Abstract
The objective of this study is to investigate the influence of water activity on the sporulation of emetic strains of Bacillus cereus and the subsequent susceptibility of sporulated B. cereus to 915 MHz microwave treatment. Water activity levels were manipulated in the sporulation medium by adjusting glycerol concentrations to 0 %, 3 %, 7.5 %, and 10 %, resulting in corresponding water activities of 0.996, 0.981, 0.971, and 0.960, respectively and sporulated at 30℃. These changes in water activity were intended to simulate the variations in water activity that can occur during the cultivation and processing of rice, where B. cereus is commonly found. Sporulation rates increased with higher water activity, achieving over 90 % of total cells after 3, 5, 5, and 6 days of incubation at water activities of 0.996, 0.981, 0.971, and 0.960, respectively. Resistance to microwave treatment increased with higher water activity levels during sporulation. Microwave treatment for 5 min yielded a reduction of 3.03 log CFU/g at a water activity of 0.960 and 1.98 log CFU/g at 0.996 during sporulation. Lower water activity led to higher % dipicolinic acid (DPA) release during microwave treatment, as measured using a spectrometer, indicating greater membrane damage. To investigate the morphology of spores, transmission electron microscopy was used. Spores produced at lower water activity levels were observed to be enveloped by the mother cell, whereas those sporulated at higher water activity levels were distinct from the mother cell. Additionally, spores sporulated at higher water activity levels exhibited higher wet density in Percoll gradient. The levels of germination were also compared after sporulation under different water activity medium using L-alanine. The spores evolved under low water activity exhibited higher germination level compared to those from high water activity conditions. These findings suggested that water activity conditions during sporulation induce variations in morphology, density, germination level, and resistance to 915 MHz microwave. Our experimental results indicated that, even with the same sporulation percentage, there can be differences in the maturation stage, indicating that the level of 915 MHz microwave treatment should be adjusted according to the sporulation conditions of B. cereus.
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