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The aim of this study was to characterize the Escherichia coli K88-specific receptors in mucus from the small intestines of 35-day-old piglets with the isogenic strains E. coli K-12(pMK005) (K88+) and E. coli K-12(pMK002) (K88-). These strains differed only in that the latter one cannot produce intact K88 fimbriae because of a deletion in the gene coding for the major fimbrial subunit. Adhesion was studied by incubating 3H-labeled bacteria with crude mucus, pronase-treated whole mucus, mucus fractionated by gel filtration, delipidated mucus, or extracted lipids immobilized in microtiter wells. In addition, E. coli strains were tested for adhesion to glycolipids extracted from mucus by overlaying glycolipid chromatograms with 125I-labeled bacteria. The recently reported finding that K88 fimbriae bind to glycoproteins in mucus from the piglet small intestine was confirmed in two ways. Pronase treatment of immobilized mucus reduced adhesion by 82%, and adhesion to delipidated mucus was 14 times greater for the K88+ than for the K88- strain. E. coli K88+ adhered to several of the fractions collected after gel filtration of crude mucus, including the void volume (Mr, > 250,000). Receptor activity specific for the K88 fimbriae was demonstrated in the lipids extracted from mucus, as the neutral lipids contained six times as much receptor activity as the acidic lipid fraction. Specificity was confirmed by demonstrating that adhesion to the total lipids could be inhibited by pretreatment of the immobilized lipids with K88 fimbriae. Relative to K-12 (K88-), the K-12 (K88+) bacterial cells bound more avidly to galactosylceramide when the neutral lipids were separated on thin-layer chromatography plates. No adhesion to lipids in the acidic fraction separated on thin-layer plates was detected. Relative to adhesion of K-12 (K88-), adhesion of K-12 (K88+) to commercially available galactosylceramide immobilized in microtiter wells confirmed the results with the thin-layer plates. It can be concluded that 35-day-old piglet mucus contains both protein and glycolipid receptors specific for K88 fimbriae, the latter being galactosylceramide.