Scientists Create Bacteria That Produce Valuable Products from Plant Fiber

December 20, 2023

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conducted by Chris Hubbuch, University of Wisconsin-Madison

Microorganisms, despite their size, often assist in addressing large-scale issues. They aid in preparing food and beverages, curing illnesses, processing waste, and even reducing pollution. They can transform plant sugars into biofuels and chemicals, which are usually obtained from fossil fuels. This ability is considered a crucial aspect in most climate change mitigation initiatives.

Researchers from the University of Wisconsin–Madison have now engineered bacteria capable of simultaneously creating two chemical products using underutilized plant fiber. These multi-skilled microbes can efficiently perform both tasks.

'To the best of my knowledge, it's one of the first instances wherein two valuable products can be simultaneously manufactured in one microbe,' comments Tim Donohue, a bacteriology professor from UW–Madison and director of the Great Lakes Bioenergy Research Center.

The discovery, outlined in a paper in Applied and Environmental Microbiology, has the potential to make biofuels more sustainable and economically viable.

'Theoretically, the strategy lowers overall greenhouse gas emissions and enhances the financial aspect,' suggests Donohue. 'The energy and greenhouse gasses required to create two products in one pot would be lesser than running two pots for each product.'

The move to replace fossil fuels with sustainable alternatives depends on maximizing value extraction from renewable biomass. Lower volume, lucrative products can suppress fuel costs, much like petrochemicals.

Lignin - a part of the plant cell wall – poses one of the biggest challenges. Even though it is the largest source of renewable aromatic carbons, lignin's irregular structure makes it tough to decompose into useful segments.

Scientists with GLBRC have been studying a bacterium named Novosphingobium aromaticivorans (often referred to as Novo), which can digest many parts of lignin and is relatively easy to genetically modify.

In 2019, a strain of Novo was engineered to generate a key ingredient of plastics - PDC. Tim Donohue's lab recently discovered another modification that allowed Novo to create a different plastic ingredient known as ccMA.

'Our carbon emission issue cannot be solved by only producing these two products,' states Ben Hall, a recent doctoral graduate who was part of the research. Through genomic modeling, Donohue's team identified potential products that biomass aromatics could produce. One of the top contenders was zeaxanthin, a type of organic pigment known as carotenoids.

These organic pigments, responsible for the distinctive colors in carrots, pumpkins, salmon, and even flamingos, are deployed as nutritional supplements, pharmaceuticals, and cosmetics. They command a market value worth tens of billions of dollars annually.

While Novo was known to have genes that could produce a less commercially valuable carotenoid, based on the bacteria's genome sequence, the team suspected that zeaxanthin was a precursor to this less valuable carotenoid. Consequently, they altered the necessary genes to halt the production process at zeaxanthin, the more valuable product.

Further, they engineered strains that could produce other high-value carotenoids—beta-carotene, lycopene, and astaxanthin when raised on a lignin-derived aromatic compound.

The engineered bacteria were also shown to synthesize these carotenoids from a blend of ground and treated sorghum stems, a solution that contains a combination of aromatics that many industrial bacteria are unable to consume.

The next experiment involved Hall combining genetic modifications that allowed the production of both PDC and carotenoids in the same bacteria.

The result was a strain capable of manufacturing both PDC and the target carotenoid, with no discernible dip in either yield. What's more, the bacterial strains accumulated carotenoids within their cells, simplifying separation from PDC, which is secreted. 'We have to separate the cells from the media anyways,' Hall explains. 'Now we have a product coming out of both.'

The next steps include testing whether engineered strains can simultaneously produce carotenoids and ccMA, which Donohue thinks they will, and to engineer strains to improve yields in industrial conditions.

While there are lucrative markets for each of these products, Donohue and Hall say the real value of the discovery is the ability to add multiple functions to this biological platform.

'To me, it's both the strategy and the products,' Donohue says. 'Now that we've done this, I think it opens the door to see if we can create other microbial chassis that make two products.'

Journal information: Applied and Environmental Microbiology

Provided by University of Wisconsin-Madison