Isolation and characterization of a thermophilic PET-degrading bacterium

Abstract:

[Objective] Polyethylene terephthalate (PET) plastics are usually discarded as waste, which seriously harms the ecological health. Considering the lack of thermophilic PET-degrading bacteria, this study aims to obtain a thermophilic PET-degrading bacterium and decipher its degradation mechanism. [Methods] We collected waste PET bottles from hot springs in Tengchong City, Yunnan Province and then extracted the biofilms from their surfaces to investigate the microbial diversity. We screened the thermophilic bacteria that could grow with PET as a nutrient source and identified them based on 16S rRNA gene sequences. According to the colonization ability and growth curve, we screened out a strain with strong degrading ability and determined the optimal pH, temperature, and NaCl concentration for its growth. Further, we investigated the degrading effects of the strain on PET and its intermediates, (bis(hydroxyethyl) terephthalate (BHET) and mono(2-hydroxyethyl) terephthalate (MHET), by measuring the product yield and degradation rate. In addition, we evaluated the interaction between the degrading bacterium and PET by observing morphology of the PET surface and determining the viable count and esterase activity. [Results] The microbial diversity was low in the biofilms of waste PET bottles. We isolated 5 thermophilic bacterial strains that could grow with PET as a nutrient source from the biofilms. Strain JQ3 using PET as the sole carbon source showed the best growth, which was identified as Bacillus thermoamylovorans. Its optimal growth conditions were pH 7.0, 50, and 0.5% NaCl. Strain JQ3 degraded PET at a rate of 0.043 mg PET/d, and the yield of terephthalic acid (TPA) peaked at 45.2 mmol/L on the 7th day. Moreover, strain JQ3 exhibited significant degradation effects on PET intermediates, degrading 85.9% of BHET within 6 h and 50.1% of MHET within 60 h. Strain JQ3 formed biofilm on the PET surface by colonization, which caused cracking and peeling of the PET surface. [Conclusion] The thermophilic PET-degrading strain B. thermoamylovorans JQ3 can degrade PET and its intermediates at high temperature (60), which provides a new strategy for PET degradation.