A team of researchers at the University of British Columbia (UBC) and the University of Wisconsin-Madison has engineered a soil bacterium to convert plant matter into muconic acid, a ubiquitous precursor to everyday products.

Making plastic from plants_image_RKothe

Their findings, recently published in Metabolic Engineering, lay the groundwork for a more environmentally friendly way to produce plastic, textiles, pharmaceuticals and more.

“In order to build a sustainable society, we need to move away from fossil fuels. But what many people don’t realize is that there’s a significant portion of fossil fuels that are not extracted for energy usage, but for chemical manufacturing,” said Anne Lalande, a graduate student in the Department of Microbiology and Immunology at UBC and first author of the study. “We’re interested in using lignin, or wood waste, to make valuable chemicals in order to displace the fossil fuel industry.”

Plant power

Lignin is a complex biopolymer that gives plants their rigidity and structure. It represents the second most abundant renewable source of carbon in the world and conveniently resembles oil in its chemical composition. Excess lignin is generated during the paper production process, yet most of it is currently burned. Using lignin as a feedstock for the chemical industry could therefore offer a more sustainable alternative to fossil fuels.

Synthesizing useful chemicals from lignin depends on two key steps - combining chemistry and biology. First, researchers at the University of Wisconsin-Madison chemically broke down lignin into mixtures of simpler chemical building blocks. While the composition of this lignin mixture can differ depending on the source of the lignin, the authors focused on a trio of vanilla-related compounds: vanillin, vanillate and acetovanillone.

In the second step, lignin mixtures made up of vanillin, vanillate and acetovanillone are converted into muconic acid. “We give these mixtures to engineered bacteria that can take all these different things and funnel them into one single product of interest,” said Lalande. “That’s something that chemistry cannot do very well, but that biology can do very well.”

Soil bacterium

Specifically, the authors identified the soil bacterium Rhodococcus aromaticivorans RHA1 as an ideal candidate to help with this step as it is non-pathogenic, fast-growing, and can metabolize a wide range of plant-derived compounds.

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The researchers genetically engineered the bacterium, incorporating RHA1-native genes and genes from other bacteria that encode enzymes which turn vanillin into vanillate and convert vanillate and acetovanillone into muconic acid. They also deleted several competing metabolic pathways that transform these compounds into other chemicals, thereby increasing the yield of their desired product.

Ultimately, the modified Rhodococcus bacterium, containing 15 engineered genes, was able to convert a lignin plant matter mixture into muconic acid with 97 percent efficiency. For these findings to be applied to industry, however, researchers will need to assess how these processes scale up in large bioreactors. Additionally, as some metabolic bottlenecks remain, the authors expect that further engineering Rhodococcus to reduce the build-up of unwanted intermediates could enhance its efficiency for muconic acid production.

Next steps

Still, the present findings represent a significant step towards a sustainable solution for synthesizing a high-value chemical. By showcasing novel genetic tools for manipulating rhodococcal bacteria strains, the authors hope that their work also helps pave the way for the development of microbial factories for a wide range of applications.

This study was supported by a grant from the Natural Sciences and Engineering Research Council of Canada (DG 171359) and by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy Bioenergy Technologies Office (BETO) under grant number DE-EE0011114.

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