The mystery of how tiny bacteria can successfully digest some of the toughest plant waste expelled by most creatures has finally been resolved, in an achievement with major implications for biomanufacturing.
Lignin, a tough, complex polymer that provides plants with their rigidity, is typically indigestible to humans. However, a common soil bacterium called Pseudomonas putida has now been shown to be up to the task, long intriguing the biomanufacturing industry.
The new research, detailed in a paper from Northwestern University researchers published in Communications Biology, finally identifies exactly how the bacteria achieve this by shifting their metabolic processes, a discovery that may be a significant boon to the development of microbial factories dedicated to producing biofuels and plastics.
A Blueprint for Digestion
One of the most significant questions about Pseudomonas putida is not just how it manages to digest lignin, but how it does so without exhausting itself, thriving on the normally indigestible polymer. In their research, the Northwestern team found that the bacterium slows down specific metabolic processes while accelerating others, thereby increasing its digestive efficiency through coordinating carbon metabolism and energy production.
“Lignin is an abundant, renewable, and sustainable source of carbon that could potentially provide an alternative to petroleum in the production [of] plastics and valuable chemicals,” said Ludmilla Aristilde, who led the study.
“Certain microbes naturally have an ability to make precursors to valuable chemicals that are lignin-based rather than petroleum-based. But if we want to take advantage of that natural ability to develop new biological platforms, we first need to know how it works. Now, we finally have a roadmap,” Aristilde continued.
Lignin Explained
Only cellulose eclipses lignin as Earth’s most abundant biopolymer. The potential in successfully harnessing lignin is immense, as among the chemical compounds produced in its breakdown are phenolic acids, which help produce valuable chemicals. While biomanufacturing may find the chemicals useful, scientists have puzzled over how bacteria could metabolize them, as it would generally cost an organism more energy than it could gain from breaking down the complex carbon compounds.
“Before we eat food, we have to shop for it, cook it and eventually chew it up,” Aristilde explained. “That whole process uses energy but consuming the food also gives us energy. There is a balance between the energy we exert to make the food versus the energy we derive from the food. It’s the same for soil microbes.”
Studying Impossible Digestion
Aristilde’s team studied the metabolic processes through a set of “multi-omics” tools, designed to follow carbon’s journey as the bacteria metabolized it, including proteomics, metabolomics, and advanced carbon-tracing techniques. The researchers applied these tools to Pseudomonas putida that they grew on four common compounds derived from lignin to develop a digestive roadmap.
“We wanted to see what happens on every street at very high resolution,” Aristilde explained. “We wanted to know where every ‘stoplight’ and ‘traffic jam’ might occur. That allowed us to determine which pathways are important to balance the energy in a way that is optimal for the cell.”
Tracing the bacteria’s metabolic processes revealed that when faced with the challenge of complex lignin compounds, Pseudomonas putida activated a previously unknown high-energy mode. In this state, the bacteria increased enzyme production for targeted metabolic reactions by hundreds or even thousands of times. Additionally, it increased efficiency and avoided bottlenecks by rerouting the flow of carbon inside it. With all of these changes, Pseudomonas putida increased its production of energy-providing ATP by sixfold when compared against more readily digestible compounds.
Biomanufacturing Futures
With the process now understood, the team began to consider how they could further optimize it. While the system was already highly efficient, the team attempted to remove some remaining bottlenecks by overexpressing specific enzymes. However, it turned out that improving upon nature was not so easy, and the entire metabolic system became unbalanced due to their alterations.
“Engineering strategies can often result in negative effects on the metabolism in a completely unexpected way,” Aristilde said. “By speeding up the flow of one pathway, it can introduce an imbalance in energy that is detrimental to the operation of the cell.”
As the industry seeks new ways to leverage the team’s discoveries in biomanufacturing applications, the failed experiment to enhance the bacteria’s efficiency is a significant outcome. It highlights the importance of continued work to fully understand Pseudomonas putida’s energy production before attempting to optimize it for commercial use in transforming plant waste into sustainable products.
“Before this study, we could not explain exactly the coordination of carbon metabolism and energy fluxes important in the rational design of bacterial platforms for lignin carbon processing,” Aristilde said. “We just had to figure it out as we went along. Now that we have an actual roadmap, we know how to navigate the network.”
The paper, “Quantitative Decoding of Coupled Carbon and Energy Metabolism in Pseudomonas putida for Lignin Carbon Utilization,” appeared in Communications Biology on August 29, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.