Fungal and Bacterial Pigments: Secondary Metabolites with Wide Applications

1Color affects every bit of life, including the clothes we wear, the furniture in our home, and the allure of food. Just think, for instance, how plants could prepare their own food without chlorophyll or how oxygen could be carried in the body without hemoglobin. It can be said that life on earth depends on pigments.

2The use of pigments as coloring agents has been practiced since prehistoric times. Archaeologists have uncovered evidence that early humans used paint for aesthetic purposes. The use of pigment in prehistoric times was further proven when pigments and grinding equipments, which were between 350,000 and 400,000 years old, were found in a cave at Twin Rivers, near Lusaka, Zambia. Pigments were used in different parts of the world. In Europe, it was practiced during the Bronze Age. In China, dyeing with plants, barks, and insects has been traced back more than 5,000 years. In India, it occurred during the Indus Valley period (2500 BC). Henna was used before 2500 BC, while saffron has been mentioned in the Bible. In Egypt, mummies have been found wrapped in colored cloth, which showed the presence of alizarin.

3The addition of color to food started in Egypt when candy makers added natural extracts to their candy. Similarly, the use of natural colorants in food was seen in Japan in the shosoin text of the Nara period (8th century) that contains references to coloring soybean and adzuki-bean cakes.

4The first synthetic color, mauvine, was developed by Sir William Henry Perkin in 1856 and this development started a revolution in the history of synthetic colorants. Since then, the synthetic color industrial revolution has rapidly proceeded. Synthetic color captured the market due to ease of production, less expensive, no unwanted flavors imparted to food, superior coloring properties, and only tiny amounts are needed to color anything. Sellers at the time offered more than 80 artificial coloring agents. Many color additives at that time had never been tested for their toxicity or other adverse effects, which ultimately led to adverse effects on the health and environment.

5Dyes such as tartrazine, cochineal red, and sunset yellow provoke allergies either on their own or in combination with other colorants. Although, some synthetic colorants that had been approved by the Food and Drug Administration (FDA) for use in foods, pharmaceuticals, and cosmetic preparations were later found to promote cancer. Some synthetic dyes have even been withdrawn from external use due to their apparent hazards. For example, benzidine dyes cause bowel cancer, while carbon black (widely used as printing ink pigment) is thought to be a potential carcinogen. From the environmental point of view, unethical discharge of untreated industrial dye effluents produce toxins and persist for long time due to long periods of stability. The drawbacks of synthetic color have increased the global demand for natural pigments.

6The main sources for natural pigments are plants or microorganisms. The use of plant pigments has many drawbacks such as non-availability throughout the year and pigment stability and solubility. Large scale plant use may lead to loss of valuable species. For these reasons, the process may not consider viable (Downham and Collins). Microorganisms such as fungi and bacteria provide a readily available alternate source of naturally derived pigments. Bacterial and fungal pigments have extensive applications (Table 1) and have an enormous advantage over plant pigments, including easy and rapid growth in low cost medium, easy processing, and growth that is independent of weather conditions.

7There are no reliable published statistics on the size of the color market; however, according to global industry analysts, the demand for organic pigments and dyes is expected to reach almost 10 million tons by 2017. Among the various available pigments, the carotenoids alone are estimated to reach $1.4 billion by 2018.

8Microbial production of β-carotene costs approximately US$1000/kg versusUS$500/kg for synthetic means. Though microbial pigments are several times more expensive, they still can compete with synthetic dyes for being natural and safe. There is an increased push to reduce the production costs for microbial pigments by using low cost substrates or strain improvements, and in the near future, there may be a monopoly market for microbial pigments.

9Textile industries remains the largest consumer of organic pigments and dyes, while faster growth is expected to occur in other industrial sector such as printing inks, paints, and coating agents. The value of the international food colorant market, which was estimated at around $1.15 billion USD in 2007, may also increase in the future due to food coloring approval for use in the food industry.

10Filamentous fungi are known to produce an extraordinary range of pigments such as carotenoids, melanins, flavins, phenazines, quinones, monascins, violacein, and indigo. The use of Monascus for ang-kak (red mold rice) production is the oldest recorded use of fungal pigment. Monascus produce yellow (ankaflavine, monascine), orange (rubropunctatine, monascorubrine), and purple (rubropunctamine, monascorubramine) pigments which are often encountered in Oriental foods, especially in Southern China, Japan and Southeast Asia. Currently, more than 50 Monascus pigments have been identified and studied. More than 50 patents around the globe have been issued concerning the use of Monascus pigments in food. Monascus pigments possess antimicrobial, anticancer, anti-mutagenic, and anti-obesity properties.

