BopA has no

28 The ability of bifidobacteria to adhere to the intestine of human host is considered to be 29 important for efficient colonization and achieving probiotic effects. Bifidobacterium 30 bifidum strains DSM20456 and MIMBb75 adhere well to human intestinal cell lines, 31 Caco-2 and HT-29. Surface lipoprotein BopA has previously been described to be 32 involved in mediating adherence of B. bifidum to epithelial cells, but experiments with 33 thioacylated, purified BopA inhibited the adhesion of B. bifidum to epithelial cells in 34 competitive adhesion assays only in very high concentrations indicating an unspecific 35 effect. In this study, the role of BopA in the adhesion of B. bifidum was re-addressed. 36 The gene encoding BopA was cloned and expressed without its lipobox and hydrophobic 37 signal peptide in Escherichia coli, and an antiserum against the recombinant BopA was 38 produced. The antiserum was used to demonstrate the abundant localization of BopA 39 on the cell surface of B. bifidum. However, blocking of B. bifidum BopA with specific 40 antiserum did not reduce adhesion of bacteria to epithelial cell lines arguing that BopA 41 is not an adhesin. Also adhesion of B. bifidum to human colonic mucin and fibronectin 42 was found to be BopA independent. The recombinant BopA was bound only moderately 43 to human epithelial cells and colonic mucus and it failed to bind to fibronectin. Thus, 44 our results contrast the earlier findings on the major role of BopA in adhesion, 45 indicating that the strong adhesion of B. bifidum to epithelial cell lines is BopA 46 independent. 47