Sorbitol required for cell growth and ethanol production by Zymomonas mobilis under heat, ethanol, and osmotic stresses

BACKGROUND During ethanol fermentation, the ethanologenic bacterium, Zymomonas mobilis may encounter several environmental stresses such as heat, ethanol and osmotic stresses due to high sugar concentration. Although supplementation of the compatible solute sorbitol into culture medium enhances cell growth of Z. mobilis under osmotic stress, the protective function of this compound on cell growth and ethanol production by this organism under other stresses such as heat and ethanol has not been described yet. The formation of sorbitol in Z. mobilis was carried out by the action of the glucose-fructose oxidoreductase (GFOR) enzyme which is regulated by the gfo gene. Therefore, the gfo gene in Z. mobilis was disrupted by the fusion-PCR-based construction technique in the present study, and the protective function of sorbitol on cell growth, protein synthesis and ethanol production by Z. mobilis under heat, ethanol, and osmotic stresses was investigated. RESULTS Based on the fusion-PCR-based construction technique, the gfo gene in Z. mobilis was disrupted. Disruption of the Z. mobilis gfo gene resulted in the reduction of cell growth and ethanol production not only under osmotic stress but also under heat and ethanol stresses. Under these stress conditions, the transcription level of pdc, adhA, and adhB genes involved in the pyruvate-to-ethanol (PE) pathway as well as the synthesis of proteins particularly in Z. mobilis disruptant strain were decreased compared to those of the parent. These findings suggest that sorbitol plays a crucial role not only on cell growth and ethanol production but also on the protection of cellular proteins from stress responses. CONCLUSION We showed for the first time that supplementation of the compatible solute sorbitol not only promoted cell growth but also increased the ethanol fermentation capability of Z. mobilis under heat, ethanol, and osmotic stresses. Although the molecular mechanism involved in tolerance to stress conditions after sorbitol supplementation is still unclear, this research has provided useful information for the development of the effective ethanol fermentation process particularly under environmental conditions with high temperature or high ethanol and sugar concentration conditions.