Nature is a symphony of vibrant colours, thanks to natural pigments present in rocks and many living organisms including flowers, fruits, animals, and even microorganisms. Natural pigments or bio-pigments, especially those from microorganisms, are potential alternatives to artificial dyes that are widely used in various industries.
What are Pigments?
Pigments are substances that reflect certain wavelengths of visible light while absorbing others. The colour of a pigment is the wavelength of light it reflects. For example, the plant and algal pigment, chlorophyll, absorbs lights in the purple/blue (400-475 nm) and orange/red (650-675 nm) regions of the spectrum but reflects green (500-570 nm), which is why plants and many algae appear green.
Anthocyanins are red, blue, and purple pigments that give currants, berries, and grapes their colour. Carotenoids are responsible for the vivid red, orange, and yellow colours of tomatoes, carrots, and bell peppers. In animals and humans, melanin gives colour to the hair, skin, and eyes, while the yellow pigment, papiliochrome, is present in the wings of swallowtail butterflies.
A swallowtail butterfly
Natural pigments are also called bio-pigments because they are of biological origin
A History of Colourants
Colours communicate with the subconscious tremendously influencing our emotional and mental state, and even our physiology. Certain colours increase our heart rates and blood pressures possibly because they evoke strong emotions and brain activity, while others are soothing and elicit a sense of peace and calm. It is not surprising that we have long since learnt to manipulate emotions, moods, and behaviours by adding colours to living spaces, clothing, and other everyday products.
Bio-pigments have been used as colouring agents since the beginning of time when ancient civilizations used insects, plants, seeds, fruits, flowers, and rocks as fabric dyes, and for tattoos, pottery paintings, and murals.
It wasn’t until William Perkin’s accidental discovery of the very first artificial dye in 1856 did artificial colourants become widely popularized in the textile industry. This was largely because these colourants were much more stable, produced more uniform and brighter hues, and were cheaper to produce than pigments from natural sources. However, artificial colourants contain many toxic compounds including Cadmium – which can cause birth defects and cancer, Lead – which may cause infertility and seizures, Mercury – which interferes with nervous systems and brain functioning, Benzene – which disrupts the immune system and damages the bone marrow, and Acetic acid – that causes serious respiratory problems and various other medical conditions. Besides, most artificial colourants are non-biodegradable causing severe water pollution when discharged into the environment through partially-treated industrial wastewater. Nowadays, natural pigments, especially those of microbial origin, are becoming more popular due to increasing awareness and concerns over the safety of artificial colourants.
A colourant is either a dye or pigment. Dyes have small particle sizes and can be dissolved in water. Pigments are insoluble in water and have larger particle sizes.
What are Microbial pigments?
Microbial pigments are produced by microorganisms such as bacteria, yeasts, and fungi, and vary in colour from muted tones to vivid hues. Microbial-derived pigments are not simply colours, but they help microorganisms survive in their environment. Some microbes produce pigments in response to environmental stresses such as ultraviolet radiation, extremely low temperatures, and acidic conditions. Others produce pigments to inhibit the growth of bacterial predators and competitors. Yet other pigments are involved in providing energy for the microbe through photosynthesis. Besides their potential use as textile and food colourants, microbial pigments possess remarkable medicinal properties and therefore have wide applications in the pharmaceutical and cosmetic industries.
Compared to plants and other sources of bio-pigments, microbial pigments are more advantageous because they are relatively cheaper to produce commercially since microorganisms are easy to cultivate. Microbes grow rapidly irrespective of weather and season, and are capable of producing copious amounts of pigments in a short period of time. They can be grown on cheap substrates such as agricultural wastes, and are amenable to genetic modification to increase pigment production.
Industrial Uses of Microbial Pigments
Microbial pigments as fabric colourants
Textile manufacturers use millions of tons of artificial pigments and dyes every year. However, interests in bio-pigments, especially those derived from microbes, have risen significantly since they are non-toxic and biodegradable, and therefore not harmful to us and the environment.
Microbial pigments also come with a variety of desirable traits, some transferable to fabrics during the dyeing process. One of such pigment is prodigiosin, a vivid red pigment produced by the marine bacterium Serratia marcescens, and species of Vibrio and Pseudoalteromonas. This pigment inhibits the growth of many disease-causing bacteria, viruses, fungi, and the malaria parasite, Plasmodium, and it has been shown to suppress the growth of certain human cancer cells! Prodigiosin can be used for dyeing silk, cotton, polyester, polyester microfiber, acrylics, cotton, wool, and nylon.
Violacein is a purple pigment produced by Chromobacterium violaceum as a defense mechanism against its predators. This pigment can prevent the growth of bacteria, nematodes, protozoans, viruses, and even cancer cells, and serves as an effective dye for silk satin, rayon, and batiks.
