Starting from their introduction in the early 1900s, the use of plastics has grown exponentially, and currently, plastics have become widespread and essential in almost every society on earth to the point that life without plastics is unimaginable for most of us. None of the commonly used plastics are considered biodegradable (meaning that they cannot be dissolved or decomposed by natural agents). As a result, they accumulate, rather than decompose, in landfills or the natural environment. The continued widespread use of plastics, which does not seem to be decreasing anytime soon, has led to plastic pollution becoming an environmental hazard for which an efficient, easily employable and environmentally-friendly solution is yet to be found. The main plastic polymers that are produced worldwide are polyurethane, polyethylene, polyamide, polyethylene terephthalate, polystyrene, polyvinyl chloride, and polypropylene. These and other plastics have become ubiquitous throughout the world, even in the deepest parts of the world’s oceans such as the Mariana Trench. Plastic pollution in oceans continues to increase with plastic debris being found in all major ocean basins, with an estimated 4 to 12 million metric tons of plastic waste generated on land entering the marine environment in 2010 alone.
It has been conjectured that the great abundance of plastics has led to the evolution of plastic degrading enzymes such as PETase (PET-digesting enzyme) in bacteria. For a long time, much of the plastics that were being produced (basically, all of the plastics mentioned above) were thought to be impossible to degrade via natural agents as the polymers are designed to be durable and stable. Furthermore, no enzymes or organisms capable of breaking them down had been identified until 1977, when some lipases (extracellular enzymes that usually cleave esters in oils and fats) were reported to be able to attack ester bonds in some aliphatic polyesters and depolymerize such materials. In recent years, a lot of discoveries later, removal of plastics from the environment using microbes and their enzymes has become a focus of much research.
For the purposes of this page, plastic-eating bacteria are defined as bacteria that can break down and digest plastics such as polyethylene terephthalate (PET), polyethylene (PE) and polyurethane (PU). The recent discovery of Ideonella sakeasis, which can use the plastic PET as a carbon and energy source, and the following modifications to its genome which have enhanced its degrading capabilities have given new hope for the management of plastic pollution through the utilization of microbes and their enzymes. A number of other plastic-eating bacteria/microbes have also garnered much attention as the number of enzymes and microorganisms that are capable of breaking down several different types of plastics keeps growing. But, however exciting the applications of these organisms and their enzymes may seem, their capabilities are still limited and require further investigation and modification so that their potential can be fully exploited.
By Meheret Ourgessa
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Legend/credit: Cumulative plastic waste generation and disposal (in million metric tons). Solid lines show historical data from 1950 to 2015; dashed lines show projections of historical trends to 2050.
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Ideonella sakaiensis was first discovered in 2016 by Japanese researchers who isolated the novel species from outside a bottle-recycling plant. The authors identified the species by screening 250 environmental samples at the polyethylene terephthalate (PET) bottle recycling site.  At the time, it was the first and only known bacteria species that had naturally evolved to be capable of breaking down and digesting plastic as a carbon and energy source. Earlier work had shown that the mesophilic fungus Fusarium oxysporum could also produce an enzyme that is able to break down PET into its constituent monomers. The discovery of I. sakaiensis was more exciting, however, as bacteria are much easier to harness for industrial uses compared with fungi and other eukaryotes. Furthermore, it was shown that the enzyme involved in hydrolyzing PET in I. sakaiensis had as much as 88 times more activity compared to the enzyme found in F. oxysporum. A hydrolase enzyme isolated from the actinomycete Thermobifida fusca was also shown to effectively depolymerize PET. But T. Fusca bacteria do not use this enzyme to catabolize PET and the activity of this enzyme is about 120 times lower.
I. sakaiensis bacteria stain Gram-negative and are aerobic bacteria belonging to the phylum Proteobacteria. The cells are non-spore forming and rod-shaped. The genus Ideonella belongs to the family Comamonadaceae of the class Betaproteobacteria. I. sakaiensis was identified as a representative of a novel species in the genus Ideonella on the basis of its physiological, biochemical, and phylogenetic data, including DNA–DNA relatedness. The DNA–DNA relatedness of I. sakaiensis with its closest phylogenetic neighbors was well below the 70 % cut-off point recommended for the assignment of the strains to the same genomic species. The bacteria grow within the pH range 5.5–9.0 (optimally between pH 7–7.5) and temperature range 15–42 ºC (optimally between 30–37 ºC) and cannot grow with 3% NaCl. The cells of the species are motile and use a polar flagellum to move. They attach to PET films using appendages which can also grow longer to connect the cells to each other. Appendages connecting the cell to PET films might assist in the delivery of secreted enzymes into the film. I. sakaiensis can degrade and assimilate PET as a sole carbon source. 
