Heterofermentative glucose metabolism by glucose transport-impaired mutants of oral streptococcal bacteria during growth in batch culture

Abstract

Spontaneous mutants defective in a membrane component of the phosphoenolpyruvate-glucose phosphotransferase system were isolated by plating cells of Streptococcus sanguis 10556, Streptococcus mutans GS5-2 and NCTC 10449 on agar containing lactose and 2-deoxyglucose. Toluenized cells of these mutants were defective in their ability to catalyse the phosphoenolpyruvate-dependent phosphorylation of 2-deoxyglucose. The parental strains were mainly homofermentative when grown in batch culture in the presence of various sugars. Nevertheless, the mutants produced acetate, formate and ethanol when cultured in the presence of glucose but were homofermentative when grown in the presence of lactose or maltose. Analysis of one mutant isolated from Strep. sanguis (mutant GS26) revealed normal levels of glucokinase, glucose-6-phosphate dehydrogenase, puruvate kinase and lactate dehydrogenase. This last enzyme was dependent on fructose 1,6-diphosphate for catalytic activity. The determination of the intracellular level of fructose 1,6-diphosphate (FDP) during growth of the cells in batch culture showed that the mutant strains contained 2 to 15 times less FDP than the parental strains. Growth experiments performed at pH 6.0 and 7.0 with Strep. sanguis and its PTS-negative mutant GS26 suggested that the regulation of pyruvate metabolism in this bacterium include the intracellular level of FDP and the initial hydrogen concentration of the growth medium. The results also suggested that, in these bacteria, an active PTS is required to maintain the intracellular concentration of FDP high enough to keep the cell homofermentative during growth in batch culture.