Pigments

PIGMENTS, various organic compounds that impart color to the tissues of animals and plants. Pigments are extremely widespread in animals and plants and account for the diversity of their colors. The greatest biological significance belongs to the group of pyrrole derivatives, which include respiratory pigments: animal hemoglobin and plant chlorophyll; the same group includes bile pigments (see Bile) and porphyrins (see). Very widespread is the group of carotenoids (see), which are unsaturated hydrocarbons or their oxygen derivatives. Yellow plant pigments of importance in dyeing are mostly derivatives of flavone. Very widespread are pigments determining the blue, red, and purple coloration of most flowers and some fruits, namely the glycoside anthocyanins, which are very close to flavone derivatives. Derivatives of anthraquinone (see) are certain pigments of importance in therapy (aloin, chrysarobin, chrysophanic acid, emodins) and histological technique (carmine). Among the pigments there are also protein substances (proteinides): hemoglobin, oxyhemocyanin (a blue, copper-containing blood pigment of some lower animals), phycocyanin, and phycoerythrin of algae. The structure of many pigments has not been elucidated or has been insufficiently elucidated; besides colored proteinides, these include: hematoxylin (see), the blue cyanocrystallin of the crustacean carapace, which upon the action of acids or when boiled with water turns into red crustaceorubin, tetronerythrin found around the eyes of grouse, the violet-red turacin of the feathers of certain birds, visual purple, melanins, the majority of pigments of normal and pathological urine, pyocyanin, and various pigments produced by microbes, as well as numerous pigments of plants and lower animals.

V. Gulevich. Bacterial pigments, coloring matter that stains a bacterial culture or—in some species—only individual strains a specific color; the pigment produced by bacteria in most species passes into the surrounding environment, staining it more or less intensively. Conditions promoting pigment production are atmospheric oxygen, a specific temperature, and a specific composition of the nutrient substrate. Under anaerobic conditions, pigment is usually not formed; for saprophytes, 20–25° and for pathogenic forms, 37° are the temperatures most favorable for its production; the medium on which pigment is formed most easily and abundantly serves as potato; for the formation of some of them, special conditions are necessary; thus, for the formation of the red pigment prodigiosin, the presence of magnesium, iron, and zinc is necessary. In the overwhelming majority of pigmented bacteria, the pigment plays no role in nutrition and represents a metabolic product diffusing from the bacterial cells into the surrounding fluid or secreted from them in the form of insoluble particles; in some bacteria (green, purple), the pigment performs a definite function in the nutrition of microbes. Fluorescent pigments can play a protective role, since they possess a toxic effect with respect to protozoa (Noack). Bacterial pigments are classified according to their solubility in water, alcohol, etc. Pigments soluble in water include the green-yellow bacteriofluorescein, insoluble in other solvents, the blue pyocyanin, and the blue syncyanin. Insoluble in water but soluble in alcohol are red prodigiosin, also soluble in ether and chloroform, and red-yellow carotene. Insoluble in either water or alcohol is the brown pigment possessed by the nitrogen-fixing species Azotobacter chroococcum. Purple bacteria contain bacteriopurpurin, consisting of red bacterioerythrin and green bacteriochlorin. Among the bacteria that form pigment, mention must be made of the blue pus bacillus (Bac. pyocyaneus, forming pyocyanin), the blue milk bacillus (Bac. syncyaneus, syncyanin), Bac. prodigiosus (prodigiosin), Staph. pyogenes aureus (carotene), Sarcina lutea (carotene), the gonococcus (carotene), purple sulfur bacteria (bacteriopurpurin), and others (see Pigment-forming bacteria, Bacteriopurpurin). Among the bacteria in which only certain strains form pigment, mention should be made of acid-fast bacilli, including the tubercle bacillus.

V. Lyubarsky.