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A novel Bacillus subtilis BPM12 with high bis(2 hydroxyethyl)terephthalate hydrolytic activity efficiently interacts with virgin and mechanically recycled polyethylene terephthalate
Summary
Researchers discovered a soil bacterium, Bacillus subtilis BPM12, that can break down PET plastic building blocks at impressively high rates and across a wide range of temperatures and pH levels. The study shows that combining mechanical shredding with biological degradation by this microbe could be a practical route to recycling more PET plastic waste, a major source of environmental microplastics, back into useful chemicals.
Biotechnological treatment of plastic waste has gathered substantial attention as an efficient and generally greener approach for polyethylene terephthalate (PET) depolymerization and upcycling in comparison to mechanical and chemical processes. Nevertheless, a suitable combination of mechanical and microbial degradation may be the key to bringing forward PET upcycling. In this study, a new strain with an excellent bis(2 hydroxyethyl)terephthalate (BHET) degradation potential (1000 mg/mL in 120 h at 30 °C) and wide temperature (20-47 °C) and pH (5-10) tolerance was isolated from a pristine soil sample. It was identified as Bacillus subtilis BPM12 via phenotypical and genome analysis. A number of enzymes with potential polymer degrading activities were identified, including carboxylesterase BPM12CE that was efficiently expressed both, homologously in B. subtilis BPM12 and heterologously in B. subtilis 168 strain. Overexpression of this enzyme enabled B. subtilis 168 to degrade BHET, while the activity of BPM12 increased up to 1.8-fold, confirming its BHET-ase activity. Interaction of B. subtilis BPM12 with virgin PET films and films that were re-extruded up to 5 times mimicking mechanical recycling, revealed the ability of the strain to attach and form biofilm on each surface. Mechanical recycling resulted in PET materials that are more susceptible to chemical hydrolysis, however only slight differences were detected in biological degradation when BPM12 whole-cells or cell-free enzyme preparations were used. Mixed mechano/bio-degradation with whole-cells and crude enzyme mixes from this strain can serve to further increase the percentage of PET- based plastics that can enter circularity.