Carbohydrate Metabolism by Streptococcus thermophilus : A Review

Despite the widespread use of Streptococcus thermophilus as a starter culture in the manufacture of many fermented dairy products, only recently has an understanding of the basic processes regarding carbohydrate metabolism been developed. Although S. thermophilus is related to other lactic streptococci by virtue of their common use in dairy fermentations, available information indicates that S. thermophilus is serologically, genetically, and physiologically distinct from the Group N, mesophilic streptococci. Carbohydrate metabolism, in particular, occurs by different processes in S. thermophilus than in the Group N streptococci ( Streptococcus lactis and Streptococcus cremoris ). The latter organisms utilize lactose by a specific phosphoenolpyruvate-dependent phosphotransferase system in which the lactose hydrolysis products, glucose and galactyose-6-phosphate, are concurrently metabolized to lactic acid. In contrast, S. thermophilus lacks phosphotransferase activity and instead possesses a lactose permease. After hydrolysis by β-galactosidase, only glucose is further metabolized and galactose is released into the extracellular medium. Most strains are unable to ferment galactose and are phenotypically galactose-negative. The rapid growth rates of S. thermophilus on lactose and slow growth rates on glucose and galactose are likely due to the differences between the lactose and monosaccharide transport activities. Galactose transport by S. thermophilus requires an exogenous energy source and is mediated by a galactose permease. Galactose is further metabolized in galactose-positive cells by the enzymes of the Leloir pathway, specifically, galactokinase, galactose-1-phosphate uridyl transferase, and uridine-5-diphospho-glucose-4-epimerase. The latter two enzymes are eonstituitively expressed; however, in galactose-positive cells galactokinase and the galactose permease are induced by galactose in the absence of lactose. The phenotypic differences between galactose-positive and galactose-negative S. thermophilus are, in part, due to differences in the galactokinase and galactose permease activities. Galactose released into the medium by lactose-grown, galactose-positive cells can be subsequently metabolized, homofermentatively, to lactic acid. However, the important practical implications of released galactose has produced the need for isolation and development of S. thermophilus strains which ferment the lactose components, glucose and galactose, completely and simultaneously.