Some natural surfaces can produce complex optical effects like rain droplets or a glass prism by selectively reflecting light of specific wavelengths and, therefore, specific colors. One optical effect, in particular, the one called iridescence, produces some of the most intense colorations in nature such as the rainbow-like coloration of soap bubbles, the inside of some shells, and the bright colors of the exoskeleton of some insects. The word iridescence originates from the Greek iris, which means ‘rainbow’, and refers to the optical property of some surfaces to change color and its intensity with the illumination or the viewing angle. Iridescent surfaces are uniquely structured in a way that causes the reflected light waves to interact physically with each other. The crests and troughs of the reflected light waves sometimes align (they are ‘in phase’) and reinforce each other, thus increasing the intensity of the reflected color. By contrast, if the reflected light waves are out of phase, they can cancel each other out and those particular colors never manifest. The final effect of this optical interference is the production of one or more predominant colors, the type and intensity changing with the angle of illumination and/or observation. Thus, iridescent colors are ‘structural’: they do not result from pigmentation but from physical interactions between light and surfaces.
Although iridescence is a widespread phenomenon in nature, I was genuinely surprised to read a recent paper by Eric Rosenfeld’s group published in Applied and Environmental Microbiology reporting that several bacteria, including some well-known laboratory strains of Pseudomonas aeruginosa and Haemophilus influenzae, are iridescent. The paper begins with a fine introduction about what is known in the field. Bacterial iridescence was first reported in 1904 but has been loosely and poorly described thereafter. Many reports used (or misused) epithets such as “shine,” “sheen,” “glistening”, “metallic effect,” “bright”, “luster,” “glow,” “glisten,” or “rainbow-like” For example, the authors showed that the ‘metallic iridescence’ previously reported for some strains of P. aeruginosa is not angle-dependent. Thus, these colonies are not truly iridescent. In some cases, fluorescence was mistaken as iridescence. I must admit that I have used some of these terms loosely in the past, blissfully ignorant of what I was observing.The researchers unified these epithets employing a rigorous classification of bacterial iridescence by using two microscopic techniques: epi-illumination (where illumination and detection take place on the same side of the sample) and trans-illumination (which detects the light transmitted through the sample). They investigated and described in detail the iridescent properties of colonies of several strains, including an iridescent strain (strain BK) of the marine bacterium Cellulophaga lytica (formerly known as Cytophaga lytica). This strain was isolated from the surface of a red anemone and grew into colonies displaying a glitter-like green coloration under direct epi-illumination. Other strains of C. lytica available in pure culture, including the only sequenced strain of the group (DSM7489), were either non-iridescent or exhibited low-intensity iridescence. In fact, the intense green iridescence displayed by C. lytica strain BK is described by the authors as ‘unmatched in the bacterial kingdom’ and to rival that observed in some insects and vertebrates.
"Iridescence of a Marine Bacterium and Classification of Prokaryotic Structural Colors"
Via Cesar Sanchez, Dr. Stefan Gruenwald