Isolation and screening of low-density polyethylene degrading bacteria

Abstract

Low-density polyethylene (LDPE) is a thermoplastic made from monomer ethylene and its extensive usage and persistent nature, has contributed significantly to several environmental issues due to its indiscriminate disposal and inadequate recycling. This thesis is aimed to identify and characterize bacterial isolates with potential LDPE biodegradation capabilities. Six partially degraded plastic samples were collected from dump sites near M Hostel at Thapar Institute of Engineering and Technology. From these, 24 bacterial isolates were screened using various assays, including zone of clearance, Polyethylene glycol-4000 (PEG) utilization in Bushnell Haas medium (1% and 5% PEG), and hydrophobicity assays. The zone of clearance assay indicated that isolates DGK1, DGK2, DGK4, DGK5, DGK7, DGK8, DGK9, DGK20, DGK21, and DGK24 exhibited plastic-degrading potential, with DGK4 and DGK7 showing the most significant zones of clearance. PEG utilization data revealed that DGK4 and DGK7 demonstrated high efficiency in degrading PEG, with utilization rates of 43.47 ± 0.96% and 43.87 ± 0.39% for 1% PEG, and 39.11 ± 1.20% and 40.13 ± 0.32% for 5% PEG, respectively. Hydrophobicity assays showed that DGK7 had the highest hydrophobicity (50.14 ± 0.28%), suggesting a strong affinity for hydrophobic materials like LDPE. Consequently, DGK4 (Gram-negative) and DGK7 (Gram-positive) were selected for LDPE biodegradation studies. Gravimetric weight loss measurements confirmed substantial degradation of LDPE by DGK4 and DGK7, with DGK7 exhibiting the highest degradation rates. For DGK4, weight loss percentages were 0.46 ± 0.09% for Plastic Sample 1 (44 µm) UV and 5.19 ± 0.17% for Plastic Sample 2 (22 µm) UV. For DGK7, weight loss percentages were 4.95 ± 0.21% for Plastic Sample 1 (44 µm) UV and 12.13 ± 0.56% for Plastic Sample 2 (22 µm) UV, highlighting DGK7 as the most effective strain in degrading LDPE. SEM analysis of DGK7 revealed significant surface erosion and morphological changes in LDPE samples, underscoring its effective degradation capabilities. These findings underscore the potential of DGK4 and particularly DGK7 as effective candidates for bioremediation of LDPE, contributing to the ongoing efforts to address plastic pollution.