The present study was aimed to isolate the microorganisms from soil, with the ability to degrade plastics including Polyethylene, Polyurethane, Poly(3-hydoxybutyrate) (PH B) and Poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), observe the extent of degradation by analysis of plastics and study of enzymes involved in the process of degradation.
Low density polyethylene (LOPE) is one of the major sources of environmental pollution. In the present study, the pieces of LOPE plastic bags were buried in soil and observed microscopically after 10 months. Fungal strains found attached on the surface of LOPE plastic pieces were identified as Fusarium sp. AF4, Aspergillus terreus AF5 and Penicillium sp. AF6. Scanning electron microscopy (SEM) showed some mechanical damage or erosions on the surface of LOPE pieces incubated with Fusarium sp. AF4. Biodegradation of LOPE was determined by Sturm test, and it was found that in case of test the total amount of C02 produced was 1.85 g/l and in control it was 1.45 g/l. No changes in the Fourier Transform Infrared Spectroscopy (FTIR) were observed after soil burial of LOPE film. Further the possibility of accelerating the biodegradation process by exposing LOPE pieces to UV radiation and treating with nitric acid before microbial treatment. FTIR analysis of the pretreated PE pieces showed some breakdown indicated by the change in spectra at the wavelength 1710 cm-1 and 831 cm-1 which decreased to 1708 cm-1 and 830 cm-1.
Bacterial strains attached on the Polyurethane (PU) film, after soil burial for 6 months, were isolated and identified as Bacilllus sp. AF8, Pseudomonas sp. AF9. Micrococcus sp. AF10, Arthrobacter sp AF11, and Corynebacterium sp. AF12. Hydrolytic zones were observed around the colonies of the bacterial isolates when plated on PU-containing mineral salt media. SEM of the PU films after microbial treatments showed pits, extensive spotting and change in color. FTIR analysis also showed the formation of some new intermediate products. The results of the Sturm test showed more CO2 production (4.46 g/l) than that in the control (2.23 g/l), when PU was degraded by bacterial consortium of the isolates.
Poly(3-hydroxybutyrate) (PHB) and Poly(3-hydroxybutyrate-co-3-hyroxyvalerate) (PHBV) degrading actinomycete strain, Streptoverticillium kashmirense AF1, was isolated from soil by enrichment technique. The degradation of PHB and PHBV by Stv. kashmirense AF1 was studied by observing the formation of clear zones of hydrolysis on the polymer containing mineral salt agar plates. The optimum production of the PHB and PHBV depolymerases was observed both at slightly alkaline and neutral pH (pH 8 and pH 7), 45°C, 1 % substrate concentration and in the presence of lactose as an additional carbon source. Two types of PHB and PHBV depolymerases were purified by column chromatography using Sephadex G-75 and their molecular weights were determined as 37 and 45 kDa and 35 and 45 kDa, respectively, by SOS-PAGE. In case of microbially treated PHB and PHBV, the FTIR spectra showed formation of some new peaks (1400-1600 cm-1). In case of PHB degradation, the Sturm test showed amount of CO2 evolved in test (6.02 g/l) was more as compared to the control (2.99 g/l). Positive degradation was also shown by the amount of CO2 evolved in case of PHBV Sturm test (7.85 g/l, test; 0.94 g/l, control).
Three PHBV degrading bacterial strains identified as Bacillus subtilis AF2, Bacillus megaterium AF3, Micrococcus sp. AF7 were isolated after soil burial of PHBV film for about 120 days. The bacterial strains found to degrade PHBV film, produced clear zones of hydrolysis on the PHBV containing agar plates when used in the form of consortium. The PHBV degraders showed optimum depolymerase production at pH 7, 37oC, in the presence of 0.4% substrate, and presence of glucose and Tween 80 during 2-5 weeks of incubation. Bacillus megaterium AF3 was found to produce more PHBV depolymerase than the other members of the consortium. PHBV depolymerase was purified from B. megaterium AF3to homogeneity by chromatography on Sephadex G-75 and its molecular weight was found to be approximately 37 kDa. Sturm test also indicated the degradation of PHBV by bacterial consortium.