Color is in the Eye of the Bac­te­ria-Be­holder

by Janie

Pig­men­ta­tion is a straight­forward color­ation: the color is the same from every view­ing an­gle. In con­trast, struc­tural color­ation is in the eye of the be­holder: the col­ors are de­pend­ent on the view­ing an­gle.

Fig­ure 1. (A and B) Pho­tographs of WT and three mu­tant colonies, taken at two dif­fer­ent an­gles to demon­strate the an­­gle-de­pen­dent co­loration. WT and M16 pro­duce vivid col­oration, M17 pro­duces some col­oration, and M5 pro­duces very lit­tle. (Scale bars in A and B, 1 cm.) (C–G) Mu­tants M22, M65, and M41 (left to right in all pho­tographs) pho­tographed from five dif­fer­ent an­gles un­der the same il­lu­mination show­ing the an­gle-de­pen­­dent va­riation in color. D is photo­graphed from di­rectly above, where­as the other ob­servation an­gles are oblique. (Scale bars in C–G, 1 cm.) Source

We prev­iously cov­ered the knack of var­i­ous bac­te­ria for syn­thesiz­ing true blue pig­ments, which fre­quently serve de­fen­sive pur­poses. Flavobac­te­ria turn out to have a pen­chant for struc­tural color­ation: they can as­sem­ble into 2D pho­ton­ic crys­tals, which are pe­ri­odic assem­blages that re­flect light only at cer­tain an­gles. Flavobac­terium strain Iri­des­cent 1 (IR1) cells, iso­lated from a har­bor in the Nether­lands, ar­range into dense hex­agons that to­gether inter­fere with light to ap­pear a flashy green. Fig­ure 1 con­firms that they owe their vivid pizz­azz to struct­ural color­ation, since the appar­ent color of each colony changes with the view­ing an­gle!

This organiza­tion is no easy feat. Self-as­sem­bl­ing into these highly reg­u­lar struc­tures con­sumes both ge­netic cod­ing ca­pacity and meta­bolic re­sources – so why do it?

A study from 2018 by Jo­hansen et al. discov­ered a link be­tween the pili-de­pen­­dent glid­ing motil­ity character­istic of Flavobac­te­ria and struc­tural color­ation. IR1 mu­tants with defec­tive motil­ity were un­able to self-or­ga­nize into pe­ri­odic hexa­gonal struc­tures and were conse­quently dull-col­ored. The au­thors then sifted out genes in­volved in struc­tural color­ation with a trans­poson mutagen­esis screen. The ma­jor­ity of these hits were in­volved in glid­ing motil­ity, plus some with no prev­iously as­signed func­tion.

Fig­ure 2. Top panel: A Flavobac­terium IR1 cells ar­range into 2D pho­tonic crys­tals, which are pe­ri­odic struc­tures that re­flect green light with strong in­ten­sity at cer­tain an­gles. B The cells in this case ap­pear green be­cause they re­flect green light but trans­mit blue and red. C If the or­derly pe­ri­od­ic­ity is dis­rupted, the in­tensity of re­flected green light is re­duced. Bot­tom panel: 1D, 2D, and 3D pho­­tonic crys­tals. Source (top), Source (bot­tom)

The pres­ence of macro­algae also in­flu­ences the bac­teria, which nat­u­rally de­grade poly­mers from al­gae in their es­tu­arine environ­ments. In the 2018 study, the IR1 colonies ad­justed their pe­ri­odic struc­ture – and hence, their appar­ent color­ation – depend­ing on what al­gal poly­mer was avail­able. The pres­ence of fu­coidan, carra­geenan, or starch re­sulted in red, blue, and pur­ple color­ation in add­ition to the nor­mal green. The re­moval or add­ition of pow­dered fu­coidan caused the loss or restor­ation, respec­tively, of struc­tural color­ation. Fu­coidan again plays a cu­ri­ous role (we prev­iously cov­ered other fu­coidan-de­grad­ing bac­teria here).

A more re­cent study from 2020 by Hamid­jaja et al. linked struc­tural color­ation to an­other trait of the IR1 Flavobacte­ria : pred­ation. Like their rel­a­tive Flavobac­terium johnson­iae, the IR1 cells can in­vade ad­ja­cent colonies and prey upon the cells in a con­tact-de­pen­dent man­ner. When the research­ers spot­ted IR1 and prey bac­te­ria onto agar, IR1 sur­rounded and in­vaded the prey colonies, and the pur­ple and red color­ation typ­i­cal of growth on me­dia contain­ing fu­coidan shifted to a bright green. This indic­ated that IR1 was self-or­ga­niz­ing into 2D pho­tonic crys­tals dur­ing pred­ation.

Fig­ure 3. In­oc­u­la­tion of IR1 ad­ja­cent to B12 on agar sup­ple­mented with fu­coi­dan, show­ing the re­sult 10 h af­ter con­tact be­tween the spread­ing colony of IR1 and the sta­tic mass of B12. IR1 sur­rounds the B12 colony (w) and cre­ates breaches (x) in the thicker edge of the B12 colony. There is a shift in IR1 from dull pur­ple/ red struc­tural col­oration to green (y). Source

As ex­pected, mu­tants that could not glide were un­able to pen­e­trate prey colonies. More surpris­ingly, mu­tants that could glide but were un­able to pro­duce struc­tural color­ation were also un­able to pen­e­trate prey colonies. So, the abil­ity of the cells to or­ga­nize into pho­tonic crys­tals is re­quired for the abil­ity to en­gage in pred­atory behav­ior. This 2020 study marks the first clear-cut link be­tween such struc­tural color­ation and biolog­ical rele­vance in bac­te­ria.

IR1 also ap­pears to pre­fer to glide along edges – like those of a glass slide pro­vided by research­ers, or the edges of prey colonies – rather than a flat ex­panse of agar. Track­ing along edges in this way may be a strat­egy for quickly sur­rounding prey. IR1 isn't picky about its food; the range of prey in­cludes both Gram-pos­i­tive and Gram-neg­a­tive bac­teria as well as the yeast Can­dida al­bi­cans, but its ap­petite also isn't indiscrim­inate. While IR1 ap­proached and sur­rounded other flavo­bacterial colonies as quickly as it did prey colonies, it did not prey on its kin, indi­cating that it prefers to act (sur­round other colonies) be­fore think­ing (dis­cern be­tween self and other).

Fig­ure 4. Im­ages of PIR4 (P, white) ap­parently mov­ing to­wards and de­grad­ing a colony of IR1 (IR1 SC green) af­ter 30 and 48 h (left and right, re­spec­tively). Scale bar in­di­cates 5 mm. Source

What of the flip­ped sit­u­a­tion, in which the hunter be­comes the hunted? The au­thors iso­lated Rhodococ­cus spp PIR4, which preys on IR1. When spot­ted on agar, PIR4 in­vaded and de­graded both wild-type IR1 and IR1 def­icient in struc­tural color­ation. Organiz­ation of IR1 into pho­tonic crys­tals of­fered no protec­tion against pred­ation – ev­i­dently, a good of­fense isn't al­ways the best de­fense.

IR1 isn't the only case of struc­tural color­ation in bac­te­ria. This trick­ery of light has also been ob­served in other Flavobac­te­ria such as Cel­lu­lophaga lyt­ica, iso­lated from anemone. Struc­tural color­ation in all of these bac­teria is an intrin­sically multi­cellular behav­ior, as it is only pos­sible at the colony level and not in indi­vidual cells. Evi­dently color­ation has snuck into the multi­cellular pred­ator's tool­kit, whether in eukary­otes like tigers and vipers or in bac­teria.