Regulation of Lactose Fermentation in Group N Streptococci

Abstract

Group N streptococci, which have the lactose phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) and phospho-β- d -galactosidase (β-Pgal), grew rapidly on lactose and converted more than 90% of the sugar to l -lactate. In contrast, Streptococcus lactis 7962, which does not have a β-Pgal, grew slowly on lactose and converted only 15% of the sugar to l -lactate. With glucose and galactose, this strain had growth rates and fermentation patterns similar to those of other S. lactis strains, suggesting that the rapid and homolactic fermentation of lactose that is characteristic of group N streptococci is dependent upon a functional PEP-dependent PTS and the presence of β-Pgal. Seventeen strains of group N streptococci were examined for the activator specificities of pyruvate kinase and lactate dehydrogenase. The properties of each enzyme from all the strains, including S. lactis 7962, were similar. Pyruvate kinase had a broad activator specificity, whereas activation of lactate dehydrogenase was specific for ketohexose diphosphate. All intermediates of lactose metabolism from the hexose phosphates to the triose phosphates activated pyruvate kinase. No activation was obtained with adenosine 5′-monophosphate. K + and Mg 2+ were required for pyruvate kinase activity but could be replaced by NH 4 + and Mn 2+ , respectively. Lactate dehydrogenase was activated equally by fructose-1,6-diphosphate and tagatose-1,6-diphosphate, the activation characteristics being pH dependent. The roles of pyruvate kinase and lactate dehydrogenase in the regulation of lactose fermentation by group N streptococci are discussed.