The color blue is an oddity. Of all pigments in nature – chemicals that selectively absorb and reflect certain wavelengths of visible light – blue ones are among the rarest. Producing blue dyes was once such a costly business that medieval Europeans considered it to be as precious as gold. Blue was a marker of wealth and status, and artists reserved its use only for the most important occasions, as in the striking ultramarine blue of the headscarf in Vermeer's Girl with a Pearl Earring.
Natural true blue pigments are exceptional. The only one known so far to be produced by animals is found in the wings of the obrina olivewing butterfly. Flowers such as hydrangeas and fruits such as blueberries partially owe their coloration to a shifty family of compounds called anthocyanins – a conditional blue. The molecules are red in acidic conditions and blue (or even black) in alkaline conditions, with a sliding scale of purple in the middle.
Then what about all the other apparent blues in nature? In living organisms, the coloration we see is due either to pigments or to the physical properties of the organism's surface that perform a bit of sleight of hand with optics. Members of the latter, which owe their appearance to microscopic structures that scatter and refract light, are called structural colors. The vast majority of "blues" in nature are structural, including bird feathers, poison dart frog skin, and people's eyes. In parrots, for example, reds and yellows are due to psittacofulvin pigments (which, interestingly, confer resistance to feather degradation by Bacillus species) but all bird blues are structural. Any kind of iridescence or metallic luster, as in peacock feathers or the carapaces of jewel beetles, owe their glitter to structural colors (see a previous STC post on iridescent bacterial colonies here). Color-changing camouflage, as in chameleons and octopuses, is also structural, thanks to manipulation of the spacing between pigment crystals. "True blue" is indeed rare.
Microbes, however, seem to be unusually talented producers of blue pigments. To find variations on the blue pigment theme, look no further than the secondary metabolites of the tiniest organisms.
First up, some redox classics from some soil-dwellers. Pyocyanin from Pseudomonas aeruginosa is a blue-green pigment that can kill competing microbes by causing redox-stress. It's also involved in a fascinating molecular ferry system for shuttling electrons throughout a biofilm. Phenazine pigments like pyocyanin from several bacterial genera can produce every color in the visible spectrum, and blue is no exception. Yet another redox-active antibiotic is actinorhodin, a blue polyketide hailing from Streptomyces coelicolor. Here's a wonderful bacterial name: coelus is Latin for "sky" and is a nod at the color of the celestial expanse, again encapsulated in the Spanish word for light blue, celeste.
Glaukothalin (from the Greek glaukos for "blue" and thalatta for "sea") is aptly named: this deep-blue pigment is from Rheinheimera baltica, a blue marine bacterium that was first isolated from the Baltic Sea, as its name hints. Glaukothalin inhibits the growth of some other marine bacteria and is cytotoxic to brine shrimp, but its ecological role remains mysterious.
A beautiful deep blue pigment from the plant pathogen Pantoea agglomerans (formerly Erwinia herbicola) is produced only at temperatures above 10°C at high cell density. Unlike R. baltica above, the Pantoea cells themselves are not blue; instead, the water-soluble pigment they produce seeps out into their agar surroundings. This dye also turns pink in acidic conditions, like the plant pigment anthocyanin.
Next up is indigoidine – note that this blue pigment is chemically distinct from indigo, as the former is a bipyridine and the latter is a biindole (more on indigo shortly!). A number of species naturally synthesize this blue, including the plant pathogen Dickeya dadantii (formerly Erwinia chrysanthemi) and various strains of Streptomyces. Roseobacter species are known to exploit the antimicrobial properties of indigoidine to kill competing species, as does one species that coats the eggs of Hawaiian bobtail squids to protect from vibrios. In Streptomyces hygroscopicus, it turns out that a single nonribosomal peptide synthetase is responsible for coordinating switches between synthesizing indigoidine and the antibiotic minimycin; it can synthesize indigoidine all by itself and also initiates the biosynthesis of minimycin. NRPSs have been engineered in fungi to scale up production of the compound.
Finally, indigo, the quintessential blue pigment used to dye textiles as far back as 6000 years ago in ancient Peru. The historical means of production was processing leaves from the plant genus Indigofera, which contain the precursor molecules indican and isatan B. But chemical synthesis took over in 1870 – a process that unfortunately calls for harsh chemicals and generates harmful waste. It turns out that microbes possess an array of redox enzymes that are capable of producing indigo sans the toxic byproducts and environmental pollution. (Note: A molecule related to indigo is responsible for the blue pigment formed when X-gal is broken down by b-galactosidase in X-gal plates!)
Microbe-derived pigments like the ones mentioned here are eco-friendly dyes with myriad applications beyond textiles. Blue pigment production by P. agglomerans is temperature-dependent, so perhaps there is potential for use as a temperature indicator for foods and medicines. Vogesella indigofera only produces indigoidine under a threshold concentration of Cr6+, so perhaps there is potential here for a heavy metal biosensor. The cyanobacterial accessory pigment phycocyanobilin has even been approved by the FDA as a food coloring.
Microbial pigments are also in the works as immunosuppressive and anticancer drugs – a fitting throwback to the methylene-blue origins of drug discovery. (As a medical student, Paul Ehrlich was fascinated by methylene blue, which stains only nerve cells. His insistence on pursuing his newfound obsession with dyes and their "chemical affinity" for certain cells – to the dismay of his medical school professors who thought it a useless distraction – led him to develop important drugs like sulfa antibiotics.)
But it's not only pigments that produce both beautiful and functional coloration in microbes. We'll follow up on this post with another on unexpected functional bonuses of structural coloration in bacteria. In the meantime, see this previous post here for another look at microbial pigmentation.