School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana; Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana.
Department of Biological Sciences, Purdue University, West Lafayette, Indiana.
Biophys J. 2018 Feb 6;114(3):609-618. doi: 10.1016/j.bpj.2017.10.032.
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.
热处理是用于食品中细菌失活的最广泛使用的方法之一。热诱导的细菌活力丧失归因于蛋白质变性、氧化应激或膜泄漏；事实上，它可能涉及这些过程的组合。我们通过直接测量沙门氏菌血清型 Typhimurium 样品的电导率来检查温和热应激（50-55°C，≤12 分钟）对细胞通透性的影响，以回答一个基本的生物物理问题，即温和热应激下细菌是如何死亡的。我们的结果表明，当暴露于热冲击时，细胞膜受损，细胞主要由于小细胞质物质向周围介质泄漏而死亡，而没有裂解（通过荧光成像证实）。我们测量了野生型与遗传修饰耐热（HR）细胞对具有不同热时间常数的脉冲和斜坡加热曲线的电导变化ΔY。此外，我们开发了一个唯象模型来关联膜损伤、细胞质泄漏和细胞活力。该模型将野生型和 HR 细胞的差异存活率和ΔY 追踪到使细胞渗透所需的有效活化能的差异，这表明 HR 细胞的特征是其细胞包膜中分子（如脂质）之间的横向相互作用更强。
