Sapel Blog
The Secret of Plastic Digestion: Why Can Insects Eat Plastic When Humans Can’t?
Global plastic production has reached about 460 million tonnes a year, and because of their very high chemical stability these materials can persist in soil and water ecosystems for several hundred years. As large pieces of plastic wear down and break apart in nature, they produce very fine particles known as microplastics. These particles pollute water and soil and enter the bodies of animals and humans through the food chain, with consequences such as bioaccumulation and environmental hazards.
The structural properties of plastics, such as their high molecular weight, strong hydrophobicity and tightly packed crystalline structure, make them extremely resistant to digestion and breakdown. In this article we look at why the human digestive system cannot digest these tough structures, while the guts of some insects, with the help of symbiotic bacteria, have found a remarkable way to break these bonds.
Structural and Chemical Barriers to Digesting Plastic
Commercial plastics have a very high molecular weight, strong hydrophobicity and a tightly packed crystalline structure. The water-repellent surface of plastic prevents water uptake and stops microbes and enzymes from attaching in the first place. On top of that, the dense packing of the chains in the crystalline regions severely limits how far water and enzymes can penetrate, so enzymatic attack is largely confined to the less dense, disordered (amorphous) regions of the material.
Chemical Barriers to Digesting Plastic in the Stomach
The backbone of polyolefins and related vinyl plastics (polyethylene, polypropylene, polystyrene and PVC), which are used to make all kinds of containers, from a 10-liter jerrycan to a 1-liter bottle, as well as toys and many other products, is made up entirely of very strong, non-polar carbon–carbon and carbon–hydrogen bonds. Because they have no oxygen-containing functional groups or hydrolyzable bonds, natural enzymes are unable to break their backbone. Condensation polymers (polyethylene terephthalate, polyurethane and polylactic acid) are different: they have ester or amide-type bonds in their backbone. These groups make hydrolysis and attack by specialized enzymes (such as polyesterases and lipases) possible. Even so, the rigid rings in their structure still slow down their breakdown under normal conditions.
Human vs. Insect Digestion: How Each Copes with Plastic
The Human Gut and Plastic
The human digestive system is designed to digest natural, organic nutrients and lacks the specialized enzymes or bacteria needed to break the strong chemical bonds of plastics. Plastic particles therefore pass through the body without being digested or absorbed and without any change to their structure. But they don’t just pass through; they also do harm. Although the plastic backbone itself is not digested, chemicals added to plastic during manufacturing (such as phthalates and bisphenol A) are released along the way. Once inside the body, these toxic compounds can cause hormonal disorders, cell damage and metabolic abnormalities. A steady intake of tiny plastic particles also upsets the vital balance of beneficial gut bacteria, and this damage to the microbial population puts the normal functioning of the immune system and digestive health at risk.
Insect Digestion: How Insects Digest and Biodegrade Plastic
Unlike humans, some insects and larvae (such as mealworms and flour beetles) host special communities of symbiotic bacteria in their guts. These microbes have a remarkable ability to tackle tough plastic structures. Bacteria living in the guts of these insects (such as strains of Citrobacter, Kosakonia, Acinetobacter and Exiguobacterium) produce specific enzymes that cut the long, heavy chains of polystyrene and polyethylene into smaller pieces. As the molecular weight of the plastic drops, it becomes absorbable, so the insect can use it as a source of energy and convert the plastic into biomass and carbon dioxide. Studying these same insects has also led to the discovery of enzymes that eat and break down plastic, and do so ten times faster.
Conclusion
Research on the digestive systems of insects and their symbiotic microbes has opened up new horizons in industrial biotechnology. The real secret behind these insects’ ability to digest plastic lies in the synergy of the enzymatic “scissors” in their gut. By modeling these enzyme systems, designing advanced catalysts and putting biorefineries to work, industry and researchers can pave the way for fast, industrial-scale biodegradation of plastics.
