Enhanced intracellular glutathione synthesis and export capability of Candida utilis by using low pH-stress strategy
When Candida utilis WSH 02-08 was cultivated in a glucose-ammonium sulfate medium without pH control, glutathione leakage occurred when the pH of the medium decreased to 1.5. However, analysis of the cell viability indicated that the cells were not lysed. To further study the effect of low pH stress on glutathione production, pH-controlled batch cultures were conducted, where the pH was switched from 5.5 to 1.2 at 24 h and stood at 1.2 for 6 h.Nearly all intracellular glutathione leaked into the medium and the cell viability decreased dramatically, conceiving the idea that a long-term exposure of strain WSH 02-08 at low pH environment led to a complete cell lysis. A critical point (treated at pH 1.2 for 3 h) was experimentally determined, where most cells were alive but suffering a low pH stress. Low pH-stressed C. utilis cells displayed an increased intracellular glutathione synthesis and export capability, which protected cells against short-term low pH treatment. This is the first report of glutathione secretion of C. utilis at low pH. Using this knowledge, a low pH-stress strategy was developed and applied to fed-batch production of glutathione.A 197.3 mg·L~-1 of glutathione was secreted into the medium.The total glucose concentration reached 737.1 mg·L~-1,increasing by 24.9% and 20.0% respectively than two control cases,and the glutathione content reached 2.67 %(mass),as compared to two control cases of 2.28%(mass)and 2.11%(mass).This study demonstrated the importance of a physiology-based fermentation strategy in the production of useful compounds.