Spaceflight-induced enhancement of 2-keto-L-gulonic acid production by a mixed culture of Ketogulonigenium vulgare and Bacillus thuringiensis

Abstract

Two bacterial strains used for industrial production of 2-keto-L-gulonic acid (2-KLG), Ketogulonigenium vulgare 2 and Bacillus thuringiensis 1514, were loaded onto the spacecraft Shenzhou VII and exposed to space conditions for 68h in an attempt to increase their fermentation productivities of 2-KLG. An optimal combination of mutants B.thuringiensis 320 and K.vulgare 2194 (KB2194-320) was identified by systematically screening the pH and 2-KLG production of 16000 colonies. Compared with the coculture of parent strains, the conversion rate of L-sorbose to 2-KLG by KB2194-320 in shake flask fermentation was increased significantly from 82·7% to 95·0%. Furthermore, a conversion rate of 94·5% and 2-KLG productivity of 1·88gl(-1) h(-1) were achieved with KB2194-320 in industrial-scale fermentation (260m(3) fermentor). An observed increase in cell number of K2194 (increased by 47·8%) during the exponential phase and decrease in 2-KLG reductase activity (decreased by 46·0%) were assumed to explain the enhanced 2-KLG production. The results suggested that the mutants KB2194-320 could be ideal substitutes for the currently employed strains in the 2-KLG fermentation process and demonstrated the feasibility of using spaceflight to breed high-yielding 2-KLG-producing strains for vitamin C production. KB2194-320, a combination of two bacterial strains bred by spaceflight mutation, exhibited significantly improved 2-KLG productivity and hence could potentially increase the efficiency and reduce the cost of vitamin C production by the two-step fermentation process. In addition, a new pH indicator method was applied for rational screening of K2, which dramatically improved the efficiency of screening.