The characteristic gold color of S. aureus sets it apart from its avirulent relatives, which are mostly unpigmented. The color reflects the production of antioxidant molecules called carotenoids—similar to those originally isolated from carrots and touted for their ability to boost the immune system and decrease tumor growth in humans. Despite the connection between color and virulence of S. aureus, a functional link had never been investigated.

Liu and colleagues now show that these pigmented molecules can also help S. aureus resist the hosts' immune defenses. Carotenoids produced by S. aureus defused the reactive oxygen species (ROS) that normally help neutrophils kill bacteria. Expression of these pigments rendered the normally colorless Streptococcus pyogenes golden and more virulent. And S. aureus that were robbed of the ability to make carotenoids could no longer resist neutrophil attack and were less pathogenic in mice.

The protective effect of the carotenoids on the bacteria was a function of their antioxidant activity, as wild-type bacteria had no advantage over carotenoid-deficient bacteria in mice whose neutrophils lacked the ROS-producing machinery. The authors suggest that drugs that inhibit carotenoid synthesis might be useful for treating S. aureus infections, which are often resistant to traditional antibiotic treatment.