11There are more than 200 fungal species reported for carotenes production. Carotenes production was often found in zygomycetes from the order Mucorales, which includes Phycomyces, Blakeslea, and Mucor. In addition to Mucorales, carotene production has been reported in the basidiomycetes genera such as Rhodosporidium, Sclerotium, Sclerotinia, Sporidiobolus, and Ustilago. Ascomycetes such as Aspergillus, Cercospora, Penicillium, and Aschersonia have also been reported for carotenes production.

12Pigments such as anthraquinones, naphthaquinones, dihydroxy naphthalene melanin, flavin, anthraquinone, chrysophanol, cynodontin, helminthosporin, tritisporin, and erythroglaucin were reported by genera such as Eurotium, Fusarium Curvularia and Drechslera.

13Recent literature extensively has reported the interest in marine organisms with respect to the production of new molecules, including new pigments. Indeed, many marine ecological niches are still unexplored. Marine environments have unique features such as low temperatures, absence of light and high pressure and salinity. These conditions induce marine microorganisms to produce unique substances. Genera such as Aspergillus, Penicillium, Trichoderma, and Eurotium have been reported for pigment production. Marine derived fungal pigments are quite similar to terrestrial derived fungal pigments; however, some pigments were obtained only from marine fungi. Yellow pigment (anthracene-glycoside asperflavin-ribofuranoside) produced by Microsporum sp. appears only in marine-derived fungus.

14Several marine-derived endophytic fungi such as Eurotium rubrum, Halorosellinia, Hortaea, Phaeotheca, and Trimmatostroma have been reported for pigment production. Apart from plants, marine fungi also make associations with algae and corals. Reports suggest that marine endophytic fungi produce pigments that help to mimic and often increase the beauty of the associated life form. Fungus like Aspergillus associates with coral skeleton (Porites lutea and Porites lobata) and imparts black bands that are quite similar to the coral color.

15Although several fungal pigments have been reported in the literature, they must satisfy several criteria regarding their toxicity, regulatory approval, stability, and capital investment required to bring the products from Petri dish to the market. Although used for centuries, many microbial pigments are still forbidden in many countries. The best example is the Monascus pigment that has been used in Asia for centuries as a food colorant but forbidden in Europe and United States due to the presence of mycotoxin. In this context, methods were developed to avoid toxin productions.

16Selection of non-pathogenic strains: to evaluate whether toxin production was strain specific, several strains were screened to check toxicity. The toxin production was observed only in some strains, indicating the toxin production was strain specific.

17Through controlling the biosynthesis of the metabolite: toxin production can be controlled through the biosynthesis process; this can be achieved when the metabolic pathway were investigated.

18Media selection: researcher observed that the addition or removal of metal ions, carbon sources, and nitrogen sources can affect toxin production; hence, selection of media plays a crucial role in controlling the toxin production.

19Apart from toxin production, microbial pigments should withstand extreme pH and temperature in order to meet industrial standards. Many fungal pigments are stable at a wide pH range.

20Pigments produced by Monascus purpureus, Isaria farinosa, Emericella nidulans, Fusarium verticillioides, and Penicillium purpurogenum showed improved dyeing ability at acidic pH (pH 5). Pigment produced by Thermomyces was stable from acidic to moderate alkaline conditions (pH 5.1 and 8.0). The pigment produced by Penicillium aculeatum which is used in soft drink found stable at neutral pH. The pigment produced by Monascus purpureus was stable even at high alkaline conditions (pH 11).

21Fungal pigments are stable at various temperatures. Pigments from Monascus purpureus, Isaria spp., Emericella spp., Fusarium spp., and Penicillium spp. used for the dyeing pre-tanned leather samples that were found stable at high temperatures. Monascus pigment when added to sausages showed 92% to 98% stability at 4 °C for three months.

22Though many fungi were reported for non-toxic and stable pigments production, but the development of fermentation derived pigments needs high capital investment in terms of media components. The best example is microbial production of β-carotene. The microbial production of β-carotene cost approximately US$1000/kg versus US$500/kg produced by synthetic means.

