Microbial plastic degradation: enzymes, pathways, challenges, and perspectives

Synthetic polymers have transformed modern life, giving rise to a wide spectrum of versatile materials commonly known as plastics. They are essential to industries including packaging, medical devices, automotive, textiles, and many consumer goods. However, significant environmental challenges have emerged because of the same properties that make plastics so useful. Of the estimated 400-450 million tons (Mt) of plastics produced each year, nearly 80 percent end up in the environment. Many of these plastics will persist in nature for hundreds or even thousands of years because they are mostly not biodegradable or poorly biodegradable. The identification of polymer-active microorganisms and enzymes that target most fossil fuel-based plastics is one of the greatest challenges microbiologists are facing today. Currently, more than 255 functionally verified plastic-active enzymes from more than 11 microbial phyla are known. Here, we summarize current knowledge on the microbial pathways and enzymes involved in the degradation of polyethylene terephthalate (PET), polyamide (PA) oligomers, ester-based polyurethane (PUR), and polycarbonates (PC), as well as some of the most widely used bioplastics. We also highlight the challenges microbiologists face in identifying microorganisms acting on highly persistent commodity polymers such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), ether-based PUR, PA, polystyrene (PS), epoxy resins, and synthetic rubber (SR), for which no truly efficient degraders are currently known. We highlight methods used to discover novel microorganisms and enzymes involved in biodegradation and measure and quantify their activities. Finally, we will review the biotechnological applications of microbial-driven plastics recycling.