Heat treatment is one of the most widely used methods for inactivation of bacteria in food products. Heat-induced loss of bacterial viability has been variously attributed to protein denaturation, oxidative stress, or membrane leakage; indeed, it is likely to involve a combination of these processes. We examine the effect of mild heat stress (50–55°C for ≤12 min) on cell permeability by directly measuring the electrical conductance of samples of Salmonella enterica serovar Typhimurium to answer a fundamental biophysical question, namely, how bacteria die under mild heat stress. Our results show that when exposed to heat shock, the cell membrane is damaged and cells die mainly due to the leakage of small cytoplasmic species to the surrounding media without lysis (confirmed by fluorescent imaging). We measured the conductance change, ΔY, of wild-type versus genetically modified heat-resistant (HR) cells in response to pulse and ramp heating profiles with different thermal time constants. In addition, we developed a phenomenological model to correlate the membrane damage, cytoplasmic leakage, and cell viability. This model traces the differential viability and ΔY of wild-type and HR cells to the difference in the effective activation energies needed to permeabilize the cells, implying that HR cells are characterized by stronger lateral interactions between molecules, such as lipids, in their cell envelope.
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