Abstract
The effect of extended cold or cold-acid storage ofEscherichia coli O157:H7 on subsequent acid tolerance, freeze-thaw survival, heat tolerance, and virulence factor (Shiga toxin, intimin, and hemolysin) expression was determined. ThreeE. coli O157:H7 strains were stressed at 4°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°C to 21°C) survival, and heat (56°C) tolerance of stressed cells were compared with those of control cells. The β-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°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 ofE. coli O157: H7 at 4°C has substantial effects on freeze-thaw tolerance but does not affect subsequent virulence gene expression.
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