Sludge from a plastic recycling plant may have provided a solution to the problem of the millions of metric tons of poly(ethylene terephthalate) that piles up annually in landfills. Scientists reported this year that a bacterium they found at a PET bottle recycling facility in Sakai, Japan, uses the plastic as its primary carbon and energy source (Science 2016, DOI: 10.1126/science.aad6359). The bacterium or its isolated enzymes could someday purposefully devour water bottles, salad domes, peanut butter jars, and any other plastic items emblazoned with a number one inside a recycle symbol.
Discovered by Kyoto Institute of Technology’s Kohei Oda, Keio University’s Kenji Miyamoto, and coworkers, the microbe was dubbed Ideonella sakaiensis. It has the distinction of being the world’s first PET-eating bacterium. Although PET is the most recycled plastic in the U.S., more than half of it still ends up in landfills where the polymer’s strong ester bonds resist breakdown.
To make a new PET bottle from a used one, the plastic can be hydrolyzed to its monomers chemically, but this process requires high temperatures and pressures. I. sakaiensis uses two enzymes to dice up the polymer at a surprisingly mild 30 °C, enabling the researchers to recover the monomers terephthalic acid and ethylene glycol.
At the moment, I. sakaiensis and its enzymes need some tweaking before they’re ready to chow down on large amounts of the world’s PET waste: The bacterium prefers to dine on amorphous PET, rather than the crystalline PET used in most products, and the enzymes work too slowly to be used industrially.