Can bacteria degrade plastic?

MICROBIAL PROCESS

Thorough biological degradation of synthetic plastic polymers requires the breakdown of the plastic polymer into smaller chains like oligomers and, eventually, monomers which can easily penetrate the cell membrane followed by assimilation and subsequent intracellular metabolic degradation. [3,4,5]

The process starts with the attachment of the bacterial cells to the plastic polymers — Biofilm-adapted Pseudomonas sp. AKS2 cells are found to have greater hydrophobicity and secretion of exopolysaccharides that aid in better cell adhesion to unmodified plastic surfaces (primarily LDPE), [6] eliminating the need for pre-addition of hydrophilic functional groups on the plastic polymers (primarily LDPE) in order to improve the cell adhesion towards the general hydrophilic cell surfaces.

Pre-treatment of some plastic polymers was still required to make their surface hydrophilic and enhance cell adhesion for biofilm formation and better accessibility of secreted extracellular enzymes to the polymer surface for degradation.

Enzymatic degradation majorly involves two prominent processes secretions to enhance adhesion and breakdown of the polymers. Secretions perform chemical or biological oxidation reactions adding a hydrophilic functional group such as alcohol or carbonyl groups on the polymer surface, and breakdown of the polymer enables the catalytic effects of the enzymes that operate and facilitate the transport through the cell membrane of only smaller molecules [7]. Further, the degradative products transported inside the cell can be metabolized through β-oxidation and the citric acid cycle (CAC) [7;8].

Extracellular enzymes such as depolymerases and hydrolases are responsible for the breakdown of large plastic polymers into smaller molecules, with the hydrolytic cleavage occurring at either the terminus of the polymer chain (exo-attack) or somewhere in the middle (endo-attack). Exo-attack breaks the polymer into oligomers or monomers, facilitating their assimilation into the cell, whereas endo-attack only reduces the molecular weight and doesn’t ensure the assimilation of the product into the cell without further degradation [7;9]. A table listing Pseudomonas sp. and associated enzymes degradation of various plastic polymers has been documented [below] [10].

The intracellular metabolism of the products has not yet been evaluated explicitly. However, the consistency in the measurement of oxygen depletion and carbon dioxide generation with growth and metabolism of plastic degradation products [3]; along with the research that the smaller chains can be potentially processed through bacterial metabolism to ultimately generate CO2 or to be exploited in other metabolic pathways for the biosynthesis of valuable products indicates a prominent pathway where the smaller chains get converted into compounds that can enter the cell and take part in TCA cycle or other metabolic pathways [Fig.1; 2; 3] [7; 10].

LIST OF SPECIES

Table 1: List of List of Pseudomonas sp. and associated enzymes for degradation of various kinds of plastics

FIELD OF APPLICATION

Poor waste management and large-scale production of plastics have tended to create a global waste challenge, leaving the planet drowning in plastic pollution [1]. Many biodegradable alternatives for plastic have been developed. They are being implemented to reduce the further addition and accumulation of plastic. Still, in order to reduce the current generated pool, scientists have tried exploring the capabilities of microorganisms to accelerate the natural process of degradation through bioremediation.

Bioremediation is a branch of biotechnology that exploits the metabolic processes of living organisms like microbes and bacteria to remove pollutants, contaminants, and toxins from the environment [2]. Synthetic plastics are ubiquitous and slowly degrading polymers in environmental wastes. Members of the pseudomonas genus are metabolically diverse and present the capability of degrading and metabolizing synthetic plastics. Pseudomonas species isolated from various ecological matrices have been experimentally identified to degrade polyethylene, polystyrene, polyurethane, polyvinyl chloride, polypropylene, polyethylene terephthalate, polyvinyl alcohol, polyethylene glycol, and polyethylene succinate at varying degrees of efficiency. Thus, the pseudomonas genus presents a great deal of potential in a commercial application for cleaning the environment using bacterias.

SCHEMATICS of the MECHANISMS

Figure 1: Proposed Pathways for degradation of Polyethylene Terephthalate (PET) [10].

