Impact of cold and cold-acid stress on poststress tolerance and virulence factor expression of Escherichia coli O157:H7

The effect of extended cold or cold-acid storage of Escherichia coli O157:H7 on subsequent acid tolerance, freeze-thaw survival, heat tolerance, and virulence factor (Shiga toxin, intimin, and hemolysin) expression was determined. Three E. coli O157:H7 strains were stressed at 4 degrees C in TSB or pH 5.5 TSB for 4 weeks. The acid (TSB [pH 2.0] or simulated gastric fluid [pH 1.5]) tolerance, freeze-thaw (-20 degrees C to 21 degrees C) survival, and heat (56 degrees C) tolerance of stressed cells were compared with those of control cells. The beta-galactosidase activities of stressed and control cells containing a lacZ gene fusion in the stx2, eaeA, or hlyA gene were determined following stress in TSB or pH 5.5 TSB at 37 degrees C and in the exponential and stationary phases. Cold and cold-acid stresses decreased acid tolerance (P < 0.05), with a larger decrease in acid tolerance being observed after cold stress than after cold-acid stress (P < 0.05). Cold stress increased freeze-thaw survival for all three strains (P < 0.05). Prior cold or cold-acid stress had no effect on virulence factor production (P > 0.05), although growth in acidic media (pH 5.5) enhanced eaeA and hlyA expression (P < 0.05). These results indicate that the prolonged storage of E. coli O157:H7 at 4 degrees C has substantial effects on freeze-thaw tolerance but does not affect subsequent virulence gene expression.