Researchers study if caterpillar holds key to plastic litter

Can worms show the way to tackle the plastic litter problem? Incongruous as it may seem, that question could be serious, given the discovery that certain caterpillars seem able to digest and biodegrade polyethylene film.

Researchers in Spain and England say they found that the so-called wax worm, normally bred for fishing bait, can munch on PE bags, creating holes in the bag, and leaving behind what could be signature degradation products of PE.

It seems wax worms, which are really caterpillars of the greater wax moth Galleria mellonella, are able to break the carbon-carbon bonds in PE's molecular backbone. These bonds are normally resistant to decay, one reason PE finds such wide use in bags and other plastic packaging.

If a caterpillar can break C-C bonds, it's theoretically possible this research opens a new path to dealing with the huge problem of plastic litter accumulating on the planet.

The discovery was serendipitous, as discoveries often are. Scientist and beekeeper Federica Bertocchini was cleaning one of her beehives when she discarded wax worms from the hive into a PE bag. To her credit, she noticed that holes began appearing in the bag after only 40 minutes.

She conducted an experiment and found that 100 of the wax worms in the bag ate 92 milligrams of the PE over 12 hours. That digestion rate was hundreds of times faster than previously seen plastic degradation caused by microbes.

The wax worm is a pest in beehives because it eats beeswax, a product made by worker bees that is used to form cells for honey storage and protection of larvae and pupae. Beeswax is a complex mixture mainly of long-chain esters and long-chain alcohols. These molecules have numerous C-C bonds in their backbone, as well as carbon-oxygen bonds in their termini.

Bertocchini and colleagues deduce the worms can break the C-C bonds in PE similar to their apparent ability to break the bonds in beeswax.

It's not known yet if the apparent PE digestion is accomplished by microbes in the wax worm's gut, or by the caterpillar itself. If researchers can isolate the enzyme that breaks the C-C bonds, it possibly could be synthesized and applied on a large scale without the need for the actual wax moth caterpillars.

One researcher not involved in the study indicated it is misleading to speculate the results of the study could point the way to combat plastic litter.

"'Biodegradation' is not a magical solution to plastics waste management,” stated Ramani Narayan, professor of chemistry and biochemical engineering at Michigan State University.

“Release of small fragments (microplastics) into the terrestrial and ocean environment has been shown to cause harm to the environment and to human health.”

Narayan indicated other routes are preferred to reduce plastic litter. Truly biodegradable plastic could be mixed with food waste for digestion in a composter. Recycling of plastic waste or converting the waste to energy may be more viable than biodegradation.

Bertocchini is based at the Spanish National Research Council. Her colleagues in the research were Christopher Howe and Paolo Bombelli at the University of Cambridge's Department of Biochemistry.

Howe said in a telephone interview that Bertocchini contacted him and Bombelli to invite them to join in the research. Howe's usual work is in bioremediation, renewable energy and photosynthesis.

Howe said the discovery echoes natural processes in oceans whereby hydrocarbons are broken down by microbes. One class of microbes biodegrades long-chain molecules and those degradation products are then consumed by other microbes.

"There is this cycling of hydrocarbons in the oceans," Howe said. Much of the cycling occurs for hydrocarbons present in very low quantities. But the microbes occasionally get to feast on a hydrocarbon smorgasborg when there is an oil spill.

"Next on the list of things to do is determine if the insect itself is biodegrading the plastic, or if its gut bacteria are doing it," Howe explained.

If the relevant enzyme could be identified, then a search could begin for the caterpillar gene responsible for making the enzyme. The gene theoretically could be spliced into an organism for mass production using standard biotechnologies.

Howe said the research is at an early stage. He and colleagues have some companies in mind that they might approach to get involved in commercial development of the technology.

Narayan suggests other mechanisms could be responsible for the appearance of ethylene glycol and other alleged signature PE degradation products.

He posits that spectroscopic signals suggesting PE breakdown could simply be due to proteins and carbohydrates in the wax worm extracts. He said the published paper gives no evidence of other compounds, such as carbon dioxide, that would result from biodegradation.

Various spectroscopic and other analyses showed PE was attacked and formed ethylene glycol. Howe said other biodegradation products with carbon-oxygen bonds also were noticed. The researchers, however, have not delved into what the wax worms were doing at the molecular level with beeswax to see how that might compare to the apparent PE biodegradation.

"The main thing is the plastics breakdown," Howe said.

Critics speculate the "signature" ethylene glycol could have arisen by some other mechanism. Also, it's not known if the approach would turn plastic litter into thousands of small bits that are themselves a pollution problem because creatures can ingest them and the particles can attract and hold other pollutants.

The research could be a game-changer in the litter problem if creatures can help solve a human problem. As South African philosopher Mokokoma Mokhonoana wrote, "Every single living thing is food to at least one living thing."