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Researchers Tailor Bacteria To Create Renewable Chemicals
July 19, 2018

As scientists and researchers across the country continue to investigate ways to increase the use of environmentally friendly and sustainable materials to create fuels and consumer products, their efforts have been stymied by an inability to efficiently and cost effectively convert plant matter into usable materials. Now, according to information provided by Sandia National Laboratories (Sandia), scientists there have demonstrated a new technology based on bioengineered bacteria that could make it economically feasible to produce many petroleum-based products from renewable plant sources.

Sandia National Laboratories scientists Seema Singh, left; and Fang Liu hold vials of vanillin and fermentation broth, which are critical for turning plant matter into biofuels and other valuable chemicals. Credit: Dino Vournas

Converting lignin, a class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae, has long been a stumbling block in efforts to generate wider use of the energy source and making it cost competitive. To overcome this obstacle, Sandia bioengineer Seema Singh and two postdoctoral researchers, Weihua Wu and Fang Liu, utilizing different mechanisms from other known lignin degraders, have engineered a strain of E. coli bacteria into an efficient and productive bioconversion cell factory.

“For years, we’ve been researching cost-effective ways to break down lignin and convert it into valuable platform chemicals,” noted Dr. Singh. “We applied our understanding of natural lignin degraders to E. coli because that bacterium grows fast and can survive harsh industrial processes.”

Recently published in the journal Proceedings of the National Academy of Sciences of the United States of America, Dr. Singh’s work, “Towards Engineering E. coli with an autoregulatory system for lignin valorization,” was supported by Sandia’s Laboratory Directed Research and Development program.

Dr. Singh and her team have solved three problems with turning lignin into platform chemicals.

The first was cost. E. coli typically do not produce the enzymes needed for the conversion process and therefore must be coaxed into making the enzymes by adding something called an inducer to the fermentation broth. While effective for activating enzyme production, inducers can be so costly that they are prohibitive for biorefineries.

According to Dr. Singh, the solution was to “circumvent the need for an expensive inducer by engineering the E. coli so that lignin-derived compounds such as vanillin serve as both the substrate and the inducer.”

However, vanillin, produced as lignin breaks down, can create a second problem as it becomes toxic to E. coli at higher concentrations.

“Our engineering turns the substrate toxicity problem on its head by enabling the very chemical that is toxic to the E. coli to initiate the complex process of lignin valorization. Once the vanillin in the fermentation broth activates the enzymes, the E. coli starts to convert the vanillin into catechol, our desired chemical, and the amount of vanillin never reaches a toxic level,” Dr. Singh explained. “It auto regulates.”

While the vanillin in the fermentation broth moved across the membranes of the cells to be converted by the enzymes, it was a slow, passive movement creating a third problem, efficiency.

“We borrowed a transporter design from another microbe and engineered it into E. coli, which helps pump the vanillin into the bacteria,” Liu said. “It sounds pretty simple, but it took a lot of fine tuning to make everything work together.”

“We have found this piece of the lignin valorization puzzle, providing a great starting point for future research into scalable, cost-effective solutions,” Dr. Singh said. “Now we can work on producing greater quantities of platform chemicals, engineering pathways to new end products and considering microbial hosts other than E. coli.”

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