Pigmented Microorganisms

microorganisms that produce pigments. When grown in a streak culture on a solid nutrient medium in a test tube, pigmented microorganisms can display a variety of colors, including yellow, orange, red, green, blue, violet, or black. The color of the colonies may be caused either by pigmentation of the cells themselves or by the release of colored substances into the medium. The color, which serves as a means for identification, is reflected in the species name of the microorganism. For example, the bacterium of the genus Sarcina that produces yellow pigment is called Sarcina lutea, that is, “yellow Sarcina.” Pigmented microorganisms are found among all taxonomic groups of microorganisms, including cocci, spore-forming and nonspore-forming bacteria, spirilla, ac-tinomycetes, yeasts, and microscopic fungi.

The composition of the nutrient medium has a great effect on the rate of pigment formation in pigmented microorganisms. The pigments vary considerably in chemical nature, for example, carotenoids are hydrocarbons, certain other pigments are phena-zine derivatives, and anthocyanins and the black pigments— melanins—are aromatic compounds. The photoautotrophic bacteria, which are capable of carrying out photosynthesis, contain bacteriochlorophylls, substances that differ in chemical structure from the chlorophyll of higher plants. Carotenoids protect pigmented cells against ultraviolet radiation, and therefore many pigmented microorganisms inhabit the air; in photoautotrophic bacteria, carotenoids take part in photosynthesis. Some pigments possess antibiotic properties.