Researchers engineer bacteria to produce plastics – Ars Technica

A bacterial energy storage system is modified to make polymers.
Plastics are great, except when it comes to making or disposing of them. Production generally requires the use of chemicals derived from fossil fuels, and so helps to continue our reliance on them. And the final products are generally not biodegradable, so they tend to stick around despite breaking down into ever smaller fragments.Biology might ultimately provide a solution, however. Researchers have identified bacteria that evolved the ability to digest some plastics. And improvements in our ability to design proteins have allowed us to make new enzymes that can chew up plastics.This week brings some progress on the other side of the equation, with a team of Korean researchers describing how they’ve engineered a bacterial strain that can make a useful polymer starting with nothing but glucose as fuel. The system they developed is based on an enzyme that the bacteria use when they’re facing unusual nutritional conditions, and it can be tweaked to make a wide range of polymers.The researchers focused on the system bacterial cells use for producing polyhydroxyalkanoates (PHAs). These chemicals are formed when the bacterial cells continue to have a good supply of carbon sources and energy, but they lack some other key nutrients needed to grow and divide. Under these circumstances, the cell will link together small molecules that contain a handful of carbons, forming a much larger polymer. When nutritional conditions improve, the cell can simply break down the polymer and use the individual molecules it contained.The striking thing about this system is that it’s not especially picky about the identity of the molecules it links into the polymer. So far, over 150 different small molecules have been found incorporated into PHAs. It appears that the enzyme that makes the polymer, PHA synthase, only cares about two things: whether the molecule can form an ester bond (PHAs are polyesters), and whether it can be linked to a molecule that’s commonly used as an intermediate in the cell’s biochemistry, Coenzyme A.Normally, PHA synthase forms links between molecules that run through an oxygen atom. But it’s also possible to form a related chemical link that instead runs through a nitrogen atom, like those found on amino acids. There were no known enzymes, however, that catalyze these reactions. So, the researchers decided to test whether any existing enzymes could be induced to do something they don’t normally do.The researchers started with an enzyme from Clostridium that links chemicals to Coenzyme A that has a reputation for not being picky about the chemicals it interacts with. This worked reasonably well at linking amino acids to Coenzyme A. For linking the amino acids together, they used an enzyme from Pseudomonas that had four different mutations that expanded the range of molecules it would use as reaction materials. Used in a test tube, the system worked: Amino acids were linked together in a polymer.The question was whether it would work in cells. Unfortunately, one of the two enzymes turns out to be mildly toxic to E. coli, slowing its growth. So, the researchers evolved a strain of E. coli that could tolerate the protein. With both of these two proteins, the cells produced small amounts of an amino acid polymer. If they added an excess of an amino acid to the media the cells were growing in, the polymer would be biased toward incorporating that amino acid.However, the yield of the polymer by weight of bacteria was fairly low. “It was reasoned that these [amino acids] might be more efficiently incorporated into the polymer if generated within the cells from a suitable carbon source,” the researchers write. So, the researchers put in extra copies of the genes needed to produce one specific amino acid (lysine). That worked, producing more polymer, with a higher percentage of the polymer being lysine.Most of the polymers incorporated a fair amount of lactic acid, which can also form ester bonds. There’s normally lots of lactic acid in the cell since it’s one of the potential products of glucose metabolism. But the researchers knocked out the gene that encodes the primary enzyme that produces with lactic acid, dramatically cutting down the amount incorporated into the polymer.The researchers also tried a variety of conditions, showing that they could create polymers that were a mixture of two different amino acid monomers, and incorporating non-amino acids into the mixture. By adding a few additional enzymes to the E. coli strain, they managed to boost the yield of the polymer by weight to over 50 percent. They also showed that you could introduce mutations to the enzyme that does the polymerization, and it would selectively incorporate more of a specific amino acid into the resulting polymer.Overall, the system they develop is remarkably flexible, able to incorporate a huge range of chemicals into a polymer. This should allow them to tune the resulting plastic across a wide range of properties. And, considering the bonds were formed via enzyme, the resulting polymer will almost certainly be biodegradable.There are, however, some negatives. The process doesn’t allow complete control over what gets incorporated into the polymer. You can bias it toward a specific mix of amino acids or other chemicals, but you can’t entirely stop the enzyme from incorporating random chemicals from the cell’s metabolism into the polymer at some level. There’s also the issue of purifying the polymer from all the rest of the cell components before incorporating it into manufacturing. Production is also relatively slow compared to large-scale industrial production.So, it’s not quite ready to take over global plastic production. But the work does do a great job of highlighting the potential of bio-based manufacturing.Nature Chemical Biology, 2025. DOI: 10.1038/s41589-025-01842-2  (About DOIs).Ars Technica has been separating the signal from
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Source: https://arstechnica.com/science/2025/03/researchers-engineer-bacteria-to-produce-plastics/