23To counter balance the production cost, researchers have shown a great interest in the use of waste or industrial side-streams for the fermentation processes in the development microbial pigments. Many fungi were reported for pigment production in low cost substrate. Monascus ruber reported for pigment production utilizing corn steep liquor as a nitrogen source instead of yeast extract. Similarly, Monascus purpureus produce pigment using grape waste. Despite many hurdles, fungal pigments made their way to the market and compete with synthetic colors. Food grade pigments from fungi, including Monascus pigments, Arpink redTM from Penicillium oxalicum, riboflavin from Ashbya gossypii, lycopene, and β-carotene from Blakeslea trispora are now available in the market. Many fungal pigments are already used for industrial production, while some are in the development stage (Table 2).

24The use of bacteria for pigment production has several advantages over fungi, such as short life cycle and ease for genetic modification. However, compared with fungal pigments, most of bacterial pigments are still at the research and development stage (Table 2); hence, work on bacterial pigments production should be intensified to make them available on the market. Pigment producing bacteria are ubiquitous and present in various ecological niches, such as soil, rhizospheric soil, desert sand, fresh water, and marine samples. They were reported in low and high temperature regions, can persist in salt regions, and even as endophytes.

25Compared with other bacterial groups, the pigment production is more likely to be present in actinobacteria. Various genera such as Streptomyces, Nocardia, Micromonospora, Thermomonospora, Actinoplanes, Microbispora, Streptosporangium, Actinomadura, Rhodococcus, and Kitasatospora produce a wide variety of pigments. The genus Streptomyces was reported for highest pigment production. Many species of this genus, like Streptomyces griseus, Streptomyces griseoviridis, Streptomyces coelicolor, Streptomyces cyaneus, Streptomyces vietnamensis, Streptomyces peucetius, Streptomyces echinoruber, Streptomyces shaanxiensis, and Streptomyces caeruleatus were reported to produce pigments.

26Similar to fungi, bacteria also produce a wide range of pigments such as carotenoids, melanin, violacein, prodigiosin, pyocyanin, actinorhodin, and zeaxanthin.

27Two fundamental biotechnological approaches are applied when producing microbial pigments; firstly a search for new sources, and secondly enhancing the yield of already recognized sources either through optimization or strain improvement. To obtain new sources, several ecological niches were screened, and many pigments producing novel bacterial strains (Table 3) were discovered suggesting their vast availability. Strain improvement through chemical and physical mutations significantly varied the pigment production. Strain improvement through ultraviolet (UV) mutation increased prodigiosin production by 2.8-fold when compared with the parent strain. Employment of UV radiation and ethyl methanesulfonate enhanced pigment production in Serratia marcescens. Cultural conditions and media optimization showed increased pigment production. Bacillus sp. showed significant pigment production when cultivated at pH 7.0 ± 0.1 and a temperature of 34°C. Similarly, Duganella sp. B2 under optimum pH and nitrogen sources showed increased violacein (4.8-folds) production.

28Recent developments in genetic engineering have made it now possible to modify the bacteria to produce the pigment of interest. Streptomyces coelicolor, which produces a blue pigment, can be genetically modified to produce a bright yellow (kalafungin), orange, or yellow–red (anthraquinones) pigment.

29Carotenoids were first isolated by Heinrich Wilhelm Ferdinand Wackenroder. All carotenoids are tetraterpenoids and there are over 600 known carotenoids, which are divided into two classes: xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons, and contain no oxygen). Among the various carotenoids, the most important carotenoids (Figure 1) are alpha and beta-carotenes, cryptoxanthin, lutein, lycopene, violaxanthin, neoxanthin, zeaxanthin, and canthxanthin.

30Commercial carotenoids are either extracted from vegetables or produced through chemical synthesis. Extraction of carotenoids from plants has many drawbacks such as seasonal and geographic variability while chemical synthesis generates hazardous wastes that can affect the environment. In contrast to these methods, the microbial production of carotenoids shows great praise for use of low-cost substrates and safety. Microorganisms producing carotenoids are many and include Flavobacterium multivorum, Rhodobacter sphaeroides, Rhodotorula mucilaginosa, Sphingomonas sp., Dunaliella sp., Blakeslea trispora, Phycomyces blakesleeanus, Mucor circinelloides, Fusarium sporotrichioides, Agrobacterium aurantiacum, Paracoccus carotinifaciens, Gordonia jacobea, Sporidobolus salmoncolor, Rhodosporium paludigenum, and Rhodotorula glutinis.

31Carotenoids producing microorganisms are diverse, isolated from soil, cave, marine, and slattern crystallizer pond environments.

