Phenotypic and genotypic characterization of enterotoxigenic Escherichia coli serotype O8:KX105 and O8:K"2829" strains isolated from piglets with diarrhea.

Twelve pathogenic and seven nonpathogenic enterotoxigenic Escherichia coli strains which were previously identified as belonging to serogroup O8:KX105 (A. Broes, J. M. Fairbrother, S. Larivière, M. Jacques, and W. M. Johnson, Infect. Immun. 56:241-246, 1988) were further examined for their phenotypic and genotypic properties. Only the 12 pathogenic strains were confirmed to possess the capsular antigen KX105. The seven nonpathogenic strains did not possess this antigen and thus were incorrectly assigned to have capsular antigen KX105. All seven nonpathogenic strains apparently possessed a previously unrecognized capsular antigen which has been designated K"2829". Studies with antisera prepared against F1 (type 1) fimbriae from three E. coli strains suggested that at least three antigenic subtypes of F1 fimbriae were represented among the O8:KX105 strains examined. By using serotyping, biotyping, and outer membrane protein profile analyses, the O8:KX105 strains were divided into at least two distinct clusters, whereas the O8:K"2829" strains were grouped into a single unique cluster. Most of the strains of the same cluster were further differentiated by testing for antibiotic resistance and colicin production and resistance and by analysis of plasmid content. With the exception of one strain which lost its enterotoxicity during storage, all of the O8:KX105 strains hybridized with the gene probes for the heat-labile (LT) and heat-stable (STb) enterotoxins. For each O8:KX105 strain, a single plasmid ranging in size from 61 to 77 megadaltons carried the LT and STb genes. All of the enterotoxigenic O8: KX105 strains fermented sorbose, whereas the nonenterotoxigenic strain did not. All of the O8:K "2829" strains hybridized with the STb probe only. For each O8:K "2829" strain, the STb genes were located on a single plasmid of 61 or 22 megadaltons. None of the strains demonstrated homology with the genes encoding the F4 (K88), F5 (K99), F6 (987P), and F41 fimbrial antigens and STaP and STaH.