Promoting bacterial colonization and biofilm formation for enhanced biodegradation of low-density polyethylene microplastics

The accumulation of plastic waste presents a significant worldwide environmental challenge. This study aimed to isolate polyethylene-degrading bacteria from marine samples containing plastic waste. Four culturable bacterial isolates: Micrococcus luteus, Bacillus cereus, Enterococcus faecalis, and Actinomyces sp. were assessed for their biofilm formation, biosurfactant, and protease production. Gamma irradiation was used to induce structural changes and promote bacterial colonization and biofilm formation on low-density polyethylene microplastics (LDPE MPs). Optimal biofilm formation was achieved in minimal media supplemented with 30% tryptic soy broth, 10% biosurfactant, and 300 µM calcium chloride. The factorial design experiment demonstrated that adding media supplementation significantly improved bacterial colonization and biofilm formation when compared to gamma irradiation. This was supported with Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) mapping, and Fourier Transform Infrared Spectroscopy (FTIR). The optimized LDPE MP degradation was achieved through a multi-step protocol: (1) samples are pre-treated to 40 kGy gamma irradiation, which resulted in 5.7% Gravimetric weight loss and structural and morphological changes, (2) incubation in biofilm inducing media overnight, and (3) further incubation in minimal media for 30 days. This approach resulted in a total weight loss of 22.5%. In conclusion, synergistic pre-treatment is recommended to promote biofilm and improve biodegradation of LDPE MPs by marine bacteria.