S-Fimbriae from Escherichia coli bind to soluble glycoproteins from human milk

Background: Escherichia coli (E. coli) strains, expressing S-fimbriae, belong to the most common gram-negative pathogens that cause sepsis and meningitis in neonates. The attachment of S-fimbriae to the cell surface is mediated by membrane glycoconjugates, which often carry N-acetylneuraminic acid. Methods: Binding studies were performed with glycoproteins from the whey fraction of human milk to investigate whether they exert a potential inhibitory effect on bacterial adhesion. Whey glycoproteins were separated according to their molecular weight by fast protein liquid chromatography gel filtration. After sodium dodecyl sulfate-polyacrylamide gel electrophoresis, proteins were transferred to nitrocellulose membranes and incubated with isolated S-fimbriae from recombinant E. coli strain HB 101 (pANN 801-4). Results: S-fimbriae recognized four whey proteins with a molecular mass of more than 200 kDa, 170 to 150 kDa, and 80 kDa. Their glycosylation pattern was investigated using the lectins Sambucus nigra, Maackia amurensis, Galanthus nivalis, and Arachis hypogaea. Thus the presence of N- and O-glycans in these proteins was confirmed. The preferential binding to N-acetylneuraminic acid containing glycoproteins was demonstrated by a complete abolishment of these reactions by incubation with acidic lactose-derived oligosaccharides. However, the cleavage of N-acetylneuraminic acid from glycoproteins by mild acid hydrolysis revealed a second binding site for S-fimbriae on milk proteins of a similar molecular weight range. Terminal galactose in human milk glycoconjugates were thought to react with S-fimbriae as well. Conclusion: These data further support the opinion that glycoproteins from human milk are potential receptor analogues for certain bacteria that may prevent microbial adhesion to the epithelial cell surface.