Role of in vivo passage on the environmental adaptation of enterohemorrhagic Escherichia coli O157:H7: cross-induction of the viable but nonculturable state by osmotic and oxidative stresses.

In an enterohemorrhagic Escherichia coli (EHEC) O157:H7 outbreak caused by salted salmon roe that occurred in Japan, 1998, a food isolate (F2) was NaCl-resistant and a patient isolate (P5) was sensitive to NaCl. We show here that hydrogen peroxide, like NaCl, induced a significant loss of culturability in P5. The BacLight assay suggested that the EHEC O157:H7 entered a viable but nonculturable (VNC) state. We used the passage through mice in an attempt to model this transition in phenotype. Mouse-passaged isogenic variants of F2 became NaCl- and oxidation-sensitive, entered the nonculturable state in response to either of these stresses, and could be resuscitated by sodium pyruvate. Since the expression of RpoS in response to these stresses correlated with the isolates' culturabilities, we concluded that in vivo passage negatively modulated RpoS expression, and the subsequent stress exposure induced the VNC state in the EHEC O157:H7 isolates.