Analysis of multiple group A Streptococcus (GAS) genomes shows that genes encoding proteins involved in carbohydrate utilization comprise some 15% of the core GAS genome. Yet there is a limited understanding of how carbohydrate utilization contributes to GAS pathogenesis. Previous genome-wide GAS studies led us to a focused investigation of MalE, a putative maltodextrin-binding protein. Analysis of 28 strains of 22 distinct M protein serotypes showed that MalE is highly conserved among diverse GAS strains. malE transcript levels were significantly increased during growth in human saliva compared to growth in a chemically defined glucose-containing medium or a nutrient-rich medium. MalE was accessible to antibody binding, indicating that it is expressed on the GAS cell surface. Moreover, growth in human saliva appeared to increase MalE surface expression compared to growth in a nutrient-rich medium. Analysis of a ⌬malE isogenic mutant strain revealed decreased growth in human saliva compared to wild-type GAS. Radiolabeled carbohydrate binding assays showed that MalE was required for the binding of maltose but not glucose. The ⌬malE isogenic mutant strain colonized a lower percentage of GAS-challenged mice compared to wild-type and genetically complemented strains. Furthermore, decreased numbers of CFU were recovered from mice infected with the ⌬malE strain compared to those infected with wild-type GAS. These data demonstrate that maltodextrin acquisition is likely to be a key factor in the ability of GAS to successfully infect the oropharynx. Further investigation into carbohydrate transport and metabolism pathways may yield novel insights into GAS pathogenesis.

Study of the maltose/maltodextrin binding protein MalE in Escherichia coli has resulted in fundamental insights into the molecular mechanisms of microbial transport. Whether gram-positive bacteria employ a similar pathway for maltodextrin transport is unclear. The maltodextrin binding protein MalE has previously been shown to be key to the ability of group A Streptococcus (GAS) to colonize the oropharynx, the major site of GAS infection in humans. Here we used a multifaceted approach to elucidate the function and binding characteristics of GAS MalE. We found that GAS MalE is a central part of a highly efficient maltodextrin transport system capable of transporting linear maltodextrins that are up to at least seven glucose molecules long. Of the carbohydrates tested, GAS MalE had the highest affinity for maltotriose, a major breakdown product of starch in the human oropharynx. The thermodynamics and fluorescence changes induced by GAS MalE-maltodextrin binding were essentially opposite those reported for E. coli MalE. Moreover, unlike E. coli MalE, GAS MalE exhibited no specific binding of maltose or cyclic maltodextrins. Our data show that GAS developed a transport system optimized for linear maltodextrins longer than two glucose molecules that has several key differences from its well-studied E. coli counterpart.Analysis of microbial carbohydrate physiology has been a fertile area of scientific research for many decades. Fundamental discoveries in the areas of transcriptional regulation, selective nutrient utilization, and molecular transport have been derived from studies of microbial carbohydrate acquisition and processing (13,27,34). For example, the uptake and utilization of maltose/maltodextrins by Escherichia coli has become a model for understanding how microbes transport and use complex sugars (3).The study of E. coli MalE, the periplasmic substrate binding protein of the maltose/maltodextrin ATP binding cassette transporter, has been a particularly fruitful area of investigation (32, 33). Although E. coli MalE binds to a variety of ␣-1,4-linked oligoglucosides, including linear, cyclic, reduced, and oxidized maltodextrins, only a portion of the bound ligands is subsequently transported into the cell (10). The interaction of purified E. coli MalE with the actively transported substrates maltose and maltotriose causes an increase in the E. coli MalE maximum emission wavelength (10, 39). The change in the fluorescence emission spectrum correlates with the closed and open forms of the E. coli . The closed form is needed for active transport to occur (11). The binding of E. coli MalE to substrates that are actively transported results in an endothermic reaction that is entropy driven (40). Conversely, the binding of E. coli MalE to nontransported substrates is exothermic and results in a decrease in the maximum fluorescence emission wavelength of E. coli MalE (9, 40). Therefore, study of E. coli MalE has generated data showing two major modes of ligand binding: one in which an endothermic reaction driven by...

Group A Streptococcus (GAS) genes that encode proteins putatively involved in polysaccharide utilization show growth phase-dependent expression in human saliva. We sought to determine whether the putative polysaccharide transcriptional regulator MalR influences the expression of such genes and whether MalR helps GAS infect the oropharynx. Analysis of 32 strains of 17 distinct M protein serotypes revealed that MalR is highly conserved across GAS strains. malR transcripts were detectable in patients with GAS pharyngitis, and the levels increased significantly during growth in human saliva compared to the levels during growth in glucose-containing or nutrient-rich media. To determine if MalR influenced the expression of polysaccharide utilization genes, we compared the transcript levels of eight genes encoding putative polysaccharide utilization proteins in the parental serotype M1 strain MGAS5005 and its ⌬malR isogenic mutant derivative. The transcript levels of all eight genes were significantly increased in the ⌬malR strain compared to the parental strain, especially during growth in human saliva. Following experimental infection, the ⌬malR strain persistently colonized the oropharynx in significantly fewer mice than the parental strain colonized, and the numbers of ⌬malR strain CFU recovered were significantly lower than the numbers of the parental strain CFU recovered. These data led us to conclude that MalR influences the expression of genes putatively involved in polysaccharide utilization and that MalR contributes to the persistence of GAS in the oropharynx.