Proteinaceous factor(s) in culture supernatant fluids of bifidobacteria which prevents the binding of enterotoxigenic Escherichia coli to gangliotetraosylceramide

We have examined the competitive binding of several species of Bifidobacterium and Escherichia coli Pb176, an enterotoxigenic E. coli (ETEC) strain, to gangliotetraosylceramide (asialo GM1 or GA1), a common bacterium-binding structure, and identified a factor(s) in the Bifidobacterium culture supernatant fluid that inhibits the binding of E. coli Pb176 to GA1. The ETEC strain we used expresses colonization factor antigen (CFA) II, which consists of coli surface-associated antigens CS1 and CS3. Competitive exclusion of ETEC from GA1 molecules by Bifidobacterium cells was found by an in vitro thin-layer chromatography overlay binding suppression assay. However, the ETEC cells were less effective in blocking the adherence of Bifidobacterium cells to GA1. These findings suggest that the two bacterial species recognize different binding sites on the GA1 molecule and that the mechanism of competitive exclusion is not due to specific blockage of a common binding site on the molecule. The neutralized culture supernatant fluids of Bifidobacterium species, including that of Bifidobacterium longum SBT 2928 (BL2928), showed remarkable inhibition of the ETEC binding to GA1. Our results suggest that the binding inhibitor produced by BL2928 is a proteinaceous molecule(s) with a molecular weight around or over 100,000 and a neutral isoelectric point. The binding inhibitor produced by BL2928 and other Bifidobacterium species is estimated to contribute to their normal anti-infectious activities by preventing the binding of pathogenic strains of E. coli to GA1 on the surface of the human intestinal mucosa.