Degradation of Low-Density Polyethylene (LDPE) by bacterial isolates

Abstract

Low-density polyethylene (LDPE), poses a significant environmental challenge due to its resistance to natural degradation. This study aimed to isolate and characterize bacterial strains capable of degrading plastic polymers, specifically PEG-4000 and LDPE. Four bacterial strains DGK4, DGK5, DGK7, and DGK8 isolated previously from waste disposal site near M Hostel, Thapar University, Patiala were examined for degradation of PEG and LDPE. Initial screening using zone of clearance assays on Bushnell Haas agar supplemented with yeast extract and 5% PEG-4000 indicated positive degradation activity by all four isolates, suggesting their ability to utilize synthetic polymers as carbon sources. Further the effect of PEG on bacterial isolates was studied in Bushnell Haas medium (with and without yeast extract) supplemented with varying concentrations of PEG (1%, 2%, 4%, and 8%). Among the isolates, DGK7 and DGK4 consistently demonstrated the highest degradation potential. DGK7 reduced PEG concentration from 1.96 ± 0.03 to 0.95 ± 0.02 at 2% PEG in enriched medium within 72 hours. DGK4 also exhibited strong degradation, with PEG reduction from 1.98 ± 0.03 to 1.29 ± 0.01 under similar conditions in Bushnell Haas supplemented with yeast extract and 5% polyethylene glycol (PEG). Gravimetric analysis of LDPE films further validated these results. After 30 days in Bushnell Haas medium supplemented with yeast extract and under UV pre-treatment, DGK4 and DGK7 caused weight loss of 10.21 ± 0.59% and 9.67 ± 0.70%, respectively, compared to non-UV treatments which resulted in 7.52 ± 0.75% and 6.05 ± 0.68%. After 60 days, DGK7 showed the highest degradation by 31.0 ± 0.89% (UV) and 20.89 ± 0.78% (non-UV), while DGK4 reached 25.6 ± 0.91% (UV) compared to 13.64 ± 0.97% (non-UV). DGK5 and DGK8 showed moderate or limited enhancement under UV conditions. Scanning electron microscopic analysis of LDPE films degraded by DGK7 revealed significant structural damage, such as pits, cracks, and surface erosion, confirming enzymatic action and microbial colonization. These findings suggest that the combination of UV treatment and microbial activity enhances LDPE degradation. Phylogenetic analysis of the two potential bacterial isolates was also carried out, and the result indicates that the 16 S rRNA sequence of isolate DGK7 had similarity with Paenibacillus sp. and isolate DGK4 had similarity with Klebsiella sp.