Isolation and selection of polyethene plastic-degrading microbial strains

Abstract

Plastic bags or “festals” are made from polyethene. Due to their molecular stability, plastics do not easily breakdown into simpler components; therefore, they are not considered biodegradable. Worldwide 500 billion to 1 trillion plastic bags are being produced each year and it persists in the environment between 20 and 1000 years before they decompose. The majority of these synthetic plastics do not degrade in the environment, and incineration of plastics generates CO2 and the highly poisonous dioxins. There is yet no report on microbial polyethene and plastic degradation in Ethiopia. Advances in making polymers in more environmentally friendly and sustainable manner can have significant beneficial consequences. The aim of this study was to isolate and screen polyethene degrading microbes from garbage dumps and soils covered with vegetation in and around Jimma town, South Western Ethiopia. Screening of isolates was based on their ability to degrade low and high density polyethene. Microbes were isolated using Streptomyces and Sphingomonas selective media as well as enrichment broth procedure using polyethene as a sole carbon and energy source. Preliminary qualitative and quantitative screening based on color clearing and weight loss of plastic respectively by pure shake flask culture assays were used to screen plastic-degrading microbes. These microbes were tested for degradation of heat pre-treated (at 70oC) and untreated polyethene plastic separately and in combination. Degradation was measured in terms of weight loss after incubation for six weeks on a shaker. Among the tested microbes Sphingomonas strain LBG-5 demonstrated the highest degradation of both heat-treated and untreated low density and high density polyethene plastics. This strain degraded about 20% and 8% weight of heat-treated low density and high density polyethene sheets respectively within a period of six weeks. Garbage dumps are found out to be good source of plasticdegrading microbes that degrade low density polyethene better than high density. Microbes degrade polyethene better when they are combined. Heat pre-treated polyethene sheets showed greater degradability than untreated ones. Degradation of polyethene is influenced by physical, chemical and biological factors such as: heat exposure, the type of plastic and the kind of microbial strain to which it is exposed.