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Pathogenic bacteria such as Salmonella enterica exhibit high desiccation tolerance, enabling long-term survival in low water activity (aw) environments. Although there are many reports on the effects of low aw on bacterial survival, the mechanism by which bacteria acquire desiccation tolerance and resistance to heat inactivation in low-aw foods remains unclear. We focused on the glass transition phenomenon, as bacteria may acquire environmental tolerance by state change due to glass transition. In this study, we determined the glass transition temperature (Tg) in S. enterica serovars under different aw conditions using thermal rheological analysis (TRA). The softening behaviour associated with the state change of bacterial cells was confirmed by TRA, and Tg was determined from the softening behaviour. Tg increased as the aw ple, while the Tg of five S. enterica serovars was determined as °C to °C at 0.87 aw, the Tg of all the five serovars increased by °C to °C at 0.43 aw. Furthermore, to verify the thermal tolerance of bacterial cells, a thermal inactivation assay was conducted at 60°C for 10 min under each aw condition. A higher survival ratio was observed as aw decreased; this represented an increase in Tg for Salmonella strains. These results suggest that the glass transition phenomenon of bacterial cells would associate with environmental tolerance.
Citation: Lee K, Shoda Meters, Kawai K, Koseki S (2020) Dating ranging from mug transition temperature, and you will desiccation as well as heat threshold for the Salmonella enterica. PLoS That 15(5): e0233638.
Copyright: © 2020 Lee mais aussi al. This can be an open availableness article distributed according to the terms of the fresh Creative Commons Attribution Licenses, and that permits unrestricted play with, shipping, and you may breeding in any typical, given the first blogger and you may provider is paid.
Funding: It really works are backed by the Japan Society into the Venture of Technology (JSPS) KAKENHI (Give JP 18H02148) provided to SK. The funders didn’t come with part during the studies build, analysis collection and you can investigation, choice to create, or preparation of your own manuscript.
Outbreaks of foodborne illnesses caused by dry foods, such as nuts [1–3], chocolate [4–8], cereals , and other foods are continuing worldwide [10–12]. Such foods have low water activity (aw) and the growth of bacteria causing foodborne illness is not observed. Therefore, microbiological hygiene control has not been regarded as important. However, cases of foodborne illness caused by dry food occur frequently, which means that pathogenic bacteria continue to survive even in a low-aw environment. Especially, various Salmonella serotypes were found in dry foods associated with outbreaks [1–12]. Indeed, several reports have shown that bacteria causing food poisoning such as enterohemorrhagic Escherichia coli (EHEC) and Salmonella, continue to survive for long periods in a low-aw environment [13–21]. On the contrary, the rate of bacterial survival decreases under high aw environments [16,22–24], and it is inferred that aw and the water content of bacteria have some influence on survival. From these previous studies, we consider that there are some associations between aw and desiccation tolerance of bacterial cells. However, the mechanism of desiccation tolerance of bacterial cells under low-aw conditions has not yet been clarified and elucidation of the cause is required.
There is a clue to elucidation of the mechanism of desiccation tolerance in other organisms. For example, extreme environment microorganisms, such as tardigrades and sleeping chironomids that utilize cryptobiosis, are resistant to various external environments such as high temperature, high pressure, as well as dry environments [25–27]. In this study, we assume that bacterial cells would vitrify as well as extreme environmental organisms are considered to acquire environmental stress tolerance. Vitrification of bacterial cells the adult hub by the glass transition phenomenon might be one of the long-term survival factors in a low-aw environment. The glass transition phenomenon refers to a state change caused by the increase or decrease in molecular movement in a substance as the temperature and moisture content change [28–30]. The state in which molecular movement is limited, due to the decrease in temperature and aw, is called a glass state and the substance shows physical properties similar to a solid. Since molecular motion is almost stopped in the glass state, the substance or organism shows high tolerance to various environmental stresses such as heat, desiccation, and pressure. In the present study, we hypothesized that bacterial cells are vitrified in low-aw environments based on the physicochemical properties of solid particles. In other words, we assumed that bacterial cells enter a glass state due to a decrease in molecular movement accompanying a decrease in aw, making it difficult for the bacteria to be influenced by external factors. This, in turn, allows for long-term survival, even in a dry environment.