Lactic acid production by escherichia coli harboring ldhA gene from rhizopus oryzae

Abstract

Lactic acid is widely used for many industries such as food, pharmaceutical, cosmetic and use as monomer for producing biodegradable plastic. Lactic acid can be produced by microbial fermentation such as the fungus Rhizopus oryzae. Although this fungus has many advantages such as optically pure L(+)-lactic acid production and simple nutrient requirement, one of the obstacles for lactic production is its morphology that can increase in broth viscosity, but decrease in oxygen transfer. Therefore, genetic engineering was used to overcome the problem of R. oryzae morphology by using Escherichia coli as the host cell. The aim of this research is to study lactic acid production from the genetically modified E. coli strain RB24, E. coli with deactivated chromosomal ldhA and pta and harboring R. oryzae ldhA gene on the plasmid. After varying some fermentation parameters in shake flask level, It was suggested that a very small amount of lactic acid was detected when 56/2 minimal medium was used for 48 hours fermentation in all conditions. This lactic acid may come from the other Lactate dehydrogenases (LldD and Dld). When rich fermentation broth was used, the highest concentration of lactic acid at 7.45 g/L was obtained after being fermented with 30 g/L of initial glucose concentration under anaerobic condition for 48 hours. When IPTG inducible plasmid was used to express R. oryzae ldhA gene, smaller amount of lactic acid than that of RB24 strain may be resulted from glucose repression. From this study, it is possible that exogenous R. oryzae LdhA may not function properly in E. coli host. This was not only generation of low amount of lactic acid, but also inhibition of phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS) for glucose consumption that was investigated by high concentration of residual glucose.