32The most prominent function of carotenoids is their contribution to harvest light energy. They absorb light and pass the excitation energy onto chlorophyll, thereby extending the wavelength range of harvested light. They protect chlorophyll from photo damage. They are used as vitamin supplements and play an important role in protection from oxidative stress. Their intake can prevent photo-aging and sun burn. Epidemiological studies have shown that people with high β-carotene intake have a reduced risk of lung cancer. Carotenoids are used commercially as food colorants, as animal feed supplements, and treatment for obesity. More recently they have been used for nutraceutical, cosmetic, and pharmaceutical purposes.

33Melanins are indolic polymers classified as eumelanins, pheomelanins, and allomelanins. Melanin is commonly found in all living systems, and their presence in almost every large taxon suggests evolutionary importance.

34Melanin production has been reported by a wide variety of microorganisms such as Colletotrichum lagenarium, Magnaporthe grisea, Cryptococcus neoformans, Paracoccidioides brasiliensis, Sporothrix schenckii, Aspergillus fumigates, Vibrio cholerae, Shewanella colwelliana, Alteromonas nigrifaciens, and many species of the genus Streptomyces.

35Melanin confers resistance to UV light by absorbing a broad range of the electromagnetic spectrum and preventing photo-induced damage. Melanin is used for mimicry, and protects against high temperatures and chemical stresses. Melanin is extensively used in cosmetics, photo protective creams, eyeglasses, and immobilization of radioactive waste such as uranium. Bacterial melanin genes have been used as reporter genes to screen recombinant bacterial strains. It has anti-HIV properties and is useful for photo voltage generation and fluorescence studies. Melanin is also used to generate monoclonal antibodies for the treatment of human metastatic melanoma.

36Prodigiosin (Figure 2) is a red pigment, first isolated from Serratia marcescens.

37The name prodigiosin has been attributed to isolation from Bacillus prodigiosus which was later renamed as Serratia marcescens. Apart from Serratia marcescens, prodigiosin production has been reported from Pseudomonas magneslorubra, Vibrio psychroerythrous, Vibrio gazogenes, Alteromonas rubra, Rugamonas rubra, and Streptoverticillium rubrireticuli. Prodigiosin producing microbes are wide spread, and they are isolated from marine samples, shallow estuarine water, tidal flat sediment, and beach sand. Prodigiosin acts as a potent therapeutic molecule, especially as an immuno-suppresser and anticancer agents. Prodigiosin also shows insecticidal, antifungal, antibacterial, and anti-malarial activities.

38Violacein is a violet colored pigment, first described from Gram-negative bacterium Chromobacterium violaceum isolated from Amazon River in Brazil. Apart from Chromobacterium violaceum, violacein production has been reported from various microorganisms such as Collimonas sp., Duganella sp., Janthinobacterium lividum, Microbulbifer sp., Pseudoalteromonas luteoviolacea, Pseudoalteromonas tunicata, and Pseudoalteromonas ulvae inhabiting different environments like soil, marine, glacier, sea surface, rhizosphere, and surface of marine sponge.

39Violacein has been reported for variety of biological activities including antiviral, antibacterial, antiulcerogenic, anti-leishmanial, anticancer, and enzyme modulation properties

40Riboflavin (Figure 3), also called vitamin B2 is water soluble pigment that exhibits a strong yellowish-green fluorescence. It was first isolated by the English chemist Alexander Wynter Blyth. The riboflavin structure was confirmed by Kuhn and Weygand, which suggests that it has two distinct parts consisting of a ribose sugar unit and a three-ring flavin structure known as a lumichrome. Riboflavin is an essential vitamin that needs to be supplemented in the human diet at a concentration of 1.1–1.3 mg per day. Riboflavin acts as a structural component of the coenzymes flavin mononucleotide and flavin adenine dinucleotide. Both coenzymes catalyze non-enzymatic oxidation-reduction reactions by functioning as dehydrogenating hydrogen carriers in the transport system involved in ATP production. For over 30 years, riboflavin supplements have been used as part of the phototherapy treatment for neonatal jaundice. Riboflavin co-treatment with β blockers showed improvement against migraine headaches. Riboflavin in combination with UV light has been shown to be effective in reducing harmful pathogens found in blood products.

41Pyocyanin (Figure 4) is a blue pigment produced by Pseudomonas aeruginosa. It is composed of two subunits of N-methyl-1-hydroxyphenazine. To synthesize pyocyanin, specific genes must be functional. MvfR is a gene which produces a transcription factor which activates phnAB genes. These genes produce the molecule quinolone which then regulates operons 1 and 2 of phzRABCDEFG which are the key to the synthesis pyocyanin. Pyocyanin has been used as bio-control agent and possess anti-bacterial and anti-fungal activity.

