Bacterial Blues

by Janie
 

Fig. 1. Girl with a Pearl Earring, Johannes Vermeer c. 1665. Source

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.
 

Fig. 2. Streptomyces coelicolor colonies. Source

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.
 
 

Fig. 3. Left: Glaukothalin isolated from Rhein­heimera sp. Strain HP1. Frontis­piece. Source. Right: Glaukothalin chemical structure. Source

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.
 
 
 
 

Fig. 4. Color changes of inoculated G plates, num­bers showing the hours of incubation at 16 °C. Source

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.
 
 
 
 
 
 
 

Fig. 5. The proposed pathway for the divergent biosynthesis of MIN and indigoidine. Source

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.
 

Fig. 6. Different enzymatic routes towards indigo, either via dioxygenation (A), direct hydroxylation to indoxyl (B), or via epoxidation (C). Source

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.