Pleiotropic effects of lactate dehydrogenase inactivation in Lactobacillus casei

In lactic acid bacteria, conversion of pyruvic to lactic acid through the activity of lactate dehydrogenase (Ldh) constitutes the final step of the homofermentative pathway. Lactobacillus casei has two characterized genes encoding Ldh activities. The ldhL gene codes for an L-Ldh, which specifically catalyzes the formation of l-lactate, whereas the hicD gene codes for a d-hydroxyisocaproate dehydrogenase (HicDH), which catalyzes the conversion of pyruvate into d-lactate. In L. casei cells fermenting glucose, a mixture of l-/d-lactate with a 97:3% ratio was formed. Inactivation of hicD led to undetectable d-lactate levels after glucose fermentation, while l-lactate levels remained constant. Inactivation of ldhL did not abolish the production of l-lactate, but the lactate final concentration decreased about 25% compared to the wild type, suggesting the presence of at least a second L-Ldh. Moreover, part of the pyruvate flux was rerouted and half of the lactate produced was in the d-isomer form. ldhL inactivation in L. casei showed additional interesting effects. First, the glycolytic flux from pyruvate to lactate was redirected and other fermentation products, including acetate, acetoin, pyruvate, ethanol, diacetyl, mannitol and CO2, were produced. Second, a lack of carbon catabolite repression of lactose metabolism and N-acetyl-glucosaminidase activity was observed. This second effect could be partly avoided by growing the cells under aeration, since NADH oxidases could account for NAD+ regeneration.