Banking on the bacterium bad guy: Sandia tailors E. coli to convert plants into renewable chemicals

In California, Sandia National Laboratories scientists have demonstrated a new technology based on bioengineered bacteria that could make it economically feasible to produce things like jet fuel, pantyhose, and plastic bottles from renewable plant sources.

Economically and efficiently converting tough plant matter, or lignin, has long been a stumbling block for wider use of the energy source and making it cost competitive. Piecing together mechanisms from other known lignin degraders, Sandia bioengineer Seema Singh and two postdoctoral researchers, Weihua Wu, now at Lodo Therapeutics Corp., and Fang Liu, have engineered E. coli 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,” Singh said. “We applied our understanding of natural lignin degraders to E. coli because that bacterium grows fast and can survive harsh industrial processes.”

It’s all about that lignin

The Digest covers news about lignin developments at an alarming rate, so while we can’t cover them all here, just know that yes, there has been tons of progress with lignin in recent years – from North Carolina State University researchers improving wood properties and lignin biosynthesis for biofuels, as reported in The Digest in April 2018, to Somethin’ from just about nothin’: The Digest 2018 Multi-Slide Guide to upgrading biorefinery waste lignin into bioplastics,” as reported in February 2018, the progress has been giving much needed hope.

We said back in February, “You know what they say, you can make anything you want from lignin except money,” but Sandia could prove us wrong.

Problem solving

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. Scientists must coax the bacteria 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.

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,” Singh said.

Vanillin is not an obvious choice to replace an inducer. The compound is produced as lignin breaks down and can, at higher concentrations, inhibit the very E. coli working to convert it. This posed the second problem: toxicity.

“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,” Singh said. “It auto regulates.”

The third problem was efficiency. 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. The researchers looked for effective transporters from other bacteria and microbes to fast track this process, Wu said.

“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.”

The bottom line

Engineering solutions like these, which overcome toxicity and efficiency issues, have the potential to make biofuel production economically viable. The external inducer-free, auto-regulating method for valorizing lignin is just one way that researchers are working to optimize the biofuel-making process.

“We have found this piece of the lignin valorization puzzle, providing a great starting point for future research into scalable, cost-effective solutions,” 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.”

Sandia’s work isn’t just impressive today…as reported all the way back in 2016 in The Digest, Sandia has been key in making strides with lignin. They made a lignin breakthrough using soil bacteria that began to transform the economics of biofuel production.

Sandia also decoded the structure and behavior of LigM, an enzyme that breaks down molecules derived from lignin, as reported in The Digest in May 2017, causing people to finally look at lignin not as a waste product but a product that can be converted into useful things like bioplastics, nylon, jet fuel, and fabrics.

The fact that Sandia is now recruiting bacterium to help further improve lignin’s conversion to useful products is no surprise, but we do think this could be the beginning of a very profitable future for lignin. Now it’s time to take it to the bank!

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