Besides the photosynthetic bacteria, many other bacteria produce characteristic pigments. Biosynthesis of such non-photosynthetic pigments is genetically controlled and, therefore, pigmentation can be used as reliable taxonomic characters in identification.
Some pigments are insoluble in aqueous media and they remain restricted to the cells, while others are soluble and they diffuse from the cells into the surrounding medium. Sometimes the pigments are present only in conidia, as in many actinomycetes, and the mycelium is hyaline.
Many bacteria are characterized by pigmented colonies produced on agar surfaces ranging in colour from red, orange or yellow. The pigments of these bacteria are insoluble and belong to the carotenoids. Examples of such bacteria are various species of Micrococcus, Corynebacterium, Mycobacterium, Nocardia etc. Serratia marcescens produces a deep red insoluble pigment, called prodigiosin, consisting of pyrrol rings.
Several bacteria, like Pseudomonas indigofera, Arthrobacter atrocyaneus etc. produce an insoluble blue pigment, called indigoidin. It is an azaquinone derivative. Pseudomonas aeruginosa produces a soluble pigment, pyocyanine, a phenazine derivative. An insoluble violet pigment, violacein, is an indole compound produced by Chromo-bacterium violaceum.
Structures of some of these pigments are given in Fig. 2.39.:
The bacterial pigments are secondary metabolic products. They are generally involved in protection of the bacterial cells from harmful effects of visible and ultraviolet light. In many bacteria, formation of pigments—particularly the yellow-orange carotenoids—depends on exposure to light. Pigmentation also depends on the medium composition. In some cases, pigments help bacteria in phototactic movement.
Protection given by carotenoid pigments against damages induced by photodynamic effects has been demonstrated in orange coloured strains of halophilic bacteria. A colourless mutant of such a bacterium is easily killed by exposure to strong sunlight, but the pigmented wild-type cells can survive and grow normally.
The killing effect of strong light on the colourless mutant is due to photo-oxidation, catalysed by some membrane-localised cell pigments, like flavines and cytochromes. These membrane components make the non-pigmented mutant cells photosensitive, while, in the wild-type, the carotenoids, also localized in the membrane, protect the cells by making them photo-insensitive.