Eat (y)our plastics!

Can plastic-degrading bacteria help us to get rid of all that plastic waste?

The world has a pretty heavy plastic problem. Since the invention of plastic in the late 19th century, more than 6.3 billion metric tons of plastic waste have been globally generated—almost one ton per person currently living on Earth. Even more shockingly, 80 percent of it has ended up in landfills, or worse, in the environment as litter, where it poses a threat to wildlife and habitats. But not only that: animals, especially fish, often mistake small microplastic particles for food, which lets plastic enter the food chain and eventually end up on our plate. Given the increasing accumulation of plastic in the natural environment it is safe to say that traditional ways of plastic recycling and disposal have been failing in managing the huge amount of plastic waste produced. In looking for alternatives, researchers might have now found little allies in fighting plastic pollution: plastic-“eating” bacteria.

In 2016, a team of Japanese researchers led by Shosuke Yoshida identified a bacterium that can completely degrade the petroleum-derived plastic PET (Polyethylene terephthalate). Reports about microorganisms capable of “chewing through” different forms of plastics have been published before, but the bacterium that the Japanese team discovered—Ideonella sakaiensis—exhibits a very special feature: it can live off PET as its sole carbon and energy source. Yoshida and colleagues found that I. sakaiensis adheres to PET and secretes an enzyme, which they termed PETase, that catalyzes the hydrolysis of PET into the reaction intermediate MHET (mono(2-hydroxyethyl) terephthalic acid). With the help of a second enzyme, MHETase, the bacterium then breaks down MHET and catabolizes the resulting benign monomers terephthalic acid and ethylene glycol for growth. Interestingly, the expression of PETase specifically increased when the researchers grew I. sakaiensis in the presence of PET. Also, PETase favored PET over the preferred substrates of other previously reported PET-hydrolyzing enzymes, degrading PET from six to 120 times faster under physiological conditions. These observations suggest that the enzyme must have specifically evolved to hydrolyze PET. 

I. sakaiensis secretes the enzyme PETase to break down PET into the reaction intermediate MHET (mono(2-hydroxyethyl) terephthalic acid). The cell then takes up MHET and degrades it with the help of a second enzyme, MHETase, into TPA (terephthalic acid) and EG (ethylene glycol) which the bacterium further catabolizes for growth.

“This is a very exciting finding,” confirms John D. Coates, director of the Energy Biosciences Institute at UC Berkeley. Coates and his lab have successfully been working on developing microbiological approaches to help fight different kinds of environmental pollution for years. “But, unfortunately, I believe that plastic biodegradation in its current state will not solve our waste problem in the near future,” Coates adds.

So what exactly keeps us from dumping a bit of I. sakaiensis into landfills and letting it chew away our plastic waste?

Well, for one, PET is only one of many plastics. In fact, it is one of the easier-to-degrade plastics. PET belongs to the category of thermoplastics, a family of plastics that can reversibly be heated, formed, and frozen. Because of these properties, thermoplastics like PET are preferentially used in the packaging industry. The second category of plastics is called thermosets. Unlike thermoplastics, thermosets contain cross-linked molecular chains, which, once heated, form a three-dimensional network keeping thermosets from reshaping when heated. Thermosets are therefore very durable and almost impossible to degrade. 

And it gets worse. Most other thermoplastics, including the widely used polypropylene and polyethylene, contain atomic bonds that are much harder to break than the ones found in PET. Those thermoplastics therefore withstand any microbial attacks and have only been successfully biodegraded, so far, when preoxidized or thermally pretreated

Plastics contain crystalline (light blue) and amorphous regions (gray). In crystalline regions, molecules (dark blue) tightly pack together in an ordered fashion, which makes those regions less susceptible for microbial attack.

In addition, the degradability of thermoplastics further depends on their structural order. Plastics contain both highly ordered crystalline regions, where molecules can pack together very regularly, and less ordered so-called amorphous regions. The higher the proportion of crystalline regions in a plastic, the harder it is to degrade. For their studies on I. sakaiensis Yoshida and his group worked with a less complex low-crystalline PET film. Still, it took the bacteria 6 weeks to fully degrade 60 milligrams of film in a test tube. Guess how long they would need in such a setup to eat their way through all the plastic waste found on earth? 1,000,000,000,000,000,000 years. Definitely too long! A more catalytically-active PETase could speed up the process, and more efficient versions of the enzyme have been reported. Still, microbial PET degradation remains very slow and is thus a long way from commercial application.

Nonetheless, there is a lot we can learn from the study on I. sakaiensis. “The data are definitely a door-opener for future research,” says Coates. First, the screening strategy used to identify PET-“eating” bacteria could be applied to identify other microorganisms capable of degrading different kinds of plastics. Second, given that PET has only been around for a few decades, PETase seemed to have evolved surprisingly fast. “This leaves some hope that there is likely more out there,” says Coates. 

Biodegradation might not solve our existing plastic waste problem, but it’s reassuring to know that research keeps working on new ways to fight plastic pollution. Hopefully, the development of new, more efficient recycling methods combined with increased production of easier-to-hydrolyze plastics, such as biodegradables, will help to reduce plastic waste. In the meantime, we can contribute by trying to minimize our own plastic consumption to keep the planet from drowning in plastic—at least for now.

Featured image by Rob Sinclair.

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