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 4degreesC 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 (-20degreesC to 21degreesC) survival, and heat (56degreesC) 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 stx(2), eaeA, or hlyA gene were determined following stress in TSB or pH 5.5 TSB at 37degreesC 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 4degreesC has substantial effects on freeze-thaw tolerance but does not affect subsequent virulence gene expression.