Effect of acetate on sorbitol fermentation by oral lactobacilli

The rate of acid production and end-products from sorbitol were measured under anaerobic conditions in washed-cell suspensions of oral strains of Lactobacillus casei subsp. casei and Lactobacillus casei subsp. rhamnosus. The enzymatic activities were assayed in cell extracts of these strains. The cells fermented sorbitol to lactate, formate, ethanol and acetate under anaerobic conditions. Exposure of the cells to air (oxygen) led to inactivation of pyruvate formate-lyase and inhibition of anaerobic sorbitol fermentation. In the presence of acetate, air-exposed cells fermented sorbitol with a concomitant consumption of acetate and production of ethanol and lactate. Acetate also enhanced acid production from sorbitol in cells kept under anaerobic conditions and resulted in formation of lactate and ethanol. Cell extracts of all the strains had NADH-coupled acetate-reducing activity, which consisted of sequential reactions of acetate kinase, phosphotransacetylase, acylating aldehyde dehydrogenase and alcohol dehydrogenase. These findings indicate that oral lactobacilli can utilize acetate as an electron acceptor for maintaining their intracellular redox balance during anaerobic sorbitol fermentation in the absence of pyruvate formate-lyase activity.