Scientists Discover Bacteria That Turn Plant Waste Into Green Energy

For many years, scientists have marvelled at certain bacteria’s ability to digest seemingly indigestible substances, including carbon from lignin. Lignin is that tough, woody material giving plants their rigidity. A new study from Northwestern University reveals a significant lignin metabolism: Pseudomonas putida, a common soil bacterium, completely reorganizes its entire metabolism to thrive on these complex carbons. This process involves the bacterium slowing down some metabolic pathways while accelerating others, allowing it to extract energy from lignin without exhausting itself.

This discovery holds substantial implications for the biomanufacturing industry. This sector has long aimed to harness Pseudomonas putida to break down lignin and convert it into valuable products like biofuels, plastics, and other useful chemicals. The new information offers a “blueprint” for researchers to build efficient and productive microbial factories. Ludmilla Aristilde, who led the study at Northwestern, highlighted lignin as “an abundant, renewable and sustainable source of carbon”. She believes it could potentially replace petroleum in producing plastics and valuable chemicals. While some microbes naturally produce precursors for these chemicals from lignin instead of oil, understanding how they achieve this was crucial. Now, researchers possess a clear “roadmap“.

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The Challenge of Lignin Digestion

Lignin is notoriously tough to digest. After cellulose, it stands as the second most abundant biopolymer on Earth. When lignin breaks down it form a mix of chemical compounds, including phenolic acids. These phenolic acids could serve as renewable feedstocks for valuable chemicals. However, scientists previously struggled to understand how bacteria managed to feed on these complex compounds. These compounds consist of a six-carbon ring with attached carbon chains, and few organisms can process them efficiently. Essentially, it requires too much energy to digest them.

Aristilde explained the energy balance using a human analogy. She noted that preparing and eating food uses energy, but consuming it also provides energy. A balance exists between the energy exerted to make food and the energy derived from it. The same principle applies to soil microbes.

How Bacteria Rewire Their Metabolism to Digest Plant Waste

To investigate how bacteria achieve this balance, Aristilde and her team grew Pseudomonas putida on four common, lignin-derived compounds. They then employed a suite of multi omics tools including proteomics, metabolomics, and advanced carbon-tracing techniques to precisely map how the bacteria moved carbon through their metabolism. Aristilde compared this metabolic network to roads in a busy urban area. They sought to examine “every street at very high resolution,” identifying “stoplights” and “traffic jams”. This detailed approach allowed them to pinpoint which pathways were crucial for balancing the cell’s energy optimally.

The team discovered that P. putida rewires its metabolism into a high-energy mode when encountering lignin. It significantly ramps up the levels of enzymes for certain metabolic reactions, sometimes by hundreds to thousands of times. This dramatic increase helps reroute digestive pathways, shifting carbon away from the “main highway” to “backup metabolic roads” to avoid bottlenecks. This metabolic remodeling allows the bacteria to produce six times more ATP—a molecule that provides energy—compared to when it consumes easier-to-digest compounds.

Delicate Balance and Engineering Implications

Despite these clever strategies that keep Pseudomonas putida balanced and functioning, the researchers found the system to be quite fragile. When they attempted to relieve bottlenecks by overexpressing certain enzymes, the approach backfired, disrupting the bacteria’s careful metabolic balance. Aristilde warned that “engineering strategies can often result in negative effects on the metabolism in a completely unexpected way”. Speeding up one pathway can introduce an energy imbalance detrimental to the cell’s operation.

This finding is particularly important for biotechnology applications, where engineers frequently modify bacteria’s metabolism to produce bio-based fuels and chemicals. Aristilde stressed the importance of understanding bacteria’s natural energy rules before pushing them to work harder. By identifying which pathways act as “speed bumps” or “energy boosters,” the biotech industry can develop smarter strategies to harness bacteria for producing sustainable products from plant waste.

Before this study, researchers could not precisely explain the coordination of carbon metabolism and energy fluxes essential for the rational design of bacterial platforms for lignin carbon processing. They often proceeded by trial and error. Now, with an actual “roadmap,” they understand how to navigate this complex network.