Mechanism of Bacillus subtilis spore inactivation induced by moderate electric fields

• Published in: Innovative food science & emerging technologies Vol. 62; p. 102349
• Main Authors: Wang, Lang-Hong, Pyatkovskyy, Taras, Yousef, Ahmed, Zeng, Xin-An, Sastry, Sudhir K.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2020
• Subjects: Bacillus subtilis; Bacillus subtilis spores; Confocal laser scanning microscopy; cortex; endospores; food industry; food science; germinant receptors; germination; heat; heat tolerance; inner membrane; lysozyme; Moderate electric fields; ohmic heating; permeability; temperature; viability; germinant receptors; inner membrane; Moderate electric fields; Bacillus subtilis spores; Confocal laser scanning microscopy
• ISSN: 1466-8564; 1878-5522
• DOI: 10.1016/j.ifset.2020.102349

Bacterial endospores are the key safety targets for inactivation within low-acid foods. Herein, we investigated the inactivation of Bacillus subtilis CGMCC 1.1087 spores (107 CFU/mL) in sterile distilled water using moderate electric fields (MEF, 300 V/cm) under various temperatures (<30, 55, 65 and 75 °C). MEF treatment at below 30 °C resulted in 0.6-log reduction of spores, while treatments for 60 min without electric fields showed no inactivation. Inactivation induced by MEF in the same treatment time increased to 1.8-, 2.0- and 2.5-log as temperature increased to 55, 65 and 75 °C. Spores treated with MEF at <30, 55, 65 and 75 °C or mild heat (55, 65 and 75 °C) scarcely lost heat resistance, suggesting that spores did not germinate during MEF or mild heat treatment. The viability of MEF-treated spores did not increase by addition of lysozyme (3 μg/mL) in recovery plates, preincubation for 1 h in a 1:1 mixture of 60 mM Ca2+ and DPA, or lysozyme treatment in hypertonic medium. Confocal laser scanning microscopy photomicrographs showed that exposure to MEF induced a marked increase in the permeability of inner membrane and cortex. These findings suggested that damage of the cortex and inner membrane, rather than spore nutrient germinant receptors or cortex lytic enzymes, are possible reasons contributing to inactivation of B. subtilis spores by MEF. This study indicates that MEF at mild temperatures (55 to 75 °C) have the potential for spore inactivation. Literature in the past few years has shown that moderate electric fields (MEF), typically associated with ohmic heating, have nonthermal effects on bacterial spores, leading to accelerated inactivation. The current work extends the range of temperatures to those well below thermally lethal conditions, and shows that some spore inactivation occurs under MEF, even when temperatures are sublethal. Little or no germination is observed, and spore inner membranes are increasingly compromised over time. The elucidation of such nonthermal effects would be significant to the food industry as it seeks increasingly nonthermal methods for inactivation of spores. •The inactivation of spores was 0.6-log reduction by MEF at 300 V/cm (T < 30 °C).•The reduction level of spores increased to 2.0-log by MEF at 300 V/cm (T = 65 °C).•Spores did not germinate after being exposed to MEF or heat treatment.•MEF did not damage the spore nutrient germinant receptors or cortex lytic enzymes.•MEF induced increased permeability in inner membrane and cortex of spores.