Discovery of a pigment produced by Streptococcus pyogenes Abstract This proposal seeks to advance a new observation that the human pathogen Streptococcus pyogenes (Group A Streptococcus, GAS) produces a pigment that has gone unreported. We hypothesize the pigment protects GAS from host-derived and/or environmental stressors (such as reactive oxygen species) and likely enhances its fitness and potentially augments its virulence. For 80 years, no reference in the literature exists of a pigment being produced by this important organism, and prior to that, only two citations allude to such a factor. We suspect this property has been overlooked because its presence only becomes apparent in the laboratory after culturing in a chemically defined medium, grown to high cell densities, and after exposure to oxygen. Understanding if and how the production of a pigment could boost the fitness of GAS could provide a new target that therapeutics or vaccines could take aim upon. The objectives of this proposal are to elucidate the composition and structure of the GAS pigment, to identify genes associated with its production and regulation, and to characterize its ability to protect GAS from stressors and immune cell killing while exploring any cytotoxic properties.