The inactivation kinetics of Escherichia coli NCTC 8164 during pressure treatment at 400 MPa in phosphate buffered saline were influenced by the initial cell concentration such that the rate of inactivation increased as cell concentration decreased. Cells in sparse populations (3 x 10⁶ /ml), were protected by cell-free supernatants from dense (3 x 10⁹ /ml) pressure-treated suspensions suggesting that material leaking from cells was responsible for the protective effect. Pressure-treated cells lost about 14% of their cellular protein but the resultant concentration of protein in the supernatant (80 micrograms/ml) was considered unlikely to be sufficient to afford significant non-specific protection. The protective factor was stable to heating at 78°C for 15 min, and was lost by dialysis indicating a molecular mass of less than 2 kDa. Both calcium and magnesium are known to have a protective effect and both leaked from cells during pressurisation. However the concentrations found in the supernatant (ca. 0.2 and 0.05 microgram atoms/L respectively) were too low to be protective. Complex growth media are more similar to foods than buffer solutions and a cell density affect was seen in Luria Bertani broth but not Tryptone Soya broth. These results show that inactivation can be highly dependent cell concentration and on small differences in the composition of the suspending medium and identification of the mechanisms of protection would help clarify the critical factors leading to cell death.

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