Bacteria become source of ‘greener’ blue jeans
- Sayuki
- Jul 31, 2015
- 4 min read
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.
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.
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