Utilization of Lactose and Galactose by Streptococcus mutans : Transport, Toxicity, and Carbon Catabolite Repression

Abstract

Abundant in milk and other dairy products, lactose is considered to have an important role in oral microbial ecology and can contribute to caries development in both adults and young children. To better understand the metabolism of lactose and galactose by Streptococcus mutans, the major etiological agent of human tooth decay, a genetic analysis of the tagatose-6-phosphate (lac) and Leloir (gal) pathways was performed in strain UA159. Deletion of each gene in the lac operon caused various alterations in expression of a P lacA -cat promoter fusion and defects in growth on either lactose (lacA, lacB, lacF, lacE, and lacG), galactose (lacA, lacB, lacD, and lacG) or both sugars (lacA, lacB, and lacG). Failure to grow in the presence of galactose or lactose by certain lac mutants appeared to arise from the accumulation of intermediates of galactose metabolism, particularly galatose-6-phosphate. The glucose-and lactose-PTS permeases, EII Man and EII Lac , respectively, were shown to be the only effective transporters of galactose in S. mutans. Furthermore, disruption of manL, encoding EIIAB Man , led to increased resistance to glucose-mediated CCR when lactose was used to induce the lac operon, but resulted in reduced lac gene expression in cells growing on galactose. Collectively, the results reveal a remarkably high degree of complexity in the regulation of lactose/galactose catabolism.Lactose, a ␤1,4-linked disaccharide of ␤-D-galactose and ␣/␤-D-glucose, is commonly found in the dairy-rich diets of most industrialized nations. Lactose is rapidly fermented by streptococci, including the cariogenic oral bacterium Streptococcus mutans (21), as well as by a variety of industrially important lactic acid bacteria (LAB) (19). Multiple pathways have been identified in bacteria for the utilization of lactose encountered in the environment. For example, Streptococcus salivarius strain 25975 (26) secretes a ␤-galactosidase that hydrolyzes extracellular lactose into galactose and glucose, although it is more common for lactose to be transported before cleavage (18). Most efficiently, and almost exclusively in Gram-positive bacteria, lactose is internalized by the phosphoenolpyruvate (PEP)-dependent sugar-phosphotransferase system (PTS), yielding lactose-6-phosphate (Lac-6-P) (36). The Lac-6-P is hydrolyzed to glucose and galactose-6-phosphate (Gal-6-P) by a cytoplasmic phospho-␤-galactosidase (LacG), and the Gal-6-P can be catabolized by the tagatose-6-phosphate pathway (18) (Fig. 1). Many bacteria, including Escherichia coli, Lactococcus lactis strain 7962, and S. salivarius strain 57.I, can internalize lactose through non-PTS transporters. Intracellular lactose is cleaved by a ␤-galactosidase enzyme and the D-galactose can directly enter the Leloir pathway ( Fig. 1) (18, 20).S. mutans has a functional lactose-specific PTS (14, 26) encoded by the lacF (EIIA) and lacE (EIIBC) genes (40). A phospho-␤-galactosidase (lacG) and the enzymes of the tagatose-6-phosphate pathway (Fig. 1B), including the two subunits of the heteromeri...

Modification of Gene Expression and Virulence Traits in Streptococcus mutans in Response to Carbohydrate Availability

“…Immunoblotting confirmed that the antibody specifically recognized a protein band with the predicted size of ManL (ϳ35 kDa) in batch-grown cells cultured in glucose or fructose and that this band was absent in cells carrying a nonpolar allelic exchange mutation of the manL gene (see Fig. S1 in the supplemental material) (45). We next probed cell lysates of S. mutans UA159 from steady-state cultures grown under glucose-limited conditions or with excess glucose and consistently found more ManL protein in cells grown under glucose-limited conditions than in cells grown with excess glucose (Fig.…”

“…Moreover, by using strain UA159, we were able to integrate the physiologic data with data from complete genome sequence and transcriptomic analysis by use of a microarray. In addition, we further characterized the physiology of a strain bearing a deletion in the glucose/mannose EIIAB permease gene, manL, which has been shown to play a dominant role in CCR of genes in S. mutans and to regulate multiple virulence attributes of the organism (20,25,45,51). The results reveal that there are profound changes in the expression of genes and the phenotypic properties of the organism that are known to affect its establishment, persistence, and virulence in response to the amount of carbohydrate in the environment.…”

Effects of the ERES Pathogenicity Region Regulator Ralp3 on Streptococcus pyogenes Serotype M49 Virulence Factor Expression

“…These include both lac operons and the fru operon, which are well characterized in different Gram-positive bacteria (4,33,34,43). GAS encodes two lactose metabolic operons.…”

Comprehensive Mutational Analysis of Sucrose-Metabolizing Pathways in Streptococcus mutans Reveals Novel Roles for the Sucrose Phosphotransferase System Permease

“…In most low-GϩC Gram-positive bacteria, CCR is effected primarily by catabolite control protein A, CcpA, which regulates gene expression by binding to conserved catabolite response elements (CRE) in the promoter regions of CCRsensitive genes, although many alternative mechanisms to selectively regulate carbohydrate uptake and catabolic pathways exist. In S. mutans, CcpA does play an important role in the global control of gene expression (26), but CcpA-independent mechanisms involving the PTS phosphocarrier protein HPr and various EII permeases play dominant roles in CCR, regulating catabolic pathways such as the fruAB operon, the fructose/mannose-PTS encoded by levDEFG (22,27), and transport and catabolism of cellobiose (28) and lactose (29). There are data to support that sucrose may be a preferred carbohydrate source that can elicit CCR in S. mutans (30)(31)(32)(33).…”