Effects of cold storage and heat-acid shocks on growth and verotoxin 2 production of Escherichia coli O157:H7

Escherichia coli O157:H7 was cold-stored (4 degrees C) either in nutritious menstruum [buffered Brain Heart Infusion (BHI) broth] or with starvation (buffered saline) at pH 7.0 or 5.5. Cultures grown in BHI broth at 37 degrees C for 24h served as non-cold-stored controls. After 4-weeks cold storage, bacterial cells were shocked by heat (45 degrees C for 5 min) and acid (pH 2.5 for 30 min at 37 degrees C) and subsequently moved to optimal conditions (BHI broth of pH 7.4 incubated at 37 degrees C). The results showed: (a) both lag-phase duration and growth rate of this pathogen at 37 degrees C significantly increased after cold-storage with starvation, but not after cold storage in the nutritious menstruum; (b) combined heat-acid shocks increased growth rates at 37 degrees C of both previously cold-stored and non-cold-stored bacterial cells; (c) final concentrations of verotoxin produced by bacterial cells at 37 degrees C were not affected by previous cold storage in the nutritious menstruum; (d) verotoxin production by bacterial cells at 37 degrees C increased after cold storage with starvation, and heat-acid shocks further enhanced that production. Further research is needed to evaluate the food safety implications of these results, i.e. whether cells of E. coli O157:H7 originating from nutrient-poor/lower-pH environments may be more harmful to humans than those from nutrient-rich/higher-pH foods.