CbpA: a novel surface exposed adhesin of clostridium difficile targeting human collagen

Clostridium difficile is the leading cause of antibiotic-associated diarrhea and pseudomembranous colitis. While toxins A and B are primarily responsible for the pathogenesis, determinants of bacterial adherence are also essential for intestinal colonization process. We focused our study on a novel member of the MSCRAMM family, named CbpA (Collagen binding protein A) for its adhesive properties towards these components of the extracellular matrix (ECM). We demonstrate that CbpA, which carries an LPXTG-like cell wall anchoring domain, is expressed on the bacterial surface of C. difficile and that the recombinant form binds at high affinity to collagen I and V (apparent KD in the order of 10-8 and 10-9 M respectively). Such evidences were confirmed by confocal microscopy studies showing the association of the protein with Type I and V collagen fibers produced in human fibroblasts and mouse intestinal tissues. However, the collagen binding activity of the wild-type C. difficile 630 strain was indistinguishable to cbpA knock-out strain. To overcome this apparent clostridial adherence redundancy, we engineered a Lactococcus lactis strain for the heterologous expression of CbpA. By this approach, we were able to demonstrate that when exposed on the surface of L. Lactis, CbpA significantly enhances the ability of the bacterium to interact with collagen and to adhere to ECM-producing cells. To prove the specificity of the interaction, we showed that the binding activity of L. lactis-CbpA strain was prevented by pre-incubating bacteria with an antiserum raised against CpbA. The results reported in this study suggest a role for CpbA in the adherence to intestinal submucosa during C. difficile colonization of the gut.