Synthetic plastics are pivotal in our current lifestyle and therefore, its accumulation is a major concern for environment and human health. Petroleum-derived (petro-)polymers such as polyethylene (PE), polyethylene terephthalate (PET), polyurethane (PU), polystyrene (PS), polypropylene (PP), and polyvinyl chloride (PVC) are extremely recalcitrant to natural biodegradation pathways. Some microorganisms with the ability to degrade petro-polymers under in vitro conditions have been isolated and characterized. In some cases, the enzymes expressed by these microbes have been cloned and sequenced. The rate of polymer biodegradation depends on several factors including chemical structures, molecular weights, and degrees of crystallinity. Polymers are large molecules having both regular crystals (crystalline region) and irregular groups (amorphous region), where the latter provides polymers with flexibility. Highly crystalline polymers like polyethylene (95%), are rigid with a low capacity to resist impacts. PET-based plastics possess a high degree of crystallinity (30–50%), which is one of the principal reasons for their low rate of microbial degradation, which is projected to take more than 50 years for complete degraded in the natural environment, and hundreds of years if discarded into the oceans, due to their lower temperature and oxygen availability. The enzymatic degradation occurs in two stages: adsorption of enzymes on the polymer surface, followed by hydro-peroxidation/hydrolysis of the bonds. The sources of plastic-degrading enzymes can be found in microorganisms from various environments as well as digestive intestine of some invertebrates. Microbial and enzymatic degradation of waste petro-plastics is a promising strategy for depolymerization of waste petro-plastics into polymer monomers for recycling, or to covert waste plastics into higher value bioproducts, such as biodegradable polymers via mineralization. The objective of this review is to outline the advances made in the microbial degradation of synthetic plastics and, overview the enzymes involved in biodegradation.

Polyethylene (PE) is the most abundant synthetic, petroleum-based plastic materials produced globally, and one of the most resistant to biodegradation, resulting in massive accumulation in the environment. Although the microbial degradation of polyethylene has been reported, complete biodegradation of polyethylene has not been achieved, and rapid degradation of polyethylene under ambient conditions in the environment is still not feasible. Experiments reported in the literature suffer from a number of limitations, and conclusive evidence for the complete biodegradation of polyethylene by microorganisms has been elusive. These limitations include the lack of a working definition for the biodegradation of polyethylene that can lead to testable hypotheses, a non-uniform description of experimental conditions used, and variations in the type(s) of polyethylene used, leading to a profound limitation in our understanding of the processes and mechanisms involved in the microbial degradation of polyethylene. The objective of this review is to outline the challenges in polyethylene degradation experiments and clarify the parameters required to achieve polyethylene biodegradation. This review emphasizes the necessity of developing a biochemically-based definition for the biodegradation of polyethylene (and other synthetic plastics) to simplify the comparison of results of experiments focused for the microbial degradation of polyethylene.

We have characterized the ability of eight bacterial strains to utilize powdered low-density polyethylene (LDPE) plastic (untreated and without any additives) as a sole carbon source. Cell mass production on LDPE-containing medium after 21 days of incubation varied between 0.083 ± 0.015 g/L cell dry mass (cdm) for Micrococcus luteus IRN20 and 0.39 ± 0.036 g/L for Cupriavidus necator H16. The percent decrease in LDPE mass ranged from 18.9% ± 0.72% for M. luteus IRN20 to 33.7% ± 1.2% for C. necator H16. Linear alkane hydrolysis products from LDPE degradation were detected in the culture media, and the carbon chain lengths of the hydrolysis products detected varied, depending on the species of bacteria. We also determined that C. necator H16 produced short-chain-length polyhydroxyalkanoate biopolymers, while Pseudomonas putida LS46 and Acinetobacter pittii IRN19 produced medium-chain-length biopolymers while growing on polyethylene powder. Cupriavidus necator H16 accumulated poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-V) polymers to 3.18% ± 0.4% of cdm. The monomer composition of the PHB-V was 94.9% ± 0.61% 3-hydroxybutyrate and 5.03% ± 0.56% 3-hydroxyvalerate. This is the first report that provides direct evidence for simultaneous bioconversion of LDPE plastic to biodegradable polyhydroxyalkanoate polymers.

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Three bacterial species isolated from whole body extracts of the greater wax moth larvae, Galleria mellonella, were evaluated for their ability to utilize low-density polyethylene (LDPE) as a sole carbon source in vitro. These bacteria were identified as Lysinibacillus fusiformis, Bacillus aryabhattai, and Microbacterium oxydans. Their ability to biodegrade LDPE was assessed by growth curves, cell biomass production, polyethylene (PE) weight loss, and the presence of LDPE hydrolysis products in the growth media. Consortia of these bacteria with three other bacteria previously shown to degrade LDPE (Cupriavidus necator H16, Pseudomonas putida LS46, and Pseudomonas putida IRN22) were also tested. Growth curves of the bacteria utilizing LDPE as a sole carbon source revealed a peak in cell density after 24 h. Cell densities declined by 48 h but slowly increased again to different extents, depending on the bacteria. Incubation of LDPE with bacteria isolated from greater wax moth larvae had significant effects on bacterial cell mass production and weight loss of LDPE in PE-containing media. The bacterial consortia were better able to degrade LDPE than were the individual species alone. Gas chromatographic analyses revealed the presence of linear alkanes and other unknown putative LDPE hydrolysis products in some of bacterial culture media.

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