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Review

Microbial Degradation of (Micro)plastics: Mechanisms, Enhancements, and Future Directions

by
Wei Gao
,
Mingxuan Xu
,
Wanqi Zhao
,
Xiaorui Yang
,
Fengxue Xin
,
Weiliang Dong
,
Honghua Jia
and
Xiayuan Wu
*
College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China
*
Author to whom correspondence should be addressed.
Fermentation 2024, 10(9), 441; https://doi.org/10.3390/fermentation10090441
Submission received: 29 July 2024 / Revised: 21 August 2024 / Accepted: 22 August 2024 / Published: 23 August 2024
(This article belongs to the Special Issue Feature Review Papers in Industrial Fermentation, 2nd Edition)
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Review Reports Versions Notes

Abstract

:
Plastic wastes, widely distributed in the environment, can be transformed into microplastics, posing a huge threat to ecosystems and human health due to their stability and adsorbability to other toxic pollutants (e.g., heavy metals and antibiotics). Recently, microbial degradation of (micro)plastics has gained widespread attention because of its green and sustainable properties. Microbial degradation of (micro)plastics is based on the cascade effects of various enzymes secreted by microorganisms, which can convert (micro)plastics into oligomers and monomers, or even mineralize them into CO2 and H2O. The microbial degradation of (micro)plastics is affected by multiple factors, such as microbial species, plastic properties, and environmental conditions. Currently, limited efficient plastic-degrading microorganisms have been discovered, and their degradation mechanisms are still unclear. Furthermore, the efficiency of microbial degradation needs to be improved for future application. Therefore, this review systematically summarizes the sources and properties of existing plastics, identifies pure cultures and mixed cultures for plastic degradation, and examines their influencing factors. In particular, the microbial degradation behaviors of (micro)plastics, including relevant enzymes, degradation efficiency, and degradation mechanisms, were thoroughly discussed. Additionally, the augmentation technologies coupling with microbial degradation, such as advanced oxidation, electrochemical, and genetic engineering technologies, were introduced and highlighted for their potential prospects. This review provides a reference for future research and development of (micro)plastic biodegradation technology.

1. Introduction

Plastic is a significant material widely applied in various fields, and its production is increasing along with the rapid development of the global economy. The global production of plastics reached 345 million tons in 2017 and increased to approximately 400 million tons by 2022, with China accounting for about one-third of the total [1,2,3]. Moreover, around 93% of discarded plastics are not recycled and are mainly converted into microplastics that are widely distributed in nature. There are about 24.4 trillion microplastic particles in the global oceans, and high concentrations of microplastic particles are also found in freshwater systems; additionally, soil systems contain about 4 to 23 times more microplastic particles than aquatic systems [4,5]. The durability, adsorption capacity, cytotoxicity, and reproductive toxicity of microplastics in the environment pose a huge threat to the ecosystem. Therefore, efficient technologies for (micro)plastic degradation or recycling are essential for the sustainable development of society [6,7,8,9].
Currently, the common techniques for (micro)plastic degradation are chemical and thermal degradation. However, these methods have several disadvantages, including high cost, low efficiency, and the production of toxic by-products [10,11,12]. Biodegradation by microorganisms, such as bacteria, fungi, and microalgae, mainly involves the enzymatic cleavage of plastic polymer chains, leading to the breakdown of the polymer into smaller molecules (e.g., oligomers, dimers, and monomers) and even CO2 and H2O [13]. Some microorganisms can achieve high-value conversion of degraded monomers while degrading plastics [14]. Compared to physical and chemical degradation, microbial degradation offers the advantages of being cost-effective, environmentally friendly, and sustainable. In the current context of economic development, with a focus on “green” initiatives, it holds great potential for widespread application [15,16]. However, the (micro)plastic biodegradation technology is still in its infancy, and only a few plastic-degrading microorganisms and enzymes have been discovered. There are few comprehensive reviews that provide systematic information for researchers in this field.
Hence, this review systematically summarizes the sources and properties of different types of plastics, identifies pure and mixed cultures for plastic biodegradation, and examines the corresponding influencing factors. It particularly focuses on the microbial degradation behaviors of (micro)plastics, including relevant enzymes, degradation efficiency, and degradation mechanisms. Additionally, the application of coupled enhancement technologies for microbial degradation, such as advanced oxidation, electrochemical, and genetic engineering technologies, is introduced, with the technologies’ future development potential highlighted.