The researchers who discovered I. sakaiensis identified two enzymes produced by the bacteria that are involved in PET degradation - PETase and MHETase (MHET stands for mono(2-hydroxyethyl) terephthalic acid, the monoester of terephthalic acid and ethylene glycol) - which catabolized PET into its monomers terephthalic acid and ethylene glycol. These monomers are then used for bacterial metabolism. Although the PETase enzyme has been described in other bacterial species (mostly belonging to the phylum Actinobacteria), MHETase seems to be unique to the genome of I. sakaiensis. Originally, the degradation of PET by I. sakaiensis was relatively slow; complete degradation of a small PET film took 6 weeks. Recently, researchers were able to genetically modify I. sakaiensis to break down PET faster and also degrade PEF(polyethylene-2,5-furandicarboxylate) which is an emerging bioderived PET replacement with improved properties. The newly modified PETase enzyme is active on the aromatic polyesters (PET and PEF), but not aliphatic polyesters. The researchers who engineered this new enzyme concluded that it is likely that significant potential remains for improving its activity through similar protein modifications further.
PET was only patented roughly 80 years ago and put into widespread use in the 1970s. (15) It is likely that the enzyme system for PET degradation and catabolism in I. sakaiensis appeared only recently, demonstrating the remarkable speed at which microbes adapt and evolve to exploit new substrates. (15) In this case, the new substrate was waste from an industrial PET recycling facility. (15) It is thought that a limited number of mutations in a hydrolase, such as PET hydrolytic cutinase, that inherently targets the natural aliphatic polymer cutin (waxy polymer that is one of two main components of the plant cuticle) may have resulted in enhanced selectivity for PET and led to the evolution of the PET degradation pathway in I. sakaiensis. (8) Genomic analysis has shown that a genomic basis to support the metabolism of MHET analogs was established much earlier than when ancestral PETase proteins were incorporated into the pathway. (8) PET enrichment in the habitat of I. sakaiensis is thought to have potentially promoted the selection of a bacterium that might have obtained the necessary set of genes through lateral gene transfer. (8)
Mechanisms of Degradation
Understanding the mechanisms by which bacteria and other microbes degrade plastic polymers or oligomers can lead to identification of enzymes involved in the processes which can be targets for biotechnological engineering to lead to more efficient microbial degradation of plastics.
One of the most well-studied mechanisms of biodegradation of plastics is the pathway for PET catabolism in I. sakeasis. It is thought that I. sakeasis first excretes PETase enzymes (classified as esterase) from the cells onto PET surfaces. (24)(8) Appendages that grow extracellularly might be involved in the delivery of PETase onto PET surfaces.(8) Extracellular PETase hydrolyzes PET to produce MHET as the major product and TPA(terephthalic acid).(24) The PET hydrolysis products are then transported into the periplasmic space through an outer membrane protein such as porin.(24) MHETase, predicted to be an outer membrane anchored lipoprotein, hydrolyzes MHET into TPA and EG(ethylene glycol).(24) TPA is taken up into the cytoplasm through the TPA transporter coupled with TPA-binding protein and then integrated via protocatechuic acid (PCA) to the tricarboxylic acid (TCA) cycle.(24)(8) Ethylene glycol is metabolized via glyoxylic acid to the TCA cycle.(24)
Interestingly, the expression of the PETase gene was dramatically upregulated in the presence of PET (but not TPA) in a culture of I. sakaiensis, raising the question of which molecule(s) induces its expression.(24) However, TPA does upregulate the expression of a gene cluster that highly identical with two TPA degradation gene clusters identified in Comamonas sp. strain E6.(24) In other bacteria acting on PET,
Researchers have also proposed a degradation pathway for the oligomers of polyurethane by a Pseudomonas strain.(4) This proposed pathway is summarized in the figure to the right.(4) There a number of enzymes involved in this pathway. One of the first enzymes identified to act on PUR was the PueB lipase from Pseudomonas chlororaphis, a different Pseudomonas species.(17) This organism codes for at least one additional enzyme active on PU, which was designated PueA.(17) Both enzymes are lipases (which catalyze the ester-linked lipids) and similar to PET degradation with microbes, PU is degraded by the secreted hydrolases, and the degradation is tightly regulated.(17) As mentioned previously, enzymes and microbes described as capable of degrading organisms were all acting on ester-linked PU.(17) There have yet to be enzymes described to act on polyurethane ethers in scientific literature.(17)
To date, no microorganisms capable of degrading the intact high-molecular-weight polyamide polymer have been reported, but there are many bacteria that act on the oligomers of polyamide.(17) Three main enzymes have been reported as essential for the initial hydrolysis of cyclic and linear 6-aminohexanoate oligomers.(17) The first one is a cyclic-dimer hydrolase (NylA), the second a dimer hydrolase (NylB), and the third an endo-type oligomer hydrolase (NylC).(17) Once the oligomers are hydrolyzed, the monomers are metabolized by different aminotransferases.(17)
As with polyamides, there is no known microorganisms capable of degrading the high molecular-weight polystyrene polymer and neither are there any enzymes, but there are many microbes that degrade the oligomers of the plastic.(17) Under aerobic conditions, styrene is oxidized by two different pathways. The first one involves attacking the vinyl side chain and the second one is through attacking a rather nonspecific aromatic ring, thereby forming primarily the intermediates 3-vinylcatechol, phenylacetic acid, and 2-phenylethanol. These intermediates are channeled into the Krebs cycle after ring cleavage. The degradation of the vinyl side chain involves the action of three key enzymes, a styrene monooxygenase(styA and styB), a styrene oxide isomerase(styC), and a phenylacetaldehyde dehydrogenase(styD). The respective genes for side-chain oxygenation are frequently located in a single conserved gene cluster, often designated styABC(D).(17) The styrene monooxygenase attacks the vinyl side chain to release epoxystyrene, which is then subjected to isomerization to form phenylacetaldehyde. Phenylacetaldehyde is then oxidized to phenylacetic acid through the involvement of a dehydrogenase. In P. putida, the phenylacetic acid is activated to phenylacetyl-coenzyme A (CoA) and then subjected to β-oxidation to yield acetyl-CoA, which is directly fed into the Krebs cycle(26). (17)The expression of the conserved cluster is regulated through either a two-component regulatory system or LysR-type regulators.
