Enhancement of Thermal Resistance by Metal Ions in Thermotolerant TISTR 548.

The thermal resistance of fermenting microbes is a key characteristic of stable fermentation at high temperatures. Therefore, the effects of various metal ions on the growth of TISTR 548, a thermotolerant ethanologenic bacterium, at a critical high temperature (CHT) were examined. Addition of Mg and K increased CHT by 1°C, but the effects of the addition of Mn, Ni, Co, Al, Fe, and Zn on CHT were negligible. To understand the physiological functions associated with the addition of Mg or K, cell morphology, intracellular reactive oxygen species (ROS) level, and ethanol productivity were investigated at 39°C (i.e., above CHT). Cell elongation was repressed by Mg, but not by K. Addition of both metals reduced intracellular ROS level, with only K showing the highest reduction strength, followed by both metals and only Mg. Additionally, ethanol productivity was recovered with the addition of both metals. Moreover, the addition of Mg or K at a non-permissive temperature in 26 thermosensitive, single gene-disrupted mutants of . TISTR 548 revealed that several mutants showed metal ion-specific growth improvement. Remarkably, K repressed growth of two mutants. These results suggest that K and Mg enhance cell growth at CHT via different mechanisms, which involve the maintenance of low intracellular ROS levels.