Eat me!

Micro-organisms have evolved processes that let them digest plastics. Could they remedy one of Earth’s most pressing problems?

- Michael Marshall

In 2016 researchers led by microbiologist Kohei Oda of the Kyoto Institute of Technology in Japan reported a surprise discovery. Oda’s team visited a recycling site that focused on items made of polyethylene terephthalate (PET), a clear plastic that is used to make clothing fibres and drinks bottles.

Like all plastics, PET is made up of long string-like molecules. These are assembled from smaller molecules strung together into chains. The chemical bonds in PET chains are strong, so it is long-lasting – exactly what you do not want in a single-use plastic.

Oda’s team took samples of sediment and wastewater that were contaminated with PET, and screened them for micro-organisms that could grow on the plastic. It found a new strain of bacterium, Ideonella sakaiensis 201-F6, which could grow on pieces of PET. The team also reported that the bacterium could use PET as its main source of nutrients, degrading the PET in the process.

The key to this ability was a pair of enzymes made by the bacteria. Enzymes are complex molecules that can speed up chemical reactions. They are crucial to life: our digestive system relies on enzymes to break down the complex chemicals in food into simpler ones that our bodies can absorb and use.

Ideonella sakaiensis 201-F6 produces two unique enzymes. The first is a PETase that breaks the long PET molecules down into smaller molecules called MHET. A second enzyme called MHETase then goes to work, producing ethylene glycol and terephthalic acid. These two chemicals are the building blocks of PET, so Ideonella sakaiensis 201-F6 can completely reverse the manufacturing process that made PET.