Potassium ion leakage impairs thermotolerance in Corynebacterium glutamicum.

Corynebacterium glutamicum, a gram-positive bacterium, can produce amino acids such as glutamic acid and lysine. The heat generated during cell growth and/or glutamate fermentation disturbs both the cell growth and fermentation. To overcome such a negative effect of the fermentation heat, we have tried to establish a high temperature fermentation. One of the approach is to create a thermotolerant strains, while the other is to create an optimum culture conditions able for the strain to grow at higher temperatures. In this study, we focused on the latter approach, where we examined the effect of potassium ion on cell growth at high growth temperatures of C. glutamicum. The supplementation of high concentrations of potassium chloride (300 mM) (or sorbitol, an osmolyte) mitigated the repressed cell growth induced by high temperature at 39 °C or 40 °C. The intracellular potassium concentration declines from 300 mM to ∼150 mM by increasing the growth temperature but not by supplementing potassium chloride or sorbitol. Furthermore, in vitro experiments revealed that the potassium ion leakage occurs at high temperatures, which was mitigated in the presence of high concentrations of extracellular potassium chloride. This suggested that the presence of high osmolyte in the culture medium could inhibit the potassium ion leakage induced by high temperature and subsequently support cell growth at high temperatures.

Thermal adaptation of acetic acid bacteria for practical high-temperature vinegar fermentation.

Thermotolerant microorganisms are useful for high-temperature fermentation. Several thermally adapted strains were previously obtained from Acetobacter pasteurianus in a nutrient-rich culture medium, while these adapted strains could not grow well at high temperature in the nutrient-poor practical culture medium, "rice moromi." In this study, A. pasteurianus K-1034 originally capable of performing acetic acid fermentation in rice moromi was thermally adapted by experimental evolution using a "pseudo" rice moromi culture. The adapted strains thus obtained were confirmed to grow well in such the nutrient-poor media in flask or jar-fermentor culture up to 40 or 39 °C; the mutation sites of the strains were also determined. The high-temperature fermentation ability was also shown to be comparable with a low-nutrient adapted strain previously obtained. Using the practical fermentation system, "Acetofermenter," acetic acid production was compared in the moromi culture; the results showed that the adapted strains efficiently perform practical vinegar production under high-temperature conditions.