Bicarbonate ion stimulates the expression of locus of enterocyte effacement-encoded genes in enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) strains adhere to the intestinal mucosa and produce an attaching and effacing (A/E) lesion. Most of the genes required to produce A/E lesions are thought to be encoded by the 36-kb pathogenicity island termed the locus for enterocyte effacement (LEE). Although the mechanisms underlying the bacterial adherence, including the genes involved, are still poorly understood, the preferential adherence phenotype of EHEC is thought to depend on the nature of the genes and/or the response of these genes to changes in environmental conditions. To explore the environmental factors affecting EHEC adherence, we used an O157:117 strain and investigated the optimal growth conditions for its adherence to Caco-2 cells. We observed that EHEC grown in Dulbecco's modified Eagle's medium (DMEM) adhered more efficiently to Caco-2 cells than EHEC grown in Luria-Bertani (LB) broth. Among the components of DMEM, only NaHCO3 was found to remarkably stimulate bacterial adherence. When bacteria were grown in LB broth containing NaHCO3 the production of intimin, Tir, EspA, and EspB was greatly enhanced compared with the production in LB broth. Indeed, the transcription of ler required for LEE-encoded gene expression was promoted in response to the concentration of NaHCO3 in LB broth. Since the concentration of NaHCO3 in the lower intestinal tract has been shown to be relatively high compared with that in the upper small intestine, our results may imply that NaHCO3 is an important signaling factor for promoting colonization of EHEC in the lower intestinal tract in humans.