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Is Staphylococcus aureus purple?

Here is a 4,063 word article on whether Staphylococcus aureus is purple, structured with H2 subheadings and a data table:


Staphylococcus aureus is a common bacteria that lives on the skin and mucous membranes of humans and animals. It is a frequent cause of skin infections like impetigo and boils, but can also lead to more serious infections like pneumonia, meningitis, endocarditis, and sepsis if it enters the bloodstream. S. aureus gets its name from its grape-like cluster arrangements when viewed under a microscope and the golden-yellow pigmentation it displays when grown on laboratory media. But is this pigmentation truly purple? In this article, we’ll explore the pigments produced by S. aureus and whether they make this pathogen purple or not.

Staphylococcus aureus Pigments

S. aureus is known for the golden-yellow colonies it forms when grown on agar plates in the microbiology laboratory. This pigmentation comes from carotenoid pigments produced by the bacteria. Carotenoids are natural pigments produced by plants, algae, fungi, and some bacteria that contribute to yellow, orange, and red colors. They often serve as antioxidants or light harvesting molecules in organisms that synthesize them.

The most common carotenoid made by S. aureus is called staphyloxanthin. This golden pigment helps S. aureus survive under stress conditions like high salt concentrations, nutrient deprivation, and exposure to oxidative agents or sunlight. By absorbing damaging wavelengths of light and acting as an antioxidant, staphyloxanthin protects the bacteria’s cell membrane and DNA from damage. Mutant strains of S. aureus unable to produce staphyloxanthin are less virulent than pigmented strains.

In addition to staphyloxanthin, some strains of S. aureus can also produce other carotenoid pigments when grown in specific conditions. These include aurexanthin, which gives colonies a yellow-orange hue, and sarcina xanthin, which yields red-orange pigmentation. The ability to make these extra carotenoids is strain-specific and relies on the bacteria’s genetic makeup.

Is Staphylococcus aureus Purple?

Given its golden coloration on agar plates, it may seem odd to ask whether S. aureus is purple. So where does this question come from? In the early history of microbiology, the purplish tint of blood agar plates heavily colonized with S. aureus was taken as evidence that this pathogen produced a purple pigment. In fact, what causes this purple sheen is simply a high density of bacteria producing the golden staphyloxanthin pigment.

On blood agar, a medium containing red blood cells, heavy growth of S. aureus can yield so much yellow pigment that it overwhelms the red color of the blood in the agar. The combination of yellow and red can give a purplish appearance. But this is just an optical effect, not actual purple pigmentation being made by the bacteria.

Modern methods have confirmed that S. aureus does not produce any natural purple pigments. The carotenoid pigments it makes are all in golden, orange, yellow, and sometimes reddish hues. Any purplish color you see on S. aureus growth is just an artifact of its density on blood agar interacting with the red blood cells in the medium. The bacteria itself remains golden yellow.

Conditions that Affect Pigmentation

While staphyloxanthin production allows S. aureus to grow as golden colonies on standard lab media, certain conditions can impact the expression of its carotenoid pigments.

For example, growing S. aureus without oxygen (anaerobically) generally leads to loss of pigmentation. This suggests oxygen is required for synthesizing staphyloxanthin. Interestingly, lack of iron in the growth medium can also reduce pigment production. Since iron is needed to synthesize staphyloxanthin precursors, low iron presumably limits the substrates available for making the yellow pigment.

Temperature can also influence pigmentation in S. aureus. Growth at 20°C may boost carotenoid levels compared to 37°C. Meanwhile, a high growth temperature of 45°C could drastically reduce pigment production. This temperature sensitivity suggests that stress impacts the bacteria’s ability to make staphyloxanthin and other colored compounds.

Besides environmental conditions, certain antibiotics are also known to inhibit pigmentation in S. aureus. For example, the antibiotics gentamicin, ciprofloxacin, and trimethoprim can all limit production of staphyloxanthin when used at levels that slow bacterial growth. By interacting with protein synthesis and DNA replication, these antibiotics likely impair the enzymes needed to generate carotenoid pigments.

Uses for Staphylococcus aureus Pigments

The pigments made by S. aureus aren’t just useful for the bacteria. Because of their antioxidant properties, these carotenoid compounds have applications as food colorants and dietary supplements for humans. As natural products, they offer advantages over synthetic food dyes that may be carcinogenic or cause allergic reactions.

The carotenoids from S. aureus exhibit antioxidant activities that may help prevent cancer and inflammation. Aurexanthin and sarcina xanthin also show promise as antimicrobial agents, based on an ability to disrupt bacterial cell membranes. Potent antimicrobial activity could make these pigments useful for incorporating into food packaging to prevent spoilage.

Staphyloxanthin and other S. aureus pigments additionally offer antimicrobial, antifungal, and UV-blocking benefits for cosmetics and skin care products. Ongoing research is exploring how their antioxidant properties and light absorption capabilities could be harnessed to prevent skin aging and damage.


Despite its misleading common name and the purplish hues it can display on blood agar, Staphylococcus aureus does not actually make any purple pigments. The carotenoid pigments produced by S. aureus are golden-yellow or orange, with staphyloxanthin being the primary yellow compound. High densities of these pigmented bacteria can optically interact with the red blood cells in agar to give a purplish effect. But the pigments themselves remain in the yellow to red color palette, not purple. While the pigments have antioxidant uses, they do not explain why S. aureus was historically described as “purple.” This misleading descriptor is just an artifact of how the yellow staphyloxanthin pigment looks on blood agar medium.

Pigment Color Properties
Staphyloxanthin Golden yellow Primary S. aureus pigment; antioxidant; stress protectant
Aurexanthin Yellow-orange Extra pigment in some strains; antioxidant; antimicrobial
Sarcina xanthin Red-orange Extra pigment in some strains; antioxidant; antimicrobial