2. Sources and Properties of (Micro)plastics

Plastic is a synthetic polymeric material composed of various elements such as carbon, hydrogen, oxygen, nitrogen, silicon, and chlorine, typically derived from natural gas, oil, coal, and their derivatives [17,18]. The overwhelming majority of plastics enter the environment after human use and are transformed into microplastics, measuring less than 5 mm, through natural processes such as solar radiation, weathering, and enzyme action, which presents a significant threat to ecosystems and human health [19,20]. Based on differences in chemical structure, plastics are primarily classified into three categories. The first category is polyester plastics, which contain ester bonds. These are more prone to degradation by microorganisms due to the ease of hydrolysis of the ester bonds. Examples include polyethylene terephthalate (PET), polylactic acid (PLA), and polyurethane (PU). The second category is polyolefin plastics, characterized by the stability of the C-C bond as well as their high hydrophobicity and crystallinity, which make them resistant to microbial degradation [21]. Examples include polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), and polypropylene (PP). The third category is polyamide plastics, represented by polyamides (PA), which have strong polar amide groups that make them highly crystalline and challenging to biodegrade. The physical and chemical properties of these three types of plastics are presented in Table 1.

3. Microbial Degradation of (Micro)plastics

3.1. Pure Cultures for (Micro)plastic Degradation

3.1.1. Bacteria

Currently, there is substantial research on the bacterial degradation of (micro)plastics, and significant progress has been made in elucidating the molecular mechanisms underlying the partial bacterial degradation. Bacteria such as Acinetobacter, Bacillus, Pseudomonas, and Klebsiella have been gradually isolated through enrichment and culture from plastic waste, sludge, and wastewater. Furthermore, a wide range of genes and enzymes involved in plastic degradation have been continuously identified (Table 2) [32,33,34,35].
For polyester plastics, in 2016, Yoshida et al. [36] first isolated a strain of Ideonella sakaiensis that was able to completely degrade PET within six weeks. This bacterium produces two hydrolytic enzymes, PETase and MHETase, which catalyze the degradation of PET into monomers, facilitating further catabolism. This represents the shortest and most efficient bacterial degradation of PET reported to date. Subsequently, Seongjoon et al. [37] proposed a detailed process for the degradation of PET into bis(2-hydroxyethyl) terephthalate (BHET), monohydroxyethyl terephthalate (MHET), terephthalate (TPA), and ethylene glycol (EG) based on structural and site-directed mutagenesis experiments. PETase secreted by I. sakaiensis first binds to the PET surface using its flat hydrophobic surface that has a substrate binding cleft. It then undergoes further degradation through a nick generation step and terminal digestion step, leading to the accumulation of four molecules: BHET, MHET, TPA, and EG. BHET can be further degraded into MHET and EG (Figure 1).
For polyolefin plastics, in 2023, Zhang et al. [38] isolated a PVC-degrading strain of Klebsiella sp. EMBL-1 from the intestines of insect larvae. The average weight loss of PVC films reached 19.57% over 90 days. The degradation genes of PVC identified were KatG, lccA, ladA, ENO, dhaA, and ilvD, and the degradation enzymes included catalase, dehalogenase, laccase, esterase, enolase, aldehyde dehydrogenase, and oxygenase. The degradation pathway of PVC was hypothesized to be as follows: PVC is first dechlorinated and depolymerized into oligomers under the action of extracellular peroxidases and dehalogenases. The oligomers are further oxidized by laccase, esterase, enolase, aldehyde dehydrogenase, and oxygenase into small molecules of alcohols, organic acids, and fatty acids, which then enter the cell. The fatty acids enter the tricarboxylic acid cycle after β-oxidation and are finally mineralized into CO2 and H2O (Figure 1). In 2024, Lina et al. [39] isolated a deep-sea Acinetobacter venetianus F1 strain that degraded PE by 12.2% within 56 days. Multi-omics analysis identified the key degradation enzymes, including hydroxylase, monooxygenase, alcohol dehydrogenase, aldehyde dehydrogenase, esterase, and lipase. The pathway for PE degradation involves the initial depolymerization of oligomers and alkanes into cells under the effect of peroxidase, followed by oxidation into alcohol via hydroxylase and monooxygenase. Subsequently, the alcohol is further oxidized into aldehyde and ketone by alcohol dehydrogenase and aldehyde dehydrogenase. Ketones are then converted to esters through Baeyer–Villiger oxidation before being cleaved by esterases and lipases to form corresponding alcohols and fatty acids. These compounds undergo β-oxidation before entering the tricarboxylic acid cycle for mineralization to CO2 and H2O.

