Nizet Lab Research - Bacterial Carotenoid Pigments

	Carotenoid Pigments in Immune Resistance

Carotenoids that give color to dietary fruits and vegetables are well recognized as potent antioxidants by virtue of their free radical scavenging properties and exceptional ability to quench singlet oxygen. We have been exploring how pathogenic bacteria might take advantage of the antioxidant effects of carotenoid pigments to extend their own lives, by inactivating chemicals deployed by neutrophils and macrophages that are lethal to most bacteria.

In GBS, the pore-forming ß-hemolysin/cytolysin (ßh/c) encoded by cylE is an important virulence factor as demonstrated in several in vivo models by our group and others. Interestingly, cylE deletion results not only in the loss of ßh/c activity, but also in the loss of a carotenoid pigment whose function had been previously uncharacterized. In our recent studies, we sought to define the mechanism(s) by which both cylE-encoded phenotypes could contribute to GBS phagocyte resistance and increased virulence potential. We found that cylE-deficient GBS was more readily cleared from a mouse's bloodstream, human whole blood, and isolated macrophage and neutrophil cultures. Survival was linked to the ability of ßH/C to induce cytolysis and apoptosis of the phagocytes. However, at lower bacterial inoculum, cylE also contributed to enhanced survival within phagocytes that we hypothesized could be attributed to the ability of carotenoid to shield GBS from oxidative damage. In oxidant killing assays, cylE mutants were shown to be more susceptible to hydrogen peroxide, hypochlorite, superoxide, and singlet oxygen. Together, these data suggest a mechanism by which the linked cylE-encoded phenotypes act in partnership like a sword (ßh/c) and shield (carotenoid pigment) to thwart the immune phagocytic defenses.

Ogston (1881) coined the genus Staphylococcus to describe grapelike clusters of bacteria (staphylo = grape, Gr.) recovered in pus from surgical abscesses (1). Shortly thereafter, Rosenbach (1884) isolated the major human pathogen in pure culture, and proposed the species name S. aureus (golden, Lat.) for its characteristic surface pigmentation in comparison to less virulent staphylococci that normally colonize the skin surface. Subsequent studies of the S. aureus pigment have unraveled an elaborate biosynthetic pathway that produces a series of carotenoids. We hypothesized that S. aureus could utilize its golden carotenoid pigment to resist oxidant based clearance mechanisms of the host innate immune system. In our studies we demonstrated a role of this hallmark phenotype in virulence. Compared to the wild-type bacterium, a S. aureus mutant with disrupted carotenoid biosynthesis is more susceptible to oxidant killing, impaired in neutrophil survival, and less pathogenic in a mouse subcutaneous abscess model. The survival advantage of wild-type S. aureus over the carotenoid-deficient mutant is lost upon inhibition of neutrophil oxidative burst or in human or murine NADPH oxidase-deficient hosts. Conversely, heterologous expression of the S. aureus carotenoid in the nonpigmented Streptococcus pyogenes confers enhanced oxidant and neutrophil resistance and increased animal virulence. Blocking S. aureus carotenogenesis increases oxidant sensitivity and decreases whole blood survival, suggesting a novel target for antibiotic therapy.