Figure 2: Proposed Pathways for degradation of Polyethylene (PE) [10].

Figure 3: Proposed Pathways for degradation of Polyvinyl Alcohol (PVA) [10].

CHALLENGES

The significant challenges for plastic biodegradation are as follows:

1. Polymer characteristics such as high molecular weight, lack of favorable conditions, and crystallinity constitute a significant challenge as these make the polymers less susceptible to microbial attack, resulting in less yield of oligomers and monomers which can pass the cell membrane to participate in the biodegradation, in turn resulting in less degradation [7; 4]

2. Synthetic plastics are highly hydrophobic with stable, functional groups like alkane and phenyl and oxidation and hydrolysis of these are necessary for microbial attack and degradation. Furthermore, pre-treatment is required for uniform degradation as amorphous regions with greater branching are more prone to the attack than crystalline regions with rigid shapes. [7; 4]

3. External factors like pH, temperature, humidity, and addition of additives that can act as inhibitors like polyurethane containing material can be accompanied by dibutyl tin dilaurate that can kill microbes acting as antimicrobials [11;12]

POSSIBLE SOLUTIONS/FUTURE NEEDS

The possible solutions coupled with the future requirements that can help solve the challenges mentioned above are, firstly, a comprehensive understanding of the enzymes involved in the degradation, as well as identification of their extracellular and intracellular localization can help in developing optimized bioengineering approaches for degradation. Furthermore, genetic engineering approaches can be applied in order to combine mutually beneficial enzymes and enzymes that can degrade different types of plastics, promoting plastic degradation by genetically modified native microbial species. A complete elucidation of the degradation pathway can help determine the rate-limiting steps and intermediate compounds that can block further metabolism; these combined can help design the plastics that can degrade more easily when released in the environment, thus reducing the waste generated.

REFERENCES

1. The plastic waste problem explained. (2021, March 22). The Alliance.

2. Bioremediation Definition. (2020, June 23). Investopedia.

3. Lucas, N., Bienaime, C., Belloy, C., Queneudec, M., Silvestre, F., & Nava-Saucedo, J. E. (2008). Polymer biodegradation: Mechanisms and estimation techniques–A review. Chemosphere, 73(4), 429–442.

4. Singh, B., & Sharma, N. (2008). Mechanistic implications of plastic degradation. Polymer degradation and stability, 93(3), 561–584.

5. Kolvenbach, B. A., Helbling, D. E., Kohler, H. P. E., & Corvini, P. F. (2014). Emerging chemicals and the evolution of biodegradation capacities and pathways in bacteria. Current opinion in biotechnology, 27, 8–14.

6. Tribedi, P., Gupta, A. D., & Sil, A. K. (2015). Adaptation of Pseudomonas sp. AKS2 in biofilm on low-density polyethylene surface: an effective strategy for efficient survival and polymer degradation. Bioresources and Bioprocessing, 2(1), 1–10.

7. Shah, A. A., Hasan, F., Hameed, A., & Ahmed, S. (2008). Biological degradation of plastics: a comprehensive review. Biotechnology advances, 26(3), 246–265.

8. Restrepo-Flórez, J. M., Bassi, A., & Thompson, M. R. (2014). Microbial degradation and deterioration of polyethylene–A review. International Biodeterioration & Biodegradation, 88, 83–90.

9. Lenz, R. (1993). Advances in polymer science (No 107: biopolymers).

10. Wilkes, R. and Aristilde, L. (2017), degradation and metabolism of synthetic plastics and associated products by Pseudomonas sp.: capabilities and challenges. J Appl Microbiol, 123: 582–593.

11. Gu, J. D. (2003). Microbiological deterioration and degradation of synthetic polymeric materials: recent research advances. International biodeterioration & biodegradation, 52(2), 69–91.

12. Cregut, M., Bedas, M., Durand, M. J., & Thouand, G. (2013). New insights into polyurethane biodegradation and realistic prospects for the development of a sustainable waste recycling process. Biotechnology advances, 31(8), 1634–1647.