PLATES XXVIII AND XXIX NEONATAL diarrhoea in piglets follows the proliferation of certain strains of Escherichia coli in the small intestine (Smith and Jones, 1963). These enteropathogenic strains synthesise enterotoxins which produce diarrhoea and dehydration that frequently result in the death of the piglets (Kohler, 1968;Smith and Gyles, 1970;Smith and Linggood, 1971). The rapid proliferation of E. coli in the small intestine appears to be attributable to the ability of the bacteria to attach themselves to the intestinal epithelium (Arbuckle, 1970; Drees and Wader, 1970a and b; Bertschinger, Moon and Whipp, 1972) and thereby avoid removal from the small intestine by peristalsis (Dixon, 1960). Sojka (1965) noted that the majority of E. coli strains isolated from cases of neonatal piglet diarrhoea possess a surface antigen designated K88, and it has been shown that this antigen is an essential virulence determinant (Smith and Linggood, 1971 ;Jones and Rutter, 1972) apparently because it enabled K88-positive E. coli to attach to the intestinal lining (Jones and Rutter, 1972).To investigate the interaction between the host mucosal surface and K88-positive E. coli a convenient test system is essential. The K88 antigen agglutinates guinea-pig red cells (Stirm et al., 1967;Jones and Rutter, 1974), and attempts to study the chemical aspects of adhesion by means of this reaction have been reported (Gibbons, Jones and Sellwood, 1975). It would have been preferable to use a natural intestinal receptor, but the technique involving adhesion of E. coli to disks of intestinal tissue (Jones and Rutter, 1972) is laborious. This paper describes a simple in-vitro technique that demonstrates adhesion of K88-positive E. coli to brush borders from pig intestinal cells; a survey of pigs in respect of the ability of their brush borders to adhere to E. coli is also described. MATERIALS AND METHODS

Evidence is presented that a dominant allele, S, is expressed as a receptor for K88 on the brushborder surface of the pig intestinal cell. The homozygous recessive (ss) lacks this receptor. The receptor enables K88 - positive coliforms to adhere to the gut of the piglet which they must do if they are to cause neonatal diarrhoea. The homozygous recessive is thus a disease resistant animal.A possible reason for the persistence of the dominant (susceptible) gene is given.

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S U M M A R YThe K99 antigen common to some bovine strains of Escherichia coli caused mannose-resistant haemagglutination of sheep erythrocytes and was shown to be responsible for the attachment of Kgg-positive bacteria to calf brush-border preparations because (i) strains grown at 18 "C did not produce K99 antigen, cause haemagglutination, or attach to brush borders; (ii) a K12 (K99+) recombinant strain showed both haemagglutinating activity and attachment to brush borders whereas, before it received the K99 plasmid, therecipient strain was negative in both respects; and (iii) cell-freeextracts of K99 antigen showed haemagglutinating activity and inhibited the attachment of Kgg-positive organisms to brush borders.K99 antigen appears to be a virulence determinant in the pathogenesis of neonatal calf diarrhoea. It is readily demonstrated by haemagglutination and brushborder attachment tests.