The article is published in Scientific India magazine (Oct. 2017 issue)
Plastics have become the ubiquitous workhorse material of the modern economy – combining unrivalled applied properties with low price. Their use has expanded twentyfold in the past five decades and is anticipated to double again within the next 20 years. It has been speculated that more than 310 million tonnes of plastic is produced annually worldwide. Almost everyone, everywhere, every day comes into contact with plastics – especially plastic packaging. Considerable amount of plastics is used for packing but only~14% is collected for recycling. Their resilience is great when you want a product to last. But as soon as cast-off, plastics linger in the environment, littering streets, unused land and oceans. Every corner of our planet has been blighted by our obsession to plastic. So far there is no sustainable way towards biodegradation of plastic, but a new study suggests an answer may interestingly lie in the metabolisms of Ideonella bacterium.
Poly(ethylene terephthalate) (PET) is a colourless polymer with an annual worldwide production of around 50 million tons. Although this polymer is made from two simple monomers linked via ester bonds, its enzymatic or biological degradation has turned been to an extent very challenging task. To date, very few species of fungi – but no bacteria – have been found to break down PET. Ideonella sakaiensis was discovered in 2016 by a team of collaborating researchers from Kyoto Institute of Technology and Keio University after they collecting samples of PET debris in a probe for bacteria which hinges on plastic for carbon growth. Ideonella sakaiensis completely degrades and assimilates PET as its only carbon source. The researchers identified this strain by running through 50 PET debris–contaminated environmental samples including sediment, soil, wastewater, and activated sludge from a PET bottle recycling site. I. sakaiensis strain-a Gram negative, aerobic beta-proteo bacteria—is the sole microorganism responsible for degrading PET.
Action of I. sakaiensis
You may think this is the rerun the old chestnut, as plastic-eating microbes have already been hyped as an ultimate solution to plastic waste problem of the planet. But there are several important differences here.
Primarily, the previous reports were of tricky-to-cultivate fungi, whereas in this case the microbe scan be easily grown. The researchers just left the PET in an incubator with bacterial culture, some nutrients and few weeks later, the plastic was gone.
Secondly, talking about the real innovation which is that the research team has identified the enzymes that Ideonella sakaiensis uses to breakdown the PET. All organisms contain enzymes that they use to catalyse necessary chemical reactions. Some enzymes help digest our food, breaking it into numerous macronutrients and micronutrients. Without the necessary enzymes the body can’t exercise certain nutrients even when provided in adequate amount through food. The mechanism of action of I. sakaiensisis a simple enzymatic process. The bacterium secretes the enzyme PETase which then converts rather generates an intermediate mono(2-hydroxyethyl) terephthalicacid (MHET). This MHET molecules are taken up by the cell and hydrolysed to give a second enzyme MHET hydrolase, an intracellular enzyme which gives the two parent monomers of PET.
The biodegradation process is relatively slow, as the degradation of a small PET thin film took 6 weeks. However, it is a ground-breaking discovery which may have a huge impact on enzyme evolution. Till date only a few number of hydrolytic enzymes have been reported which were successful in breaking the ester linkages. One of such is a hydrolase from Thermobifida fusca. PETase was reported to have 51% homology with this hydrolase.
Assimilation of PET materials by I. sakaiensis is a ground-breaking discovery which will be advantageous in elimination plastic from the environment. Another advantage is that the monomers of PET obtained by hydrolysis of MHET, especially terephthalic acid could be isolated and reused. This in turn will provide huge savings in production of new polymers without the need of exploiting petrochemical reserves for raw materials Metabolic engineering can further be applied to crease a possible integration of PETase and MHETase to obtain a common strain for enzyme cascade systems. Further probe into the field will certainly provide us some more insight to the process of degradation and recycling of other plastics highly resistant to any physiochemical treatment and could be further used for benefit of mankind.