Expanding the flavonoid production in Acinetobacter baylyi ADP1 from lignin-derived aromatics

Plants are master chemists, constantly crafting complex molecules that protect them from the sun, fight off infections, and give them their colors. Among these chemical creations are flavonoids, a diverse family of compounds found in fruits, vegetables, and flowers. Humans have long valued these substances for their potential health benefits, using them in foods, medicines, and cosmetics. For decades, the primary ways to obtain flavonoids have been to extract them from plants or to build them from scratch in a laboratory. Both methods have significant drawbacks: extracting them from crops is often inefficient and limited by the seasons, while chemical synthesis can be costly and environmentally taxing.

In recent years, scientists have turned to tiny living factories called microbes to produce these compounds. By inserting plant genes into bacteria or yeast, researchers can program these cells to manufacture specific flavonoids. However, most of these microbial factories are fed simple sugars, like glucose. To turn sugar into a flavonoid, the microbe must first build a complex aromatic ring structure from scratch, a process that requires many genetic steps and consumes a lot of the cell's energy. This makes the process long and difficult to control. A more direct approach would be to feed the microbes the aromatic building blocks they need, bypassing the initial construction phase. Nature provides these blocks in abundance within lignin, a tough, woody polymer that gives trees their rigidity. When wood is broken down, it releases a mixture of aromatic compounds that are chemically very similar to the precursors plants use to make flavonoids. The challenge has been finding a microbe that can not only eat these tough aromatic compounds but also tolerate the harsh conditions of breaking down wood, all while being easy to engineer for production.

A team of researchers at Tampere University in Finland has successfully addressed this challenge by turning to a bacterium called Acinetobacter baylyi ADP1. Unlike the common lab bacteria often used in these studies, this organism naturally possesses the machinery to consume a wide variety of aromatic compounds found in wood. The researchers set out to see if they could harness this natural ability to turn lignin-derived materials directly into valuable flavonoids. Their goal was to create a single microbial platform that could produce different types of flavonoids simply by changing the type of wood-derived food provided, without needing to rewire the bacteria's genetic code for each new product.

The team began by testing whether they could get the bacteria to produce homoeriodictyol, a specific flavonoid often used in food and medicine. This molecule is typically made from a precursor called ferulate. The researchers first tried to tweak the bacteria's existing enzymes to accept ferulate, hoping to make a small genetic adjustment that would allow the cell to process this new ingredient. They altered specific parts of the enzyme responsible for building the flavonoid structure, but these changes did not work; the bacteria could not produce the desired molecule. Realizing that minor tweaks were insufficient, the researchers shifted their strategy. Instead of modifying the existing enzyme, they looked for a different version of the enzyme from a different plant species that was naturally capable of handling ferulate. They found a match in a plant called Stenoloma chusanum. When they swapped the original bacterial enzyme for this new plant enzyme, the bacteria immediately began producing homoeriodictyol. This discovery highlighted a key principle: sometimes the most effective solution is not to force an existing tool to do a new job, but to find a tool that was already designed for it.

With this new enzyme combination in place, the researchers optimized the growing conditions to maximize production. They found that the bacteria performed best when fed a moderate amount of the precursor and grown at a specific temperature. Under these conditions, the bacteria produced 3.7 milligrams per liter of homoeriodictyol in a flask. But the team did not stop there. They wanted to see if this same setup could produce other flavonoids. They introduced a different aromatic compound, caffeate, into the culture. Remarkably, the exact same strain of bacteria, with the exact same genetic setup, switched gears and began producing a different flavonoid called eriodictyol. In this case, the bacteria produced 24.2 milligrams per liter, a significantly higher amount than before. This demonstrated that the bacteria's output was entirely dictated by the food it was given. If fed ferulate, it made one product; if fed caffeate, it made another.

To test the system under more realistic conditions, the researchers moved the bacteria into a bioreactor, a larger vessel used for industrial-scale fermentation. They fed the bacteria a mixture of three different aromatic compounds found in wood: ferulate, caffeate, and p-coumarate. The bacteria successfully converted this mixed soup into a cocktail of three different flavonoids simultaneously. While the total amount of product was lower than when feeding a single ingredient, likely because the different pathways competed for resources, the result proved that the system could handle complex mixtures. This is a crucial step, as real-world wood waste is never a single pure chemical but a complex blend.

Finally, the team tested the system with actual industrial byproducts. They took wheat straw and a type of processed wood called organosolv lignin, treated them with a chemical process to break them down, and collected the resulting liquid. This liquid, known as alkaline pretreated liquor, contains a natural mixture of aromatic compounds but also many other impurities that can be toxic to most bacteria. The researchers added this complex liquid to their bacterial cultures. Despite the messy and challenging environment, the bacteria thrived and produced flavonoids. They successfully converted the compounds found in the wheat straw and lignin liquors into the target molecules, proving that the system works not just with pure chemicals in a lab, but with the actual waste streams generated by the forestry and agriculture industries.

The study concludes that Acinetobacter baylyi ADP1 is a highly versatile platform for sustainable manufacturing. By leveraging the bacterium's natural ability to eat aromatic compounds, the researchers bypassed the need for the bacteria to build these structures from scratch. This approach allows for a flexible production system where the final product is determined by the feedstock. If a factory has a surplus of one type of wood-derived chemical, the bacteria can be fed that to make one specific flavonoid; if the supply changes, the same bacteria can be fed a different chemical to make a different product, all without further genetic engineering. This work establishes a direct link between the valorization of lignin, a material often burned for fuel, and the production of high-value natural products, offering a promising path toward a more circular and efficient bioeconomy.