Control of sugar utilization in the oral bacteria Streptococcus salivarius and Streptococcus sanguis by the phosphoenolpyruvate: glucose phosphotransferase system.

Three different Strep. salivarius (G2, G5 and G29) and two Strep. sanguis (GS3 and GS12) mutants affected in the phosphoenolpyruvate: glucose phosphotransferase system were selected on agar plates containing lactose and 2-deoxyglucose. All 5 were defective in a membrane-bound component of the transport system and grew less rapidly than the parent strain in 5 mM glucose-containing medium. Mutants G2 and G29 grew poorly in the presence of 5 mM mannose. Growth on mixed substrates revealed that the mutants and wild-type parents behaved differently. Wild-type strains in medium containing glucose plus another sugar (lactose, galactose, melibiose, raffinose or trehalose for Strep. salivarius and lactose, galactose or trehalose for Strep. sanguis) always exhausted most of the glucose before utilizing the other sugar. The mutants used the second sugar concurrently or preferentially to glucose. In medium containing glucose plus fructose or mannose, the wild types consumed both sugars concurrently whereas the mutants utilized the second sugar before glucose. Mutants G2 and G5 were insensitive to repression by fructose and released glucose into the medium when grown in the presence of 0.4 per cent lactose. Mutant G5 also released galactose. Sugar release was not detected with the wild types. The Strep. salivarius mutants contained normal levels of glucokinase and beta-galactosidase but G5 was almost totally devoid of galactokinase activity after growth on lactose. On galactose, the activity was restored. It seems that the phosphoenolpyruvate: glucose phosphotransferase system is involved in the regulation of sugar utilization in these two streptococci.