One of the noticeable things about microbiologist Christian Rinke’s laboratory is the startlingly loud crunching noise of wormlike larvae chewing their way through polystyrene, burrowing into blocks of the plastic foam. Before he discards a chewed-through block, Rinke says he raises it to his ear to check for stragglers. “If the worm is still eating in there,” he says, “you can actually hear it.”
Rinke and his colleagues have been feeding plastic to Zophobas morio beetle larvae—dubbed “superworms” for their large size—to see if the microbes and enzymes in their gut might offer insights into how to break down some of the staggering amount of plastic waste humans generate. The researchers have found that these superworms can survive on a diet of nothing but polystyrene, which is used in a vast array of products, ranging from cups to packing peanuts. The worms’ ability to process the plastic suggests it is very efficiently broken down in the creatures’ digestive tract. “They are basically like eating machines,” says Rinke, who works at the University of Queensland in Australia and co-authored a new study describing his team’s findings, published on Thursday in Microbial Genomics.To investigate how superworms’ gut microbiome reacts to a purely plastic diet, the researchers split 135 of the creatures into three groups: one was fed only wheat bran, another was fed only soft polystyrene, and the third was given nothing. All the worms were monitored for cannibalism, and members of the starved group were isolated from one another. The bran-fed larvae were significantly healthier than their plastic-fed or starved counterparts, more than doubling their weight over the three weeks they were monitored. After that, some of the worms from each group were set aside to grow into beetles. Nine out of 10 bran-fed worms successfully grew into beetles and maintained the most diverse gut microbiome of all three groups. The plastic-fed larvae made less impressive gains—but they still put on more weight than the starved worms, and two thirds of them grew into beetles. Clearly polystyrene is a poor diet for the larvae, Rinke says. But it seems they can extract at least some energy from the material.
This is likely because of a symbiotic relationship between a superworm and its gut bacteria. The worm essentially shreds the plastic so the bacteria can biodegrade it and break it down into smaller molecules that may be more easily digestible—or possibly could one day be upcycled to create new plastic, Rinke says. Knowing exactly which bacterial enzymes these gut microbes use to break down the polystyrene is the golden ticket to replicating the process on a large scale in the future. For the new study, identifying those enzymes required sequencing the genomes of the organisms in the worms’ gut. “Using metagenomics, we can actually characterize all the genes in the [digestive] microbiome,” Rinke says. Previous studies of other insects were not as comprehensive, focusing on just one or two possible gut bacteria or enzymes, according to Rinke.Uwe Bornscheuer, head of the biotechnology and enzyme catalysis department at the University of Greifswald in Germany, has been waiting for these kinds of data since it first became evident just more than a decade ago that some insect larvae could eat hard-to-degrade plastics—and could thus possibly help scientists find a way to use biodegradation to recycle them. The newly published work is “the first solid study where they looked into the metagenome,” says Bornscheuer, who was not involved with the paper but had been following this area of research
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