Staph aureus excels at evading the human immune system, and its innocuous-seeming color is just one more of its tricks.
The characteristic golden color was noted when it was first isolated from infected surgical wounds in 1881[1]
The pigment is a carotenoid, a cousin to the anti-oxidant vitamin A found in carrots and other yellow vegetables. The variant found in Staph is called staphyloxanthin:
Staphyloxanthin is a virulence factor, a compound that promotes infection[2] . White blood cells, especially neutrophils, secrete highly toxic oxidizing compounds when they encounter bacterial infections[3] . These reactive oxygen species (ROS) include superoxide, hydrogen peroxide, and hypochlorous acid, aka chlorine bleach. They are a key part of the innate immune system that helps us stop bacterial infections before they can get established.
Mutant Staph strains deficient in staphyloxanthin are susceptible to being killed by neutrophils in test tubes, and are less able to cause infections in mice. They do just fine when infecting ROS-deficient mice.
Inhibitors of staphyloxanthin synthesis are being developed as antivirulence therapies for treating infections[4] . Rather than kill bacteria outright, as antibiotics do, antivirulence therapies seek to render bacteria harmless. The hope is that these therapies will prove less disruptive to the microbiome than antibiotics, and that resistance to them will develop more slowly.
Footnotes
[1] https://www.ncbi.nlm.nih.gov/pmc…
[2] Staphylococcus aureus golden pigment impairs neutrophil killing and promotes virulence through its antioxidant activity.
[3] Neutrophil-generated oxidative stress and protein damage in Staphylococcus aureus.
[4] Novel Inhibitors of Staphyloxanthin Virulence Factor in Comparison with Linezolid and Vancomycin versus Methicillin-Resistant, Linezolid-Resistant,… – PubMed – NCBI