3.1.2. Fungi

Fungi capable of degrading (micro)plastics primarily include Aspergillus, Dendrosporum, Fusarium, Penicillium, and Xanthosporea pinnatifida (Table 2). Their mycelial structures can effectively penetrate the surface of the polymer material and reach the interior, thereby maximizing the degradation of (micro)plastics. Additionally, fungi can generate biosurfactants and various oxidative enzymes, demonstrating outstanding performance in degrading (micro)plastics [40].
The rate of (micro)plastic degradation by fungi also depends on the type of plastic [41]. Polyester and polyamide plastics contain readily hydrolyzable functional groups, which can be directly attacked by fungal hydrolases such as esterases, keratases, lipases, and proteases released by mycelium, hydrolyzing the plastic polymers into small carbon chains for metabolism. Khan et al. [42] initially isolated Aspergillus tubingensis, a PUR-degrading fungus, from the soil. This biodegradation process is divided into three steps: Aspergillus tubingensis initially adheres to the surface of the PUR membrane, where its mycelium grows and reproduces. Ultimately, it secretes esterase and lipase to degrade PUR. For polyolefin plastics, most research typically begins with pretreatment using ultraviolet light or chemical oxidants, which induce these polymers to generate carbonyl and ester functional groups that can serve as target sites for fungal peroxidases, oxidases, hydrolases, keratases, and lipases. Zhang et al. [43] isolated a strain of Aspergillus flavus PEDX3 from the intestinal tract of the wax borer and discovered that the fungus exhibited strong degradation of HDPE, with potentially efficient degradation enzymes identified as laccase multicopper oxidases (LMCOs). Gao et al. [44] successfully isolated a strain of marine fungus Alternaria alternata FB1 that could effectively colonize and degrade PE film, with a degradation rate of up to 95% after 120 days. The main degradation product was diglycolamine, and the primary degradation enzymes were peroxidase and laccase. In conclusion, the specific molecular mechanisms of (micro)plastic degradation by fungi have not yet been fully elucidated, and there is considerable research space for isolating fungi with the ability to degrade (micro)plastics from the environment.