Research performed on PE degradation by microbes so far is mainly of descriptive nature, with a few studies devoted to polyethylene degradation mechanisms or the isolation of enzymes belonging related to this process.(19) However these findings are limited and further evidence is required to identify the complete mechanisms for polyethylene degradation. (19) Similarly, the few studies that describe degradation of polyvinyl chloride and polypropylene by microbial communities do not detail the enzymes and pathways involved in the process.(17)(25)
- Geyer, R., Jambeck, J. R., & Law, K. L. "Production, use, and fate of all plastics ever made." 2017. Science Advances, 3(7), e1700782.
- Danso, D., Chow, J., & Streit, W. R. "Plastics: Environmental and Biotechnological Perspectives on Microbial Degradation." 2019b. Applied and Environmental Microbiology, 85(19).
- Readfearn, G. "'Plastic is literally everywhere': the epidemic attacking Australia's oceans" 2018. The Guardian.
- Andrei, M. "Plastics are ubiquitous in the deep ocean -- even in the Mariana Trench." 2018. ZME Science.
- Austin, H. P. et. al. "Characterization and engineering of a plastic-degrading aromatic polyesterase." 2018. Proceedings of the National Academy of Sciences, 115(19), E4350–E4357.
- Lemonick, S. "Chemistry may have solutions to our plastic trash problem." 2018. Chemical and Engineering News.
- TOKIWA, Y., & SUZUKI, T. "Hydrolysis of polyesters by lipases." 1917. Nature, 270(5632), 76–78. https://doi.org/10.1038/270076a0
- Yoshida et al "A bacterium that degrades and assimilates poly(ethylene terephthalate)." 2016. Science, 351(6278), 1196–1199.
- Wilkinson, G. "Scientists hope new enzyme will 'eat' plastic pollution." 2018. CNN.
- Hodgkin, J. and Partridge, F.A. "Caenorhabditis elegans meets microsporidia: the nematode killers from Paris." 2008. PLoS Biology 6:2634-2637.
- Bartlett et al.: Oncolytic viruses as therapeutic cancer vaccines. Molecular Cancer 2013 12:103.
- Mathiesen, K. "Could a new plastic-eating bacteria help combat this pollution scourge?" 2016. The Guardian.
- Bornscheuer, U. T. "Feeding on plastic." 2016. Science, 351(6278), 1154–1155.
- Dimarogona, M. et al (2015). "Structural and functional studies of a Fusarium oxysporum cutinase with polyethylene terephthalate modification potential." 2015. Biochimica et Biophysica Acta (BBA) - General Subjects, 1850(11), 2308–2317.
- Carrington, D. "Scientists accidentally create mutant enzyme that eats plastic bottles." 2018. The Guardian.
- Müller, R. et al "Enzymatic Degradation of Poly(ethylene terephthalate): Rapid Hydrolyse using a Hydrolase from T. fusca." 2005. Macromolecular Rapid Communications, 26(17), 1400–1405.
- [ https://doi.org/10.1099/ijsem.0.001058 Tanasupawat, S., Takehana, T., Yoshida, S., Hiraga, K., & Oda, K. "Ideonella sakaiensis sp. nov., isolated from a microbial consortium that degrades poly(ethylene terephthalate)." 2016. International Journal of Systematic and Evolutionary Microbiology, 66(8), 2813–2818.]
- [ https://doi.org/10.1073/pnas.1718804115 Austin, H. P. et al "Characterization and engineering of a plastic-degrading aromatic polyesterase." 2018b. Proceedings of the National Academy of Sciences, 115(19), E4350–E4357.]
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