Bacteria dye jeans

Biotech bugs turn indigo blue in a green way.

Jeans dyed blue by bacteria may soon be swaggering down the streets. Researchers have genetically modified bugs to churn out the indigo pigment used to stain denim. The process could be a greener rival to chemical indigo production.

Originally extracted from plants, indigo dye is now made from coal or oil, with potentially toxic by-products. Bacteria have previously been adapted as alternative indigo manufacturers, but a trace by-product renders jeans an unfashionable shade of red.

Walter Weyler and his colleagues of Genencor International in Palo Alto, California tweaked the genes of the bacterium Escherichia coli to eliminate the red pigment¹. The final colour is “indistinguishable” from the globally popular deep blue of the chemically made dye, says Doug Crabb, vice president of Genencor.

The bugs offer an environmentally friendly substitute for chemical synthesis: they use sugar as their raw material and create less waste. “Biological indigo would probably be more environmentally friendly,” agrees UK environmental consultant Michael Griffiths. But industry is unlikely to use it until it is also as cheap and effective.

Blue bugs

Biotech indigo starts with a chemical called tryptophan, which bacteria produce naturally. Tryptophan is ideal for conversion to indigo because it already contains the ring-structure at the core of the indigo molecule. A few chemical alterations convert tryptophan into the dye.

Bioindigo E. coli have an enzyme from another microbe engineered into them that converts trytophan into the ring-containing indigo precursor indoxyl; this spontaneously turns into indigo when exposed to air.

Weyler and his team tinkered with their E. coli so that they churned out high levels of the raw material tryptophan. The researchers also inserted a gene that cuts down production of the contaminating red pigment. The efficiency of the process still needs to be improved, however, Crabb concedes.

Before the chemical process was invented, people used plants such as woad and dyer’s knotweed to make indigo: soaking their leaves in water releases indigo’s chemical precursors. How these are converted to indigo is still a mystery. Biochemist Philip John of the University of Reading, UK is heading a project to re-introduce indigo- yielding crops into Europe as a natural alternative to chemical synthesis.

Biotech and plant production would both have to be souped-up to feed the world’s obsession with blue jeans: 16,000 tonnes of dye are made annually, almost all of which is used on denim. “There’s no other dye that will give that characteristic colour,” explains John, “It’s got to be indigo.”

References

1. Berry, A. et al. Application of metabolic engineering to improve both the production and use of biotech indigo. Journal of Industrial Microbiology & Biotechnology, 28, 127 – 133, (2002).