Genomes of Bacteria Living in Ants Deteriorated

The carotenoids that make carrots orange also produce the golden hue of Staphylococcus aureus. Such pigments appear to be a golden armor, shielding this pathogen from oxidants that mammalian neutrophils release, thus accounting for some of this microorganism’s virulence, according to Victor Nizet at the University of California, San Diego (USCD) and his collaborators. Perhaps these colorful virulence factors will provide a new golden opportunity for battling staph infections, which are becoming resistant to standard antibiotics. The UCSD researchers knocked out a key gene for carotenoid biosynthesis, generating colorless Staphylococcus mutants, and also inserted carotenoid genes into normally colorless Streptococcus pyogenes, making it turn yellow. “We moved the first two genes in the pathway from staph into pyogenes. That was enough to create some yellow pigments, although not the rich-colored pigments that staph has,” Nizet says. Making staph colorless renders it more susceptible to killing by human neutrophils or by hydrogen peroxide in vitro. When injected into the skin of mice, the colorless staph mutant does not produce characteristic abscesses, whereas large lesions grow within four days after injections of wildtype S. aureus. Meanwhile, the yellow strep transformant becomes more resistant to oxidants and neutrophil killing. It also gains the capacity to produce abscesses when injected into mouse skin, according to the UCSD researchers, whose study appears in the July 17 issue of The Journal of Experimental Medicine. Another carotenoid pigment gives Group B Streptococcus (GBS) a distinctive orange tinge, according to George Liu, a Nizet collaborator. While culturing GBS mutants lacking carotenoids, Liu noticed that these bacteria fail to grow in test tubes that are slightly contaminated with bleach. Liu also recognized that white blood cells release hypochlorite, the active ingredient of bleach, as a means for killing bacteria. Wild-type GBS survives inside macrophages and neutrophils. However, macrophages and neutrophils, as well as hydrogen peroxide, hypochlorite, and singlet oxygen, kill carotenoid-deficient GBS mutants, according to a report by Liu, Nizet, and their colleagues in the October 2004 Proceedings of the National Academy of Sciences. “Anyone working with a colorful