by Michael Keller

The picture below shows a flowering plant called Indigofera tinctoria, the original source of the dye called indigo. Up to the start of the 20th century, fashionable miners could thank this plant for the deep blue of their Levi’s jeans.

(Courtesy Wikipedia)

But extracting the dye from the plant was a time-consuming and expensive process, and efforts to find another way to make it started bearing fruit around 1880. Scientists of the period eventually replaced I. tinctoria with chemical processes that unlocked mass production of the once rare colorant. Instead of the plant, the new technique combined derivatives of crude oil with concentrated acids and heated the mixture in the presence of metals.

Industrial production started feeding growing demand for indigo, and manufacturers around the world were producing more than 88,000 tons of it a year by 2011. But besides creating a synthetic indigo dye, the process also uses and produces large quantities of chemicals that are toxic to humans, animals and the environment.

Now, University of California, Berkeley bioengineers say they have potentially found a way to create indigo without the petroleum and chemical byproducts. The answer to making the blue dye more green? Bacteria.

I. tinctoria produces a molecule called indican, the chemical precursor to natural indigo. Inside healthy plant leaves, indican is caged within sugar molecules that prevent it from presenting its blue side. When the leaves are damaged, the sugars are stripped away and the leaf turns blue. This phenomenon is harnessed as part of the natural dye extraction process.

The Berkeley researchers have started a project that looks to insert genes into bacteria to get the microorganisms to produce sugar-encapsulated indican. Once the chemical is extracted from the bacterial cells, they plan to mix in an enzyme that will strip away the sugar molecules, leaving the deep-blue dye.

“We thought going back to the plants would be smart,” said bioengineer John Dueber, who is leading the research. “If we can identify the enzyme the plants use to produce the sugar cage and clone its gene, we think the microbes can make large quantities of indican for dyeing jeans without the use of highly ‘dirty chemicals.’”



Dueber says the innovation would cut out the petroleum feedstock currently used to synthesize indigo, since bacteria need only be fed the simple sugar glucose to thrive. If it works, it would also cut the toxic acids and metals out of the production process, since the microbes would be essentially recreating the dye production process that naturally goes on inside the I. tinctoria plant.

The work is still in its early stages, and Dueber says some of the basic science still needs to be worked out before they could even begin to think about bringing the process to market. His lab normally focuses on conducting basic science research to improve engineered metabolic pathway efficiency in living cells. But he sees in the bacterial dye-producing factories project a big opportunity to potentially clean up a major source of pollution in the world.

“We’re still going to focus our research on academically interesting questions, but when there are industrial applications, we want to be aware of that,” Dueber says. “We’d all like to see our indigo research lead to greener blue jeans.”



(When indigo’s chemical precursor is applied to cotton cloth, a single enzyme can free indigo to dye a white bear to blue. Photo courtesy of John Dueber.)

Top Image: John Dueber (right) and Berkeley bioengineering graduate student Zach Russ examine a culture of indigo-producing E. coli bacteria. Photo: Peg Skorpinski