Bacteria become source of ‘greener’ blue jeans

Indigo plant inspires eco-friendly microbial production of denim dye

Engineered bacteria can produce indigo — without harsh chemicals — to dye denim, the fabric used in these jeans.

Engineered bacteria can produce indigo — without harsh chemicals — to dye denim, the fabric used in these jeans.

KALX999/Wikimedia Commons

They come bleached and boot cut, stonewashed and straight-leg. But what most jeans aren’t is green. And we’re not talking about their hue here.

Blue jeans get their signature color from indigo, a dye. Producing that indigo releases chemicals that can pollute water and harm fish. That’s prompted scientists to seek cleaner — as in “greener” — methods. One new approach turns bacteria into micro-factories for the chemical dye. These microbes are “taught” to produce indigo using a chemical process found in living plants.

For millennia, textile makers have been using a deep blue dye made from Indigofera, a member of the pea family. But Mother Nature doesn’t make it easy to get this indigo dye. The healthy plants actually don’t make the blue pigment. “They have green leaves and look like other plants,” explains Tammy Hsu. She’s a graduate student in bioengineering at the University of California, Berkeley.

The plants’ leaves don’t store indigo but instead contain a related compound. It’s caged within a sugar molecule to make a colorless molecule called indican. Crushing the plants’ leaves releases the protected molecule. Then, it’s free to react with oxygen in the air and pair with another of its kind to produce that prized indigo.

People used to make the blue dye by extracting it from plants. But as demand for the dye grew, so too did chemists’ know-how. By the late 1800s, German scientists had figured out how to synthesize indigo from chemicals in the lab. Their plant-free approach was faster and allowed for factories to make huge amounts. Today, factories churn out some 36,000 kilograms (about 40,000 tons) of indigo per year — just for blue jeans. That’s roughly the weight of 6,000 African elephants!

GETTING THE BLUES This video from ACS Reactions show how blue jeans get their color. ACS Reactions

But this large-scale production of the dye has its challenges. Wastes produced by the dye-making haven’t been kind to the environment. One reason: Most fabric coloring takes place in water. Yet indigo dissolves poorly in water. To get around this problem, factories use chemicals called reducing agents. These chemicals convert indigo into a form that can dissolve in water. However, this soluble molecule falls apart easily. Consequently, the procedure requires huge amounts of the reducing agent. And that agent corrodes pipes and can hurt aquatic life.

So Hsu and her mentor at the university, John Dueber, took a cue from indican. This plant chemical “has all of the properties you’d want in a dye,” says Hsu. “It’s soluble. It’s also pretty stable in water.” And although it’s not blue, it takes only one enzyme-triggered reaction to turn it into indigo, she notes.

So her team decided to equip bacteria to make indican. (With genetic engineering, researchers can introduce segments of DNA into bacteria. Hsu’s microbes now become factories that, under the new DNA’s control, make large amounts of the indigo plant’s proteins.) Later, the researchers soaked cloth with the bacteria’s indican. Then they exposed the cloth to the proper enzyme. Voilà! The fabric turned blue as the indican morphed into indigo.

Easier said than done

The first step required extracting the enzyme from plants that helps produce indican.

Hsu and another student collected 200 grams (about half a pound) of indigo plant leaves. After grinding them into a paste, they separated out protein-based portions of the mush. Then they probed this, looking for its sugar-adding activity. They found it in an enzyme called glucosyltransferase (GLU-koh-TRANS-fur-ace).

Next they examined the genetics of the plant. They were looking for the part of its DNA that provides the blueprint for making this enzyme. When they found it, they introduced that tiny piece of the DNA into bacteria. Suddenly, these microbes were able to make indican.

Now Hsu’s team soaked a piece of cloth in a solution containing the bacteria’s indican. Afterward, they added an enzyme that triggered its conversion to indigo. Within minutes, the cloth turned blue! Hsu described her team’s achievement at a scientific conference in Boston last month.

Experts are excited by the preliminary data. Thomas Bechtold is a textile chemist at the University of Innsbruck in Austria. “The results are amazing and impressive,” he told Science News for Students.

However, the new approach might still pollute, Bechtold notes. One step releases indican’s sugar molecule into the wastewater. As sugar breaks down in lakes and rivers, it will feed hungry microbes. As they gobble it up and grow, they’ll need more oxygen. They’ll extract it from the dissolved oxygen present in water. As the microbes use this oxygen, there will be less for fish and other aquatic life. Depleting lakes and rivers of oxygen also creates foul smells.

But the main hurdle to using bacterial indigo will likely be efficiency. At this point the researchers recover a mere gram (a few hundredths of an ounce) of indican from a liter (about a quart) of bacterial culture. That means, Hsu explains, “We’d need about 18 liters [4.76 gallons] of media to dye one pair of jeans.”

