Plastic degrading bacteria isolated

PET bottle

The bacteria was able to almost completely degrade a thin film of PET film in six weeks at 30 degree C. – Photo: R. Prasad

A bacterium species capable of breaking down plastic — poly(ethylene terephthalate), or PET — has been identified by a team of Japanese researchers.  The bacterium uses two enzymes in sequence to break down the highly biodegradable-resistant polymer PET.  The results are published today (March 11) in the journal Science.

Except for rare instances of two fungi that have been found to grow on a mineral medium of PET yarns, there are no reports of any bacteria biologically degrading PET or growing on the chemically inert substance.

Shosuke Yoshida, the first author of the paper from the Department of Applied Biology, Kyoto Institute of Technology, Kyoto, and others collected 250 debris-contaminated samples from a PET bottle recycling site. They looked for microorganisms that relied on PET film as a primary source of carbon for growth.  At first they identified a distinct microbial consortium that contained a mixture of bacteria species that degraded the PET film surface at 30 degree C; 75 per cent of the PET film surface was broken down into carbon dioxide at 28 degree C.

From the microbial consortium, the researchers isolated a unique bacterium — Ideonella sakaiensis 201-F6 — that could almost completely degrade a thin film of PET in a short span of six weeks at 30 degree C. “The PET film was damaged extensively and almost completely degraded after six weeks at 30 degree C,” they note.

The bacterium degrades PET using two enzymes that act on it in sequence.  First, the bacterium adheres to PET and produces an intermediate substance through hydrolysis.  The second enzyme then works with water and acts on this intermediate substance to produce the two monomers — ethylene glycol and terephthalic acid — that are used for making PET through polymerisation.

Human-made PET has been littering the environment for the last 70 years and in 2013 56 million tonnes of PET were produced worldwide. Since PET came into being only 70 years ago, a pertinent question is how this distinct bacterium evolved in the environment. Also, it is not clear what natural processes were at play for the two unique enzymes capable of breaking down PET in sequential steps to evolve.

“PET enrichment in the sampling site and the enrichment culture potentially promoted the selection of a bacterium that might have obtained the necessary set of genes through lateral gene transfer,” they write.

“Did both hydrolytic enzymes evolve during that relatively short period to enable the bacterium to access a novel carbon source and hence provide an advantage for survival? Examples for such rapid natural evolution are scarce,” says Uwe T. Bornscheuer from Institute of Biochemistry, Greifswald University, Germany in an accompanying Perspective article.

Published in The Hindu on March 11, 2016