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Infection and Immunity, July 2002, p. 3500-3509, Vol. 70, No. 7
0019-9567/02/$04.00+0     DOI: 10.1128/IAI.70.7.3500-3509.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Bicarbonate Ion Stimulates the Expression of Locus of Enterocyte Effacement-Encoded Genes in Enterohemorrhagic Escherichia coli O157:H7

Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108-8639,¹ Laboratory of Veterinary Public Health, Graduate School of Agriculture and Life Science, University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan²

Received 13 December 2001/ Returned for modification 6 February 2002/ Accepted 17 March 2002

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:H7 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.

Editor: V. J. DiRita

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