the probiotic Lactobacillus rhamnosus GG SpaCBA and SpaFED pilin subunits

Lactobacillus rhamnosus GG is a well-established Gram-positive probiotic strain, whose health-benefiting properties are dependent in part on prolonged residence in the gastrointestinal tract and are likely dictated by adherence to the intestinal mucosa. Previously, we identified two pilus gene clusters (spaCBA and spaFED) in the genome of this probiotic bacterium, each of which contained the predicted genes for three pilin subunits and a single sortase. We also confirmed the presence of SpaCBA pili on the cell surface and attributed an intestinal mucus-binding capacity to one of the pilin subunits (SpaC). Here, we report cloning of the remaining pilin genes (spaA, spaB, spaD, spaE, and spaF) in Escherichia coli, production and purification of the recombinant proteins, and assessment of the adherence of these proteins to human intestinal mucus. Our findings indicate that the SpaB and SpaF pilin subunits also exhibit substantial binding to mucus, which can be inhibited competitively in a dose-related manner. Moreover, the binding between the SpaB pilin subunit and the mucosal substrate appears to operate through electrostatic contacts and is not related to a recognized mucus-binding domain. We conclude from these results that it is conceivable that two pilin subunits (SpaB and SpaC) in the SpaCBA pilus fiber play a role in binding to intestinal mucus, but for the uncharacterized and putative SpaFED pilus fiber only a single pilin subunit (SpaF) is potentially responsible for adhesion to mucus.