3.1.3. Microalgae

Microalgae possess excellent application prospects for degrading marine (micro)plastic pollution due to their superior adaptation to the ocean compared to bacteria and fungi, as well as their capacity for photosynthesis and autotrophic nutrition [45]. Microalgae found to be capable of degrading (micro)plastics mainly include cyanobacteria, green algae, and diatoms (Table 2) [46]. In the process of microalgal-mediated (micro)plastic degradation, the adhesion of various microalgae is frequently the initial step. When microalgae adhere to plastic surfaces, they generate ligninolytic enzymes and extracellular polysaccharidases, which play a crucial role in plastic degradation and contribute to reducing the activation energy necessary for breaking the chemical bonds of polymers [47]. Sarmah et al. [48] isolated two strains of fast-growing cyanobacteria, Phormidium lucidum and Oscillatoria subbrevis, which were capable of colonizing PE effectively and degrading LDPE by up to 30% after 42 days without any pretreatment with oxidants. Gowthami et al. [49] screened and identified a strain of green alga, Picochlorum maculatum, which was able to degrade LDPE efficiently, by up to 20% after 45 days, through the generation of extracellular polymers that adhered to LDPE and subsequently degraded it. Currently, there are relatively fewer studies on the degradation of (micro)plastics by microalgae, but considering the high economic recycling value of microalgae, it is worthy of further investigation.
Table 2. Pure microbial degradation of (micro)plastics.
Table 2. Pure microbial degradation of (micro)plastics.
Microorganism CategoryPlastic Microorganism and SourceKey EnzymeDegradation ConditionPlastic Weight LossReference
BacteriaPETIdeonella sakaiensis/SoilPETase, MHETase.30 °C, 300 strokes/min, aerobic100%/42 d[36]
PVCKlebsiella sp. EMBL-1/Larval gutCatalase-peroxidase, Dehalogenases, Enolase, Aldehyde dehydrogenase, Oxygenase.30 °C, 150 rpm/min, aerobic19.57%/90 d[38]
PEAcinetobacter venetianus F1/Sediments from the Haima cold seepsMonooxygenase, Oxygenase, Dehydrogenase, Esterase, Lipase, Hydrolases, Reductases.28 °C, 160 rpm/min, aerobic12.2%/56 d[39]
FungiPURAspergillus tubingensis/SoilEsterase, Lipase.37 °C, 150 rpm/min, aerobic90%/60 d[42]
PVCPhanerocheate chrysosporium/LaboratoryLignin peroxidase.25 °C, pH = 5, aerobic31%/28 d[50]
PEAlternaria alternata/Plastic debris in coastal areasLaccase, Peroxidase,
Oxidoreductase.		25 °C, alkali, aerobicNot available[44]
MicroalgaeLDPEPhormidium lucidum/Domestic sewageLaccase, Peroxidase.aerobic30%/42 d[48]
LDPEOscillatoria subbrevis/Domestic sewageLaccase, Peroxidase.aerobic30%/42 d[48]

3.2. Mixed Cultures for (Micro)plastic Degradation

3.2.1. Natural Mixed Cultures

Currently, there are fewer studies on the degradation of (micro)plastics by natural mixed cultures compared to pure bacterial cultures (Table 3). Researchers have found that by enriching natural mixed cultures from waste, they were able to degrade PE to varying degrees, with the dominant genera identified mainly as Bacillus sp., Paenibacillus sp., and Pseudomonas sp. [33,51,52]. Additionally, recent research has discovered that certain insects can survive by consuming different plastic films, revealing that the gut flora of these insects has significant degrading effects on plastics. For instance, Xu et al. [53] enriched a natural mixed bacterial consortium, EF1, from the gut of Tenebrio molitor larvae and identified Stenotrophomonas, Enterococcus, and Acinetobacter as the dominant genera. This resulted in a degradation rate of 6.13% for PVC film after 30 days of treatment with EF1.

