Identification and Characterization of a Novel Genomic Island Integrated at selC in Locus of Enterocyte Effacement-Negative, Shiga Toxin-Producing Escherichia coli

Abstract

The selC tRNA gene is a common site for the insertion of pathogenicity islands in a variety of bacterial enteric pathogens. We demonstrate here that Escherichia colithat produces Shiga toxin 2d and does not harbor the locus of enterocyte effacement (LEE) contains, instead, a novel genomic island. In one representative strain (E. coliO91:H⁻ strain 4797/97), this island is 33,014 bp long and, like LEE in E. coli O157:H7, is integrated 15 bp downstream of selC. ThisE. coli O91:H⁻ island contains genes encoding a novel serine protease, termed EspI; an adherence-associated locus, similar to iha ofE. coli O157:H7; an E.coli vitamin B₁₂ receptor (BtuB); an AraC-type regulatory module; and four homologues of E.coli phosphotransferase proteins. The remaining sequence consists largely of complete and incomplete insertion sequences, prophage sequences, and an intact phage integrase gene that is located directly downstream of the chromosomal selC. Recombinant EspI demonstrates serine protease activity using pepsin A and human apolipoprotein A-I as substrates. We also detected Iha-reactive protein in outer membranes of a recombinant clone and 10 LEE-negative, Shiga toxin-producing E. coli (STEC) strains by immunoblot analysis. Using PCR analysis of various STEC, enteropathogenic E. coli, enterotoxigenicE. coli, enteroaggregativeE. coli, uropathogenic E.coli, and enteroinvasive E.coli strains, we detected the ihahomologue in 59 (62%) of 95 strains tested. In contrast,espI and btuB were present in only two (2%) and none of these strains, respectively. We conclude that the newly described island occurs exclusively in a subgroup of STEC strains that are eae negative and contain the variantstx_2d gene.