Microbial degradation, recycling and upcycling of PET wastes: A Mini-Review

Abstract

More than 350 million tons of plastics are produced globally per annum. The major kinds of plastics include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyurethane (PU), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). PET is one of the predominant plastics used in textiles and packaging. PET wastes have been recycled using mechanical and chemical methods. However, these techniques are costly, non-eco-friendly, and may generate toxic by-products. Hence, intensive research is underway to develop biological methods of PET depolymerization. Several PET-digesting enzymes have been isolated from bacteria, actinobacteria and fungi. However, bacterial enzymes suffer from low efficiency, slow reaction rates, recalcitrance to high crystallinity PET, and lack of robustness to range of pH and temperatures. In contrast, actinobacterial enzymes show higher thermostability but they also have their own limitations. More recently, improved enzymes have been produced using rational, AI directed and even directed evolution strategies. Some of these enzymes have demonstrated rapid depolymerization of low-to-medium crystallinity PET wastes. A French group has shown efficient depolymerization of PET bottles and recycling into virgin PET bottles. A British team has produced food, vanillin, from PET waste using an engineered microbe for the first time and other groups have shown the feasibility of upcycling (valorization) of PET waste into various value-added products. This minireview will highlight the recent developments in microbial degradation, recycling, and upcycling of PET wastes.