3.2.2. Artificial Mixed Cultures

Most artificial mixed-bacterial systems for degrading (micro)plastics are typically composed of Bacillus sp. and Pseudomonas sp. as the core strains, with additional strains added (Table 4). Both Bacillus sp. and Pseudomonas sp. have individual capabilities for degrading plastics, and when interacting with other microorganisms, they can mitigate the effects of toxic products on a single degrading bacterium, thereby promoting effective plastic degradation.
For polyester plastics, in 2021, Gao et al. [54] utilized thousands of plastic waste samples collected in Huiquan Bay as bacterial sources. They isolated three strains of PET-degrading bacteria, including Exiguobacterium sp., Halomonas sp., and Ochrobactrum sp., then combined them in a ratio of OD600 of 1:1:1. This artificial bacterial system degraded the PET film into small pieces after 2 weeks. The treated PET film exhibited a notable trend towards depolymerization, and the primary degradation products were identified as TPA and MHET. Qi et al. [55] developed an artificial hybrid bacterial system consisting of Rhodococcus jostii and two metabolically engineered Bacillus subtilis strains; initially, they constructed a Bacillus system with two engineered Bacillus subtilis strains that secreted PETase and MHETase, leading to the degradation of PET film up to 13.6% after 7 days; subsequently, Rhodococcus jostii was added to this system for further degradation of the intermediate product TPA. As a result, the weight loss of PET film reached 31.2% after 7 days, representing an increase in mass loss of about 17.6% compared to the Bacillus system alone.
For polyolefin plastics, in 2023, Wang et al. [56] isolated three strains of Rhodopseudomonas sp. P1, Rhodanobacter sp. Rs, and Microbacterium sp. M1 from bacterial sources obtained in soil ecosystems for polyethylene mulch film (PMFs) degradation. These strains were then combined with Bacillus aryabhattai 5-3, a typical PMF-degrading bacterium, to form a mixed culture system using PMFs as the sole carbon source. Based on the property changes of the PMFs (roughness, chemical functional groups, and hydrophobicity), the most effective combination was finally found to be the two-bacteria system of Rhodanobacter sp. Rs and Bacillus aryabhattai 5-3. Xiang et al. [57] isolated three predominant PS-degrading bacteria, Stenotrophomonas maltophilia, Bacillus velezensis, and Acinetobacter radioresistance, from soil and river sediments. Among them, the co-culture of Stenotrophomonas maltophilia and Bacillus veleziensis achieved a 43.5% weight loss of PS after 60 days, which was significantly higher than that achieved by either bacterium alone.
Table 4. Artificial mixed bacterial degradation of (micro)plastics.
Table 4. Artificial mixed bacterial degradation of (micro)plastics.
PlasticMicrobial Community ConstructionCulture SourceDegradation ConditionPlastic Weight LossReference
PETRhodococcus jostii,
Bacillus subtilis		Laboratory30 °C, aerobic31.2%/60 d[55]
PETExiguobacterium sp.,
Halomonas sp.,
Ochrobactrum sp.		Surface sedimentary samplesCulture for four weeks, aerobicNot available[54]
PERhodopseudomonas sp. P1,
Rhodanobacter sp. Rs,
Microbacterium sp. M1,
Bacillus aryabhattai 5-3		Soil28 °C, 180 rpm for 60 d, aerobicNot available[56]
PEAcinetobacter sp. NyZ450,
Bacillus sp. NyZ451		Larval gut23 °C, 180 rpm/min, aerobic18%/30 d[58]
PSStenotrophomonas, maltophilia,
Bacillus velezensis		Soil and river sediments30 °C, 110 rpm/min, aerobic43.5%/60 d[57]

3.3. Factors Impacting Microbial Degradation of (Micro)plastics

The efficiency of microbial degradation of (micro)plastics is influenced by three primary factors: the specific microbial species involved, the properties of the plastic, and the prevailing environmental conditions. Variations in the degradation capacity of different microorganisms towards plastics are primarily attributed to differences in substrate specificity exhibited by their secreted degradation enzymes. Fungi demonstrate superior degradation potential compared to other microorganisms due to their robust adsorption, colonization, survival abilities, and capability to produce a diverse array of specific degradation enzymes. Polyester plastics exhibit greater susceptibility to microbial degradation than other types due to the presence of easily hydrolyzable ester bonds. Environmental factors, such as temperature, humidity, pH, light exposure, and oxygen level, can influence plastic degradation by impacting both the growth and metabolism of microorganisms and altering functional group structures on the plastic surface. Specifically, optimal humidity, temperature, and pH levels are helpful to promote the proliferation and metabolic activity of microorganisms. In this context, increased production and activity of enzymes can catalyze the breakdown of plastic polymers into simpler polymers and low-molecular-weight organic acids. In natural settings, light and oxygen can catalyze the generation of numerous reactive oxygen species (ROS) on (micro)plastic surfaces, thereby accelerating their aging processes. Chen et al. [59] investigated PS degradation in situ within intertidal sediments and sediment column samples. They observed that anaerobic–aerobic alternating conditions led to an increase in the weight loss rate for PS over 60 days by a factor ranging from 0.7 to 5.8 compared to static anaerobic or aerobic conditions. Therefore, the meticulous regulation of environmental parameters is essential to increase the rate of plastic biodegradation. This highlights the importance of the combination of controlled environmental conditions and plastic properties for advancing plastic waste management and treatment through sustainable biotechnology.

