Bacteria are pigmented or colored. Pigmented bacteria are also known as chromobacteria. Bacterial pigments are water soluble or insoluble; water soluble pigments are diffused in the growth medium. Chemically, bacterial pigments are pyrrole, phenazine, carotenoid, xanthophylls and quinine or quinone derivatives. The pigment molecules are synthesized in cell wall or periplasmic space. We can visualize pigmentation in bacteria in specific growth medium or by staining bacterial cells with a dye to observe under microscope. It has been proved that only aerobic and facultatively aerobic bacteria are pigmented because, molecular oxygen is essential for pigmentation. Therefore, anaerobic bacteria are nonpigmented. Pigment synthesis is also dependent on light, pH, temperature and media constituents like indicator dyes. They display all the colors from rainbow including light or dark tinges and unusual colors like black, white, brown, golden, silver and fluorescent green, yellow or blue. For example:
Purple: Spirillum rubrum
Violet: Chromobacterium violacein
Indigo: Janthinobacterium lividum
Blue: Streptomyces coelicolor (actinorhodin edible)
Green: Chlorobium tepidum
Yellow: Xanthomonas campestris (xanthomonadins)
Orange: Sarcina aurentiaca
Red: Serratia marcescens (prodigiosin)
Brown: Rhizobium etli
Black: Prevotela melaninogenica
Golden: Staphylococcus aureus
Silver: Actinomyces sp.
White: Staphylococcus epidermidis
Cream: Proteus vulgaris
Pink: Micrococcus roseus
Maroon: Rugamonas rubra
Fluorescent blue/green: Pseudomonas aeruginosa (Pyocyanin)
Fluorescent yellow: Pseudomonas fluorescens (Pyoverdin/fluorescein)
Detection of bacterial pigments on indicator media: Various types of indicator or differential media are used during the culturing of bacteria. These media are made by using suitable pH indicator dyes such as methylene blue, eosin, methyl red and chemicals like sodium sulphite, potassium tellurite, which change color when bacteria are cultured in them. When bacteria are cultured on indicator medium, growth is observed as colored colonies. MacConkey agar, EMB agar, McLeod agar and TCBS agar are some of the examples of routine indicator media used in Microbiology. On MacConkey agar lactose fermenting bacteria show pink pigmented colonies while as lactose non-fermenters are colorless. These media are very useful in identification and biochemical characterization of particular bacteria.
The question arises: Why do bacteria produce pigments?
Pigmentation is very useful for bacteria as well as for our industries. In bacteria, pigment formation is associated with morphological characteristics, cellular activities, pathogenesis, protection and survival. Autotrophic cyanobacteria contain a green colored pigment, known as chlorophyll (similar to plant chlorophyll). They also contain accessory pigments phycobilin and chlorophyll b which are required in photosynthesis. Other photosynthetic bacteria have pigments bacteriochlorophyll, proteorhodopsin and bacteriorhodopsin similar to chlorophyll. It seems that in autotrophic bacteria, pigments are needed to carry out the process of photosynthesis. Pigments of photosynthetic bacteria carry out photosynthesis similarly like plant chlorophyll.
Pigments are produced by bacteria to absorb UV radiation or to quench oxygen free radicals. In both the cases bacterial pigment play important role of the cell protection. Some bacterial pigments are antibiotics which are active against phytopathogenic fungi, bacteria, and yeasts; also active against human pathogenic Gram positive or negative bacteria and fungi. Pigments prodigiosin (Serratia), erythromycin (from Streptomyces), pyocyanin, pyoverdin and pyochelin from Pseudomonas, spirilloxanthin of Spirillum are potent antibiotic pigments.
Bacterial pigments help in survival in stress conditions. For example, in rhizosphere region iron is always present in limited amount; rhizobacteria like pseudomonads produce iron chelating compounds or siderophores. Siderophores scavenge traces of iron and make available to host plant. They also eradicate pathogenic fungi and bacteria by depriving them for iron. Pigments produced by Pseudomonas spp. like pyoverdin and pyochelin act like siderophore. Extremophiles are very colorful. Bright pigmentation of extremophilic bacteria offers protection from oxidative stress. Pigments also maintain membrane integrity and stability. The pigments of extremophiles are also required in respiratory or photosynthetic functions.
Pigments confer antibacterial and heavy metal resistance. Pathogenic staphylococci are multidrug resistant because of their pigment which acts as barrier for antibiotics acting on cell wall and plasma membrane. Bacteria showing heavy metal resistance are usually pigmented as they have been exploited for remediation of soil and water polluted by heavy metals like arsenic, copper, cadmium, mercury and nickel. Pigmented bacteria have also been used as biosensors to detect environmental pollution like oil spills or pesticide and heavy metal recalcitrance.
Many important applications of bacterial pigments are enlisted:
Resistance to phagocytosis
Heat resistance and acid stability
Unpalatability to protozoa
In vitro antibody formation enhancers
In paint formulations
Alternatives to color additives of plant origin
In textile dyeing
Source of vitamin A
Indicators of oil spill
Biosensors and markers of water, soil and air pollution
Pigments are important characteristics of particular genus and are very helpful in the identification and classification of microorganisms. The best example of pigmentation is Xanthomonas spp. All the species of Xanthomonas produce yellow colored pigments known as xanthomonadins. Taxonomically, xanthomonadin synthesis is an important trait because they have similar chromatographic and absorption spectra which form the basis of classification of xanthomonads.
The molecular genetic studies of pigment synthesis present vital scope for scaling up industrial importance of useful pigmented bacteria.
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