Caterpillars might not break down plastic after all

RESEARCHERS in Germany say that research earlier this year showing that waxworms can chemically break down polyethylene does not show “sufficient proof” of biodegradation.

In April, Federica Bertocchini at the University of Cambridge, found that waxworms, the caterpillar of the wax moth, Galleria mellonella, could break down polyethylene shopping bags. It followed a chance discovery when waxworms, which generally feed on beeswax, invaded her beehives and she temporarily stored them in plastic bags, which developed holes. Bertocchini and her team used spectroscopic analysis which identified ethylene glycol on the digested plastic bags, showing that the waxworms were digesting the plastic, not just breaking it into smaller pieces. As well as using live caterpillars, the team used a mixture of smashed-up caterpillars and achieved the same spectra indicating the presence of ethylene glycol.

However, a team Johannes Gutenberg University of Mainz, led by organic chemistry professor Till Opatz, say that they have re-examined the results from the spectroscopic analysis and that it is missing two characteristic absorbance spikes for ethylene glycol. They believe the spike found by Bertocchini and the team may be down to animal fat and protein residue on the bags, rather than ethylene glycol. They smeared minced pork and egg yolk on carrier bags and obtained very similar spectra to those obtained from bags upon which the smashed-up caterpillars had been smeared.

Opatz and the team suggested that the breakdown of the plastic bags may be due to the caterpillars physically breaking down the bags, leaving the plastic chemically unaltered.

“We cannot rule out the possibility that the wax moth Galleria mellonella might indeed be capable of the chemical (rather than physico-mechanical) destruction of polyethylene but the present publication does not provide sufficient proof for this claim. In our opinion, future studies need to include ¹³C labelling experiments to probe the formation of ¹³C-labelled metabolites in the worms resulting from the digestion of ¹³C–polyethylene, ideally as a function of the time of treatment/exposure,” Opatz and the team conclude.

They say that ¹³C labelled polyethylene should form part of future testing with the waxworms.

Current Biology doi.org/cc7h

Bertocchini and the team have since responded to the research (doi.org/cdc6). In correspondence published in Current Biology, they write that the spectroscopy peaks they identified as relating to ethylene glycol are consistent with those described in other studies. They dispute Opatz’ assertion that the peaks they observed when the bags were treated with smashed up caterpillars came from fat and protein traces, as residue was carefully washed away and similar peaks were obtained when live caterpillars were used.

“We agree that labelling studies would be very desirable to confirm polyethylene breakdown and identify more accurately the nature of the breakdown products, although in our view the fact that polyethylene is broken down at all is more significant for our initial study than the precise nature of the breakdown products,” they conclude.