The Effects of Sub-Lethal Pressure Treatment on Escherichia coli O157:H7

Abstract

The resistance of Escherichia coli O157:H7 to high pressure varies between strains. In the first experiment a pressure resistant strain, NCTC 12079, was used to investigate the effect of sub-lethal pressure treatment on the heat resistance of E. coli O157:H7. UHT skimmed milk was inoculated with a stationary phase culture, pressure treated at 200 MPa for 30 min and heat treated at 55, 58 and 60 °C. Pressure treatment caused an increase in the heat sensitivity of E. coli O157:H7 at all three temperatures. This has important implications for the safety of pressure treated foods which are intended for subsequent heat treatment. In the second experiment the effect of sub-lethal pressure treatment on verotoxin production was investigated. Samples were first pressure treated at 400 MPa or held at ambient pressure as a control. Verotoxin production was assayed at various stages during storage at 25 °C in laboratory medium. Pressure treatment caused an increase in the time to detection of verotoxin activity. This effect may be explained in terms of cell numbers rather than any effect on the production or the activity of verotoxins, as pressure treatment caused an increase in the lag period during subsequent storage. This would indicate that although injured, pressure treated E. coli O157:H7, which are allowed to proliferate, are still capable of producing toxins and therefore likely to be capable of causing illness.