Sorbate-induced cross-protection in necrotoxigenic Escherichia coli

Three strains of necrotoxigenic Escherichia coli when pretreated with potassium sorbate at pH 7, when sorbate is predominantly in the anionic form, showed an elevated resistance to sodium chloride (27% w/v) compared to a control (no sorbate at pH 7). However, when the strains were subjected to sorbate at pH 5 and 4·75, when a large proportion of sorbate is protonated, the resistance to sodium chloride was reduced. Prior stress with sorbate at pH 7 also induced heat resistance (56°C for 80 min, one strain only tested). By using dialysis, the observed heat resistance was found to be mediated by an extracellular component that was identified tentatively as a protein with a molecular mass greater than 10 kDa. Although this protein was heat resistant at 75°C for 20 min, it was inactivated at 100°C for 20 min. Since necrotoxigenic strains of E. coli have been isolated from cheese and the preservatives sorbate and salt are sometimes used in cheese, this work emphasizes the need for further studies in this area to allow a more accurate risk assessment.