Evaluation of thermal inactivation of Escherichia coli using microelectrode ion flux measurements with osmotic stress

Aims: To elucidate the potential use of microelectrode ion flux measurements to evaluate bacterial responses to heat treatment.

Methods and Results: Escherichia coli K12 was used as a test bacterium to determine if various heat treatments (55°C to 70°C for 15 min) affected net ion flux across E. coli cell membranes using the MIFE™ system to measure net K⁺ fluxes. No difference in K⁺ fluxes was observed before and after heat treatments regardless of the magnitude of the treatment. Applying hyperosmotic stress (3% NaCl w/v) during flux measurement led to a net K⁺ loss from the heat-treated E. coli cells below 65°C as well as from non-heated cells. In contrast, with E. coli cells treated at and above 65°C, hyperosmotic stress disrupted the pattern of K⁺ flux observed at lower temperatures and resulted in large flux noise with random scatter. This phenomenon was particularly apparent above 70°C. Although E. coli cells lost the potential to recover and grow at and above 62°C, K⁺ flux disruption was not clearly observed until 68°C was reached.

Conclusions: No changes in net K⁺ flux from heat-stressed E. coli cells were observed directly as a result of thermal treatments. However, regardless of the magnitude of heat treatment above 55°C, loss of viability indicated by enrichment culture, correlated with disrupted K⁺ fluxes when previously heated cells were further challenged by imposing hyperosmotic stress during flux measurement. This two-stage process enabled evaluation of the lethality of heat treated bacterial cells within 2 h and may be an alternative and more rapid method to confirm the lethality of heat treatment.

Significance and Impact of the Study: The ability to confirm the lethality of thermal treatments and to specify minimal time/temperature combinations by a non-culture dependent test offers an alternative system to culture-based methods.