With Americans buying about 450 million pairs of jeans each year, factory workers would be drowning in bacteria!

But it’s still early days. The Berkeley team received a five-year grant to work on this project, and they’ve only completed the first year.

Power Words

(for more about Power Words, click here)

agent  A compound or activating form of energy (such as light or other types of radiation) that has a role to play in getting something done.

amino acids Simple molecules that occur naturally in plant and animal tissues and that are the basic constituents of proteins.

bacterium (plural bacteria) A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

biochemistry  A field that marries biology and chemistry to investigate the reactions that underpin how cells and organs function. People who work in this field are known as biochemists.

bioengineering  The application of technology for the beneficial manipulation of living things. Researchers in this field use the principles of biology and the techniques of engineering to design organisms or products that can mimic, replace or augment the chemical or physical processes present in existing organisms. This field includes researchers who genetically modify organisms, including microbes. It also includes researchers who design medical devices such as artificial hearts and artificial limbs.

chemical bonds Attractive forces between atoms that are strong enough to make the linked elements function as a single unit. Some of the attractive forces are weak, some are very strong. All bonds appear to link atoms through a sharing of — or an attempt to share — electrons.

chemical reaction  A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

corrode  A process whereby metals react with gases or other materials in their environment and undergo a type of degradation. The rusting of iron, for instance, is one example of corrosion, driven by exposure to moisture.

culture  (in biology) The community of cells or tissue that is intentionally grown outside the body (or the wilds) for research purposes, usually in a laboratory.

DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

enzymes  Molecules made by living things to speed up chemical reactions.

genetic engineering  The direct manipulation of an organism’s genome. In this process, genes can be removed, disabled so that they no longer function, or added after being taken from other organisms. Genetic engineering can be used to create organisms that produce medicines, or crops that grow better under challenging conditions such as dry weather, hot temperatures or salty soils.

graduate student  Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

green chemistry  A rapidly growing field of chemistry that seeks to develop products and processes that will pose little or no harm to living things or the environment.

indigo  A deep blue dye made from Indigofera, a plant belonging to the pea family.One of this dye’s best known contemporary uses: tinting the denim used to make blue jeans.

insoluble Incapable of being dissolved into a fluid or gas. Salt and sugar can dissolve in water, for example, but some other substances, including some of those with large molecules such as proteins, do not.

mentor  An individual who lends his or her experience to advise someone starting out in a field. In science, teachers or researchers often mentor students or younger scientists by helping them to refine their research questions. Mentors can also offer feedback on how young investigators prepare to conduct research or interpret their data. 

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

organic (in chemistry) An adjective that indicates something is carbon-containing; a term that relates to the chemicals that make up living organisms. (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.

pigment     A material, like the natural colorings in skin, that alter the light reflected off of an object or transmitted through it. The overall color of a pigment typically depends on which wavelengths of visible light it absorbs and which ones it reflects. For example, a red pigment tends to reflect red wavelengths of light very well and typically absorbs other colors. Pigment also is the term for chemicals that manufacturers use to tint paint.

precursor  A substance from which some later thing is made. It may be a compound that will change into something else as a result of some chemical or biological reaction.

preliminary  An early step or stage that precedes something more important.

proteins  Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better-known, stand-alone proteins.Medicines frequently work by latching onto proteins.

redox  A short-hand term in chemistry for reactions that involve reduction and/or oxidation (the reducing and oxidizing of chemicals). These are changes that occur with the gain and/or loss of an electron. When something is reduced, one of its atoms gains an electron — to become stable — by stealing it from another atom or molecule. A compound used to offer up that needed electron is known as a reducing agent.

solubility A measure of the ability of one chemical to dissolve into another, creating a chemical solution.

soluble  Some chemical that is able to dissolve into some liquid. The resulting combo becomes a solution.

synthesize  (n. synthesis) To produce something — such as to build a substance chemically — by combining raw ingredients (simpler chemical building blocks).  

synthetic  An adjective that describes something that did not arise naturally, but was instead created by people. Many have been developed to stand in for natural materials, such as synthetic rubber, synthetic diamond or a synthetic hormone. Some may even have a chemical makeup and structure identical to the original.

synthetic biology A research field in which scientists work on developing custom life forms in the lab. Because they make synthetic organisms, scientists who work in this field are known as synthetic biologists.

textile   Cloth or fabric that can be woven of nonwoven (such as when fibers are pressed and bonded together).

wastewater  Any water that has been used for some purpose (such as cleaning) and no longer is clean or safe enough for use without some type of treatment. Examples include the water that goes down the kitchen sink or bathtub or water that has been used in manufacturing some product, such as a dyed fabric.

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