D-lactic acid production from cellooligosaccharides and β-glucan using l-LDH gene-deficient and endoglucanase-secreting Lactobacillus plantarum

Abstract

In order to achieve direct fermentation of an optically pure D: -lactic acid from cellulosic materials, an endoglucanase from a Clostridium thermocellum (CelA)-secreting plasmid was introduced into an L: -lactate dehydrogenase gene (ldhL1)-deficient Lactobacillus plantarum (ldhL1) bacterial strain. CelA expression and its degradation of beta-glucan was confirmed by western blot analysis and enzyme assay, respectively. Although the CelA-secreting ldhL1 assimilated cellooligosaccharides up to cellohexaose (although not cellotetraose), the main end product was acetic acid, not lactic acid, due to the conversion of lactic acid to acetic acid. Cultivation under anaerobic conditions partially suppressed this conversion resulting in the production of 1.27 g/l of D: -lactic acid with a high optical purity of 99.5% from a medium containing 2 g/l of cellohexaose. Subsequently, D: -lactic acid fermentation from barley beta-glucan was carried out with the addition of Aspergillus aculeatus beta-glucosidase produced by recombinant Aspergillus oryzae and 1.47 g/l of D: -lactic was produced with a high optical purity of 99.7%. This is the first report of direct lactic acid fermentation from beta-glucan and a cellooligosaccharide that is a more highly polymerized sugar than cellotriose.