4. Coupled Enhanced Technologies

4.1. Advanced Oxidation Technology

Advanced oxidation technologies (AOPs) utilize strong oxidants or physical processes such as light and electricity to produce reactive oxygen species (ROS), thereby achieving efficient degradation of organic pollutants [60,61,62]. The primary AOP technologies for enhancing the degradation of (micro)plastics through indirect or direct coupling with microbial degradation are photocatalysis and Fenton reactions (Figure 2). Indirect coupling involves the pre-application of AOP technology to modify the surface structure of (micro)plastics, making them more susceptible to microbial degradation. For instance, Montazer et al. [63] conducted UV pretreatment on PE and then identified a PE-degrading bacterium IRN19, resulting in a weight loss of 26.8 ± 3.04% within 28 days. Similarly, Tribedi et al. [64] compared the degradation of PE in soil with and without UV pretreatment and found that the degree of PE degradation in the pretreated group was nearly double that of the untreated group, with a weight loss rate of 6% within 28 days. Direct coupling involves utilizing microorganisms and AOP technology simultaneously to more efficiently degrade (micro)plastics. Xing et al. [65] discovered that thermophilic iron-reducing bacteria in thermophilic compost could facilitate the production of ⋅OH in compost through Fe2+ generation, thereby synergizing with microbial degradation to expedite the decomposition of (micro)plastics. Furthermore, Yang et al. [66] utilized Shewanella putrefaciens 200 to construct a microbial-driven Fenton reaction under alternating anaerobic-aerobic incubation conditions for enhancing PS degradation, resulting in a weight loss rate of 6.1 ± 0.6% within just 14 days—a significant improvement compared to microbial degradation alone by a factor of 11.5.

4.2. Electrochemical Technology

The bioelectrochemical system (BES) is a technology for treating pollutants that combines microbial metabolism and electrochemical reactions [67]. Microbial metabolic activity plays a crucial role in determining the degradation of (micro)plastics. Therefore, integrating the redox properties of BES with microbial degradation to enhance microbial activity represents an effective approach for promoting the degradation of (micro)plastics (Figure 2). Research on the degradation of (micro)plastics by BES is still in its early stages, with limited relevant literature reports. Wang et al. [68,69] investigated the impact of PE microplastics on BES wastewater treatment and observed that PE reduced methane production in the electrochemical anaerobic digestion of wastewater and the electrochemical activity of BES. This reduction was attributed to decreased microbial activity and diversity, extracellular electron transport, and the abundance of methanogenic genes; the degradation of PE in BES within wastewater has not yet been studied. The degradation of microplastics PVC, PE, and PLA in soil using BES has been examined, and the results indicated that the closed-circuit group achieved a decrease percentage of 8.94% in PLA weight after 120 days, which increased by 3.01–3.54 times compared to that from the open-circuit group. Partial degradation of PE was also observed, while PVC degradation was not significant. This suggests that PLA and PE could serve as potential carbon sources to enhance electricity production from soil-based BES systems, whereas PVC may inhibit electricity generation; electrical stimulation enhances the enrichment of bacteria responsible for degrading microplastic surfaces, thereby improving the abundance of genes related to microbial degradation and metabolism while reducing chemical stability [70]. BES involves the direct coupling of microorganisms and electrochemistry. Considering the impact of electrochemical catalysis on the degradation and high-value conversion of plastics, an alternative direction for future research could be to explore the indirect coupling of electrochemical catalysis as a front-end pretreatment or terminal high-value conversion method for microbial degradation of (micro)plastics [71,72].

4.3. Genetic Engineering Technology

Genetic engineering technology is a method used to artificially manipulate the genetic information of microorganisms in a targeted manner, aiming to optimize their initial performance or impart specific functions. Microorganisms primarily degrade (micro)plastics by secreting extracellular enzymes, and one effective way to enhance the degradation of (micro)plastics is through genetic engineering to improve the immobilization, functