Datingrams anywhere between glass change heat (T
To determine the relationship between Tg and heat resistance (log reduction) of S. enterica serovars, we illustrate both the relationship as shown in Fig 4. There seems to be Tg dependency on heat resistance, which means bacterial inactivation effect increases with decrease in Tg for the tested S. enterica serovars except for S. Stanly. The correlation coefficient of S. Typhimurium, S. Chester, S. Oranienburg, S. Stanley, and S. Enteritidis is -0.54, -0.75, -0.80, 0.01, and -0.99, respectively. g plays an important role in heat resistance of S. enterica cells in dry conditions.
g) and inactivation effect (log reductions) of Salmonella Typhimurium (0, solid line), S. Chester (?, dashed line), S. Oranienburg (?, dotted line), S. Stanley (?, fine dotted line), and S. Enteritidis (?, dot dashed line).
Previous studies have reported that aw has some influence on bacterial survival. Long-term survival of bacteria was demonstrated in low-aw conditions (? 0.87 aw), whereas bacterial death was promoted in high aw conditions [17,23,24]. The relationship between bacterial cell Tg and aw is likely a major factor influencing differences in survival across a range of aw levels. The Tg of Salmonella tested at aw ? 0.87 was 30°C or higher in the present study. These cells would be in a glass state under room temperature conditions, because room temperature (normally 20–22°C) is lower than those of Tg (30°C). In a glass state, the molecular movement in bacterial cells is almost stopped and, thus, unlikely to be affected by external environments. It is inferred that bacteria acquire desiccation tolerance by glass transition accompanying a decrease in aw. Under high-aw conditions, it is presumed that Tg would be considerably low, glass transition would not occur, and the rubber state would be maintained. Since molecular movement is not limited in the rubber state, bacterial cells would not acquire desiccation tolerance. We assume that this state change is a key factor in the survival differences among bacteria. We preliminary examined thermal inactivation effect on some S. enterica serovars, and we confirmed apparently higher inactivation effect of 6–7 log cycle reductions in aw 0.99 than those of lower aw levels on the same heat treatment (data not shown). Furthermore, since a negative correlation was demonstrated between Tg and aw in Salmonella cells (Fig 4), there is a possibility that the bacteria will exhibit stronger desiccation tolerance as the aw ong bacterial species, the difference in desiccation tolerance will depend on the Tg.
This study also showed that the thermal inactivation effect decreased in low-aw conditions (Fig 3). It has been reported that the thermal inactivation effect of low-aw food [34–37]. The difference in thermal inactivation among different aw levels is likely involved in the changing physical state properties of bacterial cells as well as in their survival differences under dry conditions. As described above, bacterial cells in a low-aw environment will be in a glass state and exhibit high tolerance to environmental stresses such as heat, pressure, and desiccation. For example, extreme environment microorganisms, such as tardigrades and sleeping chironomids that utilize cryptobiosis, are also resistant to high temperature, high pressure, as well as dry environments [25–27]. Bacterial cells would vitrify, similar to extreme environmental organisms acquiring environmental stress tolerance. Therefore, we attribute the reduced thermal bacterial inactivation in low-aw conditions to a change in physical properties due to glass transition of bacterial cells.
The differences in bacterial survival (Fig 3) could be attributed to the difference in Tg of each bacterium. S. enterica Stanley was shown to have a higher Tg than the other Salmonella strains at high aw (Table 1), which might mean differences in the ability to maintain the glass state. In other words, S. Stanley would have stronger heat-tolerance than the other Salmonella strains. In a previous study, S. Stanley was reported to have a higher long-term survival ratio in dry conditions and S. Typhimurium showed the lowest Tg at high aw, which was associated with a low survival rate . The difference in Tg among bacterial species/serovars would attribute to innate (genetically) or acquired characteristics of each bacterial species/serovars. In particular, acquired characteristics might be due to habituation to various harsh conditions during survival process. Based on all of these findings, we believe that bacterial acquisition of environmental tolerance and the glass transition phenomenon are closely related. Although the mechanism by which aw exerts its influence on bacterial survival under desiccation and thermal conditions has not been clearly elucidated, the present study demonstrates that the glass transition phenomenon of bacterial cells may play an important role in stressful environments. Furthermore, we have successfully demonstrated that glass transition temperature will have an influence on the strength of desiccation and thermal tolerance of bacteria. To elucidate the exact reason for the difference in Tg among bacterial species/serovars, further genetical and/or bacteriological investigation will be needed in the future.