Salt-loving bacteria opens new possibilities for marine microplastic research
By combining the DNA of different bacteria, scientists have engineered a microbe that might make it easier to study marine microplastics.
By: Eva Cornman
It started with a knock on the door. After the National Science Foundation put out a call for research proposals addressing the problem of microplastics, Nathan Crook’s next door neighbor, a fellow professor at North Carolina State University, came knocking. Can you do anything bio-related on this?, he asked. “Bio-related” being his specialty, Crook said yes.
That “bio-related” research gave rise to a paper that was released in the chemical engineering journal AIChE earlier this September. Bioengineers at North Carolina State University took a stab at what’s become a major problem: microscopic pieces of plastic accumulating in the environment. “I decided I would do what I do best and engineer a microbe to do something about it,” says Crook, a biomolecular engineer and author on the study who admits to having a long standing interest in the microplastic problem.
But engineering a microbe is no small feat. Researchers have been studying ways to degrade polyethylene terephthalate (PET), one of the most widely used plastics worldwide, for decades. In 2016, a group at Kyoto University discovered bacteria growing on waste outside a PET bottle recycling facility. The bacteria, which they named Ideonella sakaiensis, or Is, were using PET as a fuel source, degrading the particles in the process.
This was big news. Some researchers tried plastic digestion using Is itself. Others injected the genes responsible for Is’s PET degrading enzymes into microbes like E. coli that are good at scooping up and expressing genes that aren’t their own. But microbes like E. coli and Is are not so good at surviving in salty marine environments, the very places where microplastics can accumulate and harm marine life.
So, Crook and Tianyu Li, a PhD student in Crook’s lab and lead author on the new paper, set their sights on a different bacteria: Vibrio natriegens. Not only is V. natriegens extremely fast-growing, but it’s also skilled at surviving in salty environments. “I found there are a lot of really great characteristics of Vibrio natriegens,” says Li. “It’s just people haven’t really well studied it nor applied it.”
But V. natriegens doesn’t like to express proteins other than its own, making it challenging for it to express the Is proteins that would break down PET. To overcome this challenge, Li linked the Is proteins to proteins that V. natriegens already naturally expressed on its cell membrane. A protein hitchhiker, in a sense.
This hitchhiker method turned out to be one of the most important findings from the study. Using this method, V. natriegens expressed the PET degrading enzyme. However, after incubating the bacteria with PET particles and measuring the plastic levels, the research team found that the engineered microbes degraded PET, but not very efficiently. Based on their results, Li and Crook estimated that it would take about 24 years for their engineered microbe to degrade 1 g/L of PET particles.
Linda Zhong-Johnson, a PhD student at MIT who also studies PET degrading enzymes but was not involved in this study, says that the research “is a good proof of concept, but it’s not optimized.” Li and Crook got V. natriegens to express the Is protein, but this microbial model is still not operating efficiently enough within salt conditions to make it a viable technology.
To address this problem, Li and Crook are currently working on mutating the enzyme over and over to see if those changes result in better performance. But for now, they believe that they’ve opened new possibilities for marine microplastic research.
“One of the more fundamental results from this work is that we have a new surface anchor for Vibrio that works pretty well,” says Crook. V. natriegens, which Crook thinks of as the “natural choice” when considering marine microplastics, can now be used in research with greater ease due to the “hitchhiker” surface anchor method that Li figured out. For interested scientists, Crook says, “they won’t have to do a lot of the guess work like Tianyu did.”
But he still urges caution when considering the applications of this new research. “The main risk in a lot of the science reporting is that this is the answer to the plastic pollution crisis. It’s not.” So for the foreseeable future, V. natriegens will have to keep munching on plastic in the lab rather than plastic in the sea.