Microbial pigments in medicine and cosmetics
Natural products extracted from plants, animals, and microorganisms were historically used as medicines to cure many ailments. But their complex extraction methods and high cost of development caused pharmaceutical companies to instead pursue creating chemical-based or synthetic drugs. In recent years however, advancements in science and technology coupled with an urgent need for new treatment options for cancer, infectious diseases, and antimicrobial-resistant infections, has led to renewed interest in natural products.
Microbial pigments with strong antioxidant properties include melanin, carotenoids, violacein, anthocyanins, and anthraquinones. Antioxidants prevent or limit cell damage caused by unstable molecules called free radicals. Cell damage may increase the risks of diabetes, cancer, autoimmune disorders, inflammatory, and cardiovascular diseases.
Prodigiosin has sparked great interest in the pharmaceutical industry and among medical practitioners due to its potential use as an immunosuppressant or anti-rejection medication in patients who have had organ transplant surgery. In addition, it purportedly cures diabetes and prevents the proliferation of ovarian, leukemia, renal, lung, and colon cancer cells.
Species of Streptomyces naturally produce an arsenal of molecules of medical importance such as antivirals, antifungals, antihypertensives, antitumorals, and antibiotics. In fact, they are well known for producing more than half of the antibiotics currently being used in the healthcare system. The desert soil bacterium, Streptomyces hygroscopicus produces a yellow-coloured pigment which is active against several leading antimicrobial-resistant bacteria that cause life-threatening infections in hospitals worldwide.
In addition to their antioxidant property, anthraquinones, a group of pigments produced by the fungus Stemphylium lycopersici and many other fungi, have antiviral and antibacterial effects as well.
The bacterium, Pseudomonas aeruginosa, produces pyocyanin – a blue-green pigment, that kills other microorganisms including those that cause infections in wounds and the urinary tract. Pyocyanin has also been reported to destroy pancreatic cancer cells therefore preventing cell proliferation.
Many species of fungi and marine bacteria produce melanin as protection against UV radiation. Microbial melanin is being used in sunscreen cosmetics, eyeglasses, and hair dyes.
Microbial pigments as food colouring
Colours affect how we perceive food taste and aroma. We are subconsciously drawn to brightly coloured foods as these are perceived as being nutritious, fresh, and tasty. Food companies add colourants to processed and fresh foods to augment their visual appeal, influence customer behaviour, and ultimately increase their market value.
However, artificial food colourants have been linked to cancer, allergies, and behavioural disorders especially in children. Nowadays, microbial pigments such as phycocyanin, prodigiosin, riboflavin, canthaxanthin, melanin, violacein, lycopene, astaxanthin, and beta-carotene are increasingly being used as food colourants.
Phycocyanin, a protein-rich blue pigment produced by Cyanobacteria, adds colour to and improves the nutritional quality of milkshakes, ice cream, yogurts, candies, and jellies. Riboflavin, the B2 vitamin, is naturally present as a yellow pigment in all plants and several microorganism including yeasts and bacteria. Riboflavin helps our body convert carbohydrates to energy and is available as dietary supplements. It is also used to enhance the colour and nutritional value of processed foods such as breakfast cereals, dairy products, baby foods, and sauces.
The yellowish-orange pigment, beta-carotene, is frequently used as a colourant for butter, margarine, cheese, ice cream, biscuits, and soups. Beta-carotene is an important source of Vitamin A which is essential for good eyesight and prevents night blindness. It is produced by several microbes including algae, fungi, yeasts, and present in brightly coloured fruits and vegetables.
Lycopene, a red pigment present in tomatoes, pink grapefruits as well as some fungi, is used as a meat colouring agent in several countries. Many studies have indicated that lycopene may help prevent cardiovascular diseases, and some type of cancer. The fungus, Monascus, produces a group of pigments called azaphilones which have antiviral, antibacterial, and anticancer properties. Azaphilones are used as food colourant in wines, yoghurt, sausages, ham, and meats.
Microbial pigments in animal feed
We are not the only ones that benefit from eating foods supplemented with microbial pigments. In nature, salmon and crustaceans such as lobsters, crabs, and shrimps get their reddish-pink hues by consuming yeasts and micro-algae that contain the pigment astaxanthin. For this reason, astaxanthin is added to fish feed to enhance the colours of farmed seafood. Being an antioxidant, astaxanthin is believed to improve the immunity, growth, and survival of fish.
Canthaxanthin is another potent antioxidant added to fish and poultry feed to improve the colour of seafood and egg yolks. This orange-red pigment is produced by the bacterium Bradyrhizobium and several species of green micro-algae.
Current Limitations and Future Outlooks
Microorganisms are an infinite source of natural pigments which provide more benefits to us and the environment than artificial colourants. However, microbial pigments are sensitive to temperature, pH, oxygen, and light which may lead to shade variation or fading over time. Scientists are working on optimizing culture conditions of pigment-producing microbes to increase pigment yields. Various technologies such as genetic and metabolic engineering are also being used to increase pigment production, while nano-emulsions and micro-encapsulation are being applied to improve the stability of microbial pigments.
References
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