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Exploring Rhodococcus jostii RPET as a Polyethylene Terephthalate Bio-upcycling Platform
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Scientists found a hardy bacterium that can break down PET plastic (the kind used in water bottles and clothing) and turn the leftover chemicals into useful products like lipids and lycopene, a valuable antioxidant. This matters because it offers a more sustainable alternative to simply landfilling or incinerating plastic waste, potentially reducing the plastic pollution that eventually breaks down into microplastics found in our water, food, and even our bodies. The bacterium worked well even with real-world, impure plastic waste, suggesting this approach could realistically scale up beyond the lab.
Among the plastics currently in use, polyethylene terephthalate (PET) is among the most widely produced worldwide, with applications including woven fabrics, packaging resins, and thermoplastic resins. The escalating issue of plastic pollution has become a matter of global concern. Complete recycling of PET has long been accomplished through chemical depolymerization, which effectively converts PET into its monomers, terephthalic acid (TPA) and ethylene glycol (EG), or other intermediates such as dimethyl terephthalate (DMT). Nevertheless, sustainable strategies for converting waste PET into higher-value products remain limited. This dissertation investigates the feasibility of employing a bio-platform for PET bio-upcycling by characterizing a Rhodococcus strain, assessing its compatibility with depolymerization products, and elucidating its associated metabolic pathways. Through screening and growth analysis, a mutated strain RPET of Rhodococcus jostii was identified among several candidates belonging to the genera Rhodococcus, Pseudomonas, and Corynebacterium. The RPET strain exhibited proficient assimilation of both TPA and EG as carbon sources during growth and utilization analyses, and demonstrated the ability to withstand very high concentrations of TPA and EG. As a Rhodococcus strain, RPET is also capable of producing several value-added products, including lipids and lycopene. These attributes establish RPET as a resilient microbial platform for PET monomer conversion and underpin the foundation for bio-upcycling of PET. To integrate the bio-upcycling process into the existing infrastructure, multiple industrial PET depolymerization strategies were evaluated for their compatibility with microbial upcycling, including alkaline hydrolysis utilizing sodium hydroxide, enzymatic hydrolysis employing industrial PETase, and aminolysis with ammonium hydroxide. The resulting hydrolysates were directly employed as feedstocks to assess the growth performance and robustness of RPET. Across all three methods, the RPET achieved high growth rates and produced several value-added products simultaneously following genetic engineering. Moreover, the growth and production potential of RPET remained unaffected, despite the use of real post-commercial PET in hydrolysate production, thereby demonstrating its significant stability against potential impurities derived from PET waste. DMT is an essential chemical in the manufacturing and chemical recycling processes of PET. A comprehensive understanding of its bioassimilation is crucial for the upcycling of PET and the bioremediation of aromatic esters. RPET and another strain of Rhodococcus have been identified as DMT-degraders. Growth experiments, transcriptional analysis, and gene knockout studies were conducted to elucidate the key genes involved in DMT metabolism. Functional validation with knockout mutants confirmed the roles of genes encoding a putative DMTase and a putative MMTase. Additionally, a genetically engineered RPET strain successfully produced lipids and lycopene using DMT and glucose as feedstocks. These findings broaden the applications of RPET and enhance its capacity for upcycling. Collectively, these studies provide a promising foundation for PET hybrid upcycling utilizing Rhodococcus jostii RPET and contribute to the advancement of our sustainable strategies.
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