Biodegradation of low density polyethylene (LDPE) by a new biosurfactant-producing thermophilic Streptomyces coelicoflavus NBRC 15399T

Abstract

In the present study, we tested 83 actinomycete isolates for low density polyethylene (LDPE) degradation in the laboratory, and these isolates were previously isolated for biosurfactant production from the oil contaminated soil sample near Naval Dockyard in Visakhapatnam. Among them, 20 were positive using 2,3,5-triphenyl tetrazolium chloride (TTC) as a viability indicator and able to grow in a mineral salts medium containing LDPE as a sole source of carbon. Of these, NDYS-4 isolate showed prominent result with redox probe 2,3,5-triphenyltetrazolium chloride (TTC) as a viability indicator, which forms pink colour insoluble triphenylformazan (TPF) on mineral salts media containing emulsified polyethylene as a carbon source within five to seven days, and it was also grown prominently on mineral salt agar plates containing LDPE in comparison to other isolates, and it was selected for detailed analysis. The weight loss of polyethylene films was approximately 30% after four weeks of incubation with selected isolate. The viability and metabolic activity of this isolate growing on the polyethylene surface was confirmed using a TTC reduction test. The metabolic activity was also correlated with a concomitant increase in the protein density of the biomass on the surface of the polyethylene. The microbial degradation of LDPE was also analyzed by the change in pH of the culture media and microscopic analysis. Based on the results, this degradation may be mediated by enzymatic activity and may also involve cell-surface hydrophobicity. It was also able to grow on other polymers such as polyvinyl acetate (PVA), polycaprolactone (PCL), polyethylene oxide (PEO) and polyethylene glycol (PEG) using TTC as a viability indicator and also showed prominent growth on mineral salts agar plates containing polyethylene as a source of carbon. Based on the results of phenotypic characteristics, phylogenetic analysis, biochemical characteristics and 16S rRNA gene sequence homology, the isolate NDYS-4 was identified as Streptomyces coelicoflavus NBRC 15399^T. It could be concluded that the PE degrading actinomycete selected in this study showed diverse and varying capacities to degrade polyethylene and other polymers and can be exploited for cleaning up polyethylene contaminated sites.

Key words: Biosurfactant, low density polyethylene, 2,3,5-triphenyl tetrazolium chloride, mineral salts media, hydrophobicity, sequence homology, phylogenetic analysis.