42The textile industry uses approximately 1.3 million tons of synthetic dyes and dye precursors. About 200,000 tons of dyes are lost as effluents every year during the dyeing and finishing operations. Unfortunately, most of these dyes escape conventional wastewater treatment processes and persist in the environment as a result of their high stability against light, temperatures, water, detergents, chemicals, soap, and other parameters such as bleach and perspiration. In this context, there is a great concern about using eco-friendly dyes. Microbial pigments are eco-friendly colorants applicable to dyeing textile fabrics. Many microbial pigments were used to dye different types of fabric. Prodigiosin from Vibrio spp. can dye wool, nylon, acrylics, and silk. By using tamarind as a mordant, pigment from Serratia marcescens can color up to five types of fabric, including acrylic, polyester microfiber, polyester, silk, and cotton. Anthraquinone from Fusarium oxysporum can be used to dye wool fabrics. Recently, reported dyeing of wet blue goat nappa skin with the Penicillium minioluteum pigment. A red pigment from Talaromyces verruculosus shows an adequate color tone for cotton fabric without any cytotoxic effect.

43Microbial pigments produce different color tones in different textiles. Pigment from Janthinobacterium lividum show a bluish-purple color tone on silk, cotton, and wool, while dark blue is seen with nylon and vinylon. Similarly, the dyeing ability of yellow pigment from Thermomyces was evaluated for cotton, silk, and wool fabrics. It was observed that silk fabric showed high affinity for Thermomyces pigments when compared with other fabrics. Deep blue and red pigments from Streptomyces strains NP2 and NP4 also showed significant changes in dyeing ability with respect to the material used. Polyamide and acrylic fibers were stained vibrantly, while cotton and cellulosic fibers were stained weakly.

44In addition, as a colorant, microbial dyed textiles, showed antimicrobial properties. Textile fabric dyed by prodiginines obtained from Vibrio sp. showed antibacterial activity against Staphylococcus aureus and Escherichia coli. In the view of the extensive availability of the microbial pigments, their affinity towards different textiles, cost effectiveness, and nontoxic nature, microbial pigments may increase their market appeal and could replace such synthetic colors which are toxic to mankind and nature.

45The increasing emergence of multidrug resistant bacteria worldwide and the lack of antibiotics to combat such pathogens continue to be a major concern for the medical community. Microbial pigments serve as antimicrobial agents against a wide range of pathogens. Pigments such as carotenoids, melanins, flavins, quinones, monascins, violacein, and indigo have been reported as good antimicrobial agents. Pigments such as pyocyanin and pyorubin obtained from Pseudomonas aeruginosa have shown distinct antibacterial activity against Citrobacter sp., which are usually associated with urinary tract and wound infections. Pigments produced from Micrococcus luteus KF532949 showed promising antimicrobial activity against wound associated pathogens such as Staphylococcus sp., Klebsiella sp., and Pseudomonas sp.. Pigment obtained from Streptomyces hygroscopicus, even showed good antimicrobial activity against drug resistant pathogens such as methicillin and vancomycin resistant strains of Staphylococcus aureus and β-lactamase producing strains of Escherichia coli, Pseudomonas aeruginosa, and Klebsiella sp.. Pigment from Monascus ruber showed antimicrobial activity against food borne bacteria. Further, inhibition of human pathogenic bacteria such as Staphylococcus aureus, Klebsiella pneumoniae, and Vibrio cholera was observed by the pigment of an endophytic fungal species Monodictys castaneae.

46Efforts in understanding the mechanism of antibacterial activity of some pigments have also been made. The mode of antibacterial action of prodigiosin produced from Vibrio sp. DSM 14379 against Escherichia coli was evaluated. It was found that the prodigiosin treated Escherichia coli cells showed membrane leakage, decreased respiration, and inhibition of protein and RNA synthesis. In view of the above, microbial pigments apart from coloring agents, can be used as novel drugs.

47The development of foods with an attractive appearance is an important goal in the food industry. To make the food appealing, either synthetic or natural colors are added. In recent days, food producers are turning from synthetic to natural colors, due to negative health issues associated with some synthetic colors. Natural colorants from microbes play a significant role as food coloring agents, because of its cheap production, easier extraction, high yield, and no lack of raw materials and seasonal variations. Many pigments from microbial sources such as red pigment from Monascus sp., astaxanthin from Xanthophyllomyces dendrorhous, Arpink redTM from Penicillium oxalicum, riboflavin from Ashbya gossypii, β-carotene from Blakeslea trispora, and lycopene from Erwinia uredovora and Fusarium sporotrichioides were added to the food to increase its appeal. Pigment like canthaxanthin used in foods, particularly in products such as cheese, candy, fish, meat, fruits, beverages, snacks, beer, and wine. Pigments like riboflavin (i.e., vitamin B2) are used in beverages, instant desserts and ice creams. Carotenoids can act as a sunscreen to maintain the quality of food by protecting them from intense light.

48An increase in free radicals in the body enhances the chances of occurrence of chronic diseases such as cancer, diabetes, cardiovascular, and autoimmune disorders. To avoid this, antioxidants are used. Antioxidants are molecules that delay or inhibit cellular damage by donating electrons to a rampaging free radical and neutralizing them via their free radical scavenging properties. Microbial pigments such as carotenoid, and naphthaquinone demonstrated antioxidant activities. Similarly, anthraquinones from the endophytic fungus Stemphylium lycopersici and melanin from Streptomyces glaucescens NEAE-H were reported as antioxidants. Pigment like xanthomonadin showed antioxidant activity and protection against photo damage. Similarly, the antioxidant activity of carotenoid pigment from an antarctic bacterium Pedobacter was evaluated. The pigment possessed strong antioxidant capacity and protected the bacterium against oxidative damage. The above reports suggest that microbial pigments used as antioxidants may prevent the incidence of many diseases such as cancer and heart disease.

49Cancer is one of the most-deadly diseases known to man. The cure for certain types of cancers is considered to be like the Holy Grail since most of the existing treatments are not effective enough to provide full protection. Efforts to use microbial pigments as anticancer agents have laid the foundation for successful treatments. Many microbial pigments possess anticancer activity. Pigments such as prodigiosin from Pseudoalteromonas sp. 1020R have cytotoxicity against U937 leukemia cells. Melanin from Streptomyces glaucescens NEAE-H has been reported for anticancer activity against skin cancer cell line. Derivatives of anthraquinone from mangrove endophytic fungus Alternaria sp. ZJ9-6B has been reported for anti-cancer activity against human breast cancer cell lines. Pigments obtained from Monascus spp. showed remarkable anticancer activity against different cancer cells. Pigments from Monascus, such as monascin, showed inhibitory activity against mouse skin carcinogenesis, while ankaflavin showed inhibitory activity against Hep G2 and A549 human cancer cell lines. Similarly, monaphilone A and monaphilone B, exhibits anti-proliferative effect against HEp-2 human laryngeal carcinoma cell lines. Pigment like prodigiosin has been tested for anticancer activity against more than 60 cancer cell lines and showed a good anticancer activity due to the presence of multiple cellular targets. In the view of the above, microbial pigments can be a potential therapeutic agents to treat cancer.

50Apart from colorants, antioxidants, antimicrobial agents, and anticancer agents, microbial pigments are used as bio-indicators. Fluorescent pigments from bacteria can be used to check the progress of specific reactions. A key example is phycoerythrin, which is used to predict the rate of peroxy radical scavenging in human plasma. The pigment initially shows fluorescence, however, dark spots appear where the pigment reacts with radicals.

51Pigments are used to detect heavy metals for example, Vogesella indigofera produce blue pigment under normal environmental growth condition; however, when exposed to heavy metal like hexavalent chromium, the pigment production did not observed. Microbial pigments can also be used to monitor temperature variation. Pantoea agglomerans produce deep blue pigment only at temperatures of ≥10°C and hence can be used as temperature indicator for the low-temperature-storage management of foods and clinical materials.

52Synthetic dyes have caused considerable environmental and health problems. In contrast, microbial pigments are eco-friendly and used in the textile industry, as food colorants, antioxidants, bio-indicators, and antimicrobial and anticancer agents. Though extensive research has been done to bring microbial pigments from the Petri dish to market, still their output cannot fulfill market demand if synthetic dyes withdrawn. Efforts in finding new microbial sources for pigment production and decrease in production cost through optimization, strain improvement and genetic engineering have to be carried out to eradicate toxic synthetic dyes.

53All authors listed, have made substantial, direct and intellectual contribution to the work, and approved it for publication.

54The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.