Heat inactivation of Escherichia coli K12 MG1655: Effect of microbial metabolites and acids in spent medium

AimThe effect of spent medium, obtained after different time–temperature pre-histories, on the heat inactivation of Escherichia coli K12 MG1655 is studied.Methods and resultsStationary E. coli cells were heated in BHI broth (initial pH 7.5) at different time–temperature scenarios, i.e., (1) 30 °C to 55 °C at 0.14 °C/min, (2) 30 °C to 42 °C at 0.14 °C/min and (3) 30 °C to 42 °C at 0.8 °C/min. After the heat treatment, spent medium was filter-sterilized, non-stressed cells were added and inactivation experiments took place at 54 °C and 58 °C. In all scenarios, increased resistance was observed. The main characteristics of the spent medium – compared to the unmodified BHI broth – are (1) the presence of proteins (proven via SDS-PAGE) and (2) a lower pH of approximately 6. Possibly, the increased resistance is due to these proteins and/or the lower pH. Further experiments revealed that each factor separately may lead to an increased heat resistance.ConclusionsIt can be concluded that this increased heat resistance resulted from both the presence of the heat shock proteins in the spent medium and the lowered pH. Experiments, which separate both effects, showed that mainly the lower pH resulted in the increased thermotolerance.Significance and impact of studyThis study may lead to a better understanding and control of the heat stress adaptation phenomenon as displayed by E. coli at lethal temperatures. Therefore, it contributes to an improved assessment of the effect of temperature during thermal processes in the food industry.

► In this study we investigate the effect of metabolites and acids on the heat inactivation of Escherichia coli.
► Spent medium with metabolites was obtained after different time–temperature pre-histories.
► We observed increased heat resistance in presence of extracellular metabolites and/or acids.
► Heat tolerance was dependent on the different types and amount of extracellular metabolites.