Up-regulation of both intimin and eae-independent adherence of shiga toxigenic Escherichia coli O157 by ler and phenotypic impact of a naturally occurring ler mutation

Shiga toxigenic Escherichia coli (STEC) strains are important human pathoge ns which are capable of causing diarrhea, hemorrhagic colitis, and the pote ntially fatal hemolytic-uremic syndrome (HUS). An important virulence trait of certain STEC strains, such as those belonging to serogroup O157, is the capacity to produce attaching and effacing (A/E) lesions on enterocytes, a property encoded by the locus for enterocyte effacement (LEE). LEE contain s the ene gene, which encodes intimin, an outer membrane protein which medi ates the intimate attachment of bacteria to the host epithelial cell surfac e, and eae is routinely used as a marker for LEE-positive STEC strains. How ever, the O157:H- STEC strain 95SF2 carries eae but did not produce A/E les ions on HEp-2 cells, as judged by a fluorescent actin staining assay. In th is assay, 95SF2 adhered poorly to the HEp-2 tells, and those that did bind exhibited abnormal cell division. In contrast, the O157:H7 STEC strain EDL9 33 adhered strongly and produced typical A/E lesions. We have demonstrated that 95SF2 carries a defective LEE regulatory gene, ler, with a single base change with respect to that published for ler of EDL933, resulting in an I le(57)-to-Thr substitution. Ler shows homology to H-NS-like regulators, whi ch are modulators of transcription, and the mutation occurs in a domain imp licated in oligomerization. 95SF2 was able to adhere and produce A/E lesion s on HEp-2 cells when EDL933 ler was expressed from a multicopy plasmid, Co nversely, introduction of a plasmid carrying 95SF2 ler into EDL933 abolishe d adherence and capacity to form A/E lesions. Studies with eae deletion der ivatives of 95SF2 and EDL933 demonstrated that the ler-mediated adherence t o HEp-2 cells is largely independent of intimin. We have also demonstrated that EDL933 ler, but not 95SF2 ler, increases the level of intimin in O157 STEC.