Identification and Characterization of the Locus for Diffuse Adherence, Which Encodes a Novel Afimbrial Adhesin Found in Atypical Enteropathogenic Escherichia coli

ABSTRACTThe O26 serogroup of enteropathogenic Escherichia coli(EPEC) is one of the serogroups most frequently implicated in infant diarrhea and is also common among enterohemorrhagic E. coli(EHEC) strains. The most common O26 strains belong to EPEC/EHEC serotype O26:H11 and are generally Shiga toxin (Stx) positive. Stx-negative E. colistrains that are negative for the EPEC EAF plasmid and bundle-forming pilus (Bfp) are classified as atypical EPEC. Here, we report a novel adhesin present in an stx-negative bfpA-negative atypical EPEC O26:H11 strain isolated from an infant with diarrhea. A cloned 15-kb genomic region from this strain, designated the locus for diffuse adherence (lda), confers diffuse adherence on HEp-2 cells when expressed in E. coliK-12. Sequence analysis of ldarevealed a G+C content of 46.8% and 15 open reading frames sharing homology with the E. coliK88 faeand CS31A clpfimbrial operons. The ldaregion is part of a putative 26-kb genomic island inserted into the proPgene of the E. colichromosome. Hybridization studies have demonstrated the prevalence of the minor structural subunit gene, ldaH, across E. coliserogroups O5, O26, O111, and O145. A second plasmid-encoded factor that contributed to the Hep-2 adherence of this strain was also identified but was not characterized. Null mutations that abolish adherence to HEp-2 cells can be restored by plasmid complementation. Antiserum raised against the major structural subunit, LdaG, recognizes a 25-kDa protein from crude heat-extracted protein preparations and inhibits the adherence of the E. coliDH5α lda+clone to HEp-2 cells. Electron microscopy revealed a nonfimbrial structure surrounding the bacterial cell.