Structure, biosynthesis and function of cell wall in streptococci
Streptococcus pyogenes (Group A Streptococcus, GAS) is a human pathogen of global significance for which a vaccine is not currently available. The key antigenic surface polymer of GAS, the Lancefield group A carbohydrate (GAC), is composed of a repeating →3)α-Rha(1→2)α-Rha(1→ disaccharide backbone with N-acetyl-β-d-glucosamine (GlcNAc) side-chain modifications attached to the 3-position of the α-1,2-linked Rha. GAC is negatively charged polysaccharides through decoration with glycerol phosphate (GroP) moieties attached to the GlcNAc side-chains of GAC at the C6 hydroxyl group. We are working toward defining the mechanisms of GAC polyrhamnose backbone assembly and transport, and exploring how cell wall modifications alter physical properties of cell wall.
S. mutans is a major etiological agent of human dental caries. Similar to other streptococci, it decorates peptidoglycan with polyrhamnose-based glycopolymers modified with glycosyl side-chains. Based on differences in the glycopolymer structures S. mutans is classified into four serotypes — c, e, f and k. 70–80% of strains found in the oral cavity were classified as serotype c followed by e (about 20%), f and k (less than 5% each). The serotype c-specific carbohydrate (SCC) of S. mutans is composed of a polyrhamnose backbone with α-linked glucose (Glc) side-chains. The goals of this project are to determine the chemical structure of SCC and the role of cell wall modifications in physiology and pathogenesis of S. mutans.
Mechanisms of resistance of Gram-positive bacteria to antimicrobials
The ability of bacteria to cause infections is associated with resistance to host immune defense mechanisms. The key components of innate immunity are cationic antimicrobial peptides and proteins (AMPs). To reach the phospholipid membrane, the cationic AMPs have to traverse the Gram-positive cell envelope which is characterized by the presence of anionic glycopolymers — lipoteichoic acid anchored to plasma membrane and the peptidoglycan-attached wall teichoic acid (WTA). Cell envelope components, peptidoglycan and teichoic acids, modulate the resistance of bacteria to AMPs. For example, modification of teichoic acids with positively-charged d-alanine significantly increases the resistance of Gram-positive bacteria to hGIIA, lysozyme, histones and other AMPs. The goal of this project is to understand the biological mechanisms underlying the resistance of streptococci and enterococci to AMPs.
Regulation of cell division in streptococci
Bacterial cells come in a variety of shapes and sizes. When a bacterial cell grows and reaches a certain size, it divides at cell equators into two daughter cells, each of which retains the original shape. We discovered that in Streptococcus mutans, glycerol phosphate (GroP) modification is attached to cell wall polysaccharide SCC and its spatially regulates cell division. We showed that structurally-diverse SCCs display a specific distribution on the S. mutans cell surface with cell equators and poles being populated by ‘immature SCCs’, which are deficient in the GroP modification. These immature SCCs inform the proper positioning of the major cell separation autolysin AtlA and FtsZ through its cell wall binding regulator MapZ. We aim to elucidate the mechanism linking MapZ localization to the modifications present on SCC.