Group A Streptococci (GAS), or Streptococcus pyogenes, is a human pathogen responsible for substantial worldwide morbidity and mortality. GAS peptidoglycan is decorated with the Lancefield group A carbohydrate (GAC) which consists of a polyrhamnose backbone with N-acetylglucosamine (GlcNAc) side-chains. Understanding the molecular structure and function of GAC is of paramount importance for developing new approaches to combat GAS because GAC plays an essential role in GAS viability and virulence. Moreover, GAC has been previously targeted for vaccine development and used in clinical diagnostics. Using molecular genetics, enzymology, analytical chemistry and mass spectrometry analysis we have elucidated the molecular mechanism of GAC modification with the N-acetylglucosamine (GlcNAc) side-chains. We have discovered a new modification, i.e. glycerol phosphate (GroP) moiety, to the GAC and Streptococcus mutans cell wall polysaccharide and showed that the GroP modification is required for GAS and S. mutans tolerance to toxic levels of Zn2+ and for maximal sensitivity to the bactericidal enzyme, human group IIA secreted phospholipase A2 (hGIIA). We have identified and structurally characterized the enzyme, known as GacH, responsible for the modification. Finally, we discovered that the GroP modification is likely to be a common feature in many lactic acid bacteria.
We are currently working toward defining the mechanisms of GAC polyrhamnose backbone assembly and transport, and exploring how cell wall modifications alter physical properties of cell wall. Current efforts are also aimed at dissecting the function of cell wall modifications in resistance to antimicrobials and in GAS pathogenesis.