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Background  Enterohemorrhagic  E. coli  (EHEC), a subgroup of Shiga toxin (Stx) producing  E. coli  (STEC), may cause severe enteritis and hemolytic uremic syndrome (HUS) and is transmitted orally via contaminated foods or from person to person. The infectious dose is known to be very low, which requires most of the bacteria to survive the gastric acid barrier. Acid resistance therefore is an important mechanism of EHEC virulence. It should also be a relevant characteristic of  E. coli  strains used for therapeutic purposes such as the probiotic  E. coli  Nissle 1917 (EcN). In  E. coli  and related enteric bacteria it has been extensively demonstrated, that the alternative sigma factor σ S , encoded by the  rpoS  gene, acts as a master regulator mediating resistance to various environmental stress factors.    Methods  Using  rpoS  deletion mutants of a highly virulent EHEC O26:H11 patient isolate and the sequenced prototype EHEC EDL933 (ATCC 700927) of serotype O157:H7 we investigated the impact of a functional  rpoS  gene for orchestrating a satisfactory response to acid stress in these strains. We then functionally characterized  rpoS  of probiotic EcN and five r poS  genes selected from STEC isolates pre-investigated for acid resistance.    Results  First, we found out that ATCC isolate 700927 of EHEC EDL933 has a point mutation in  rpoS , not present in the published sequence, leading to a premature stop codon. Moreover, to our surprise, one STEC strain as well as EcN was acid sensitive in our test environment, although their cloned  rpoS  genes could effectively complement acid sensitivity of an  rpoS  deletion mutant.    Conclusion  The attenuation of sequenced EHEC EDL933 might be of importance for anyone planning to do either  in vitro  or  in vivo  studies with this prototype strain. Furthermore our data supports recently published observations, that individual  E. coli  isolates are able to significantly modulate their acid resistance phenotype independent of their  rpoS  genotype.

Impact of the rpoS genotype for acid resistance patterns of pathogenic and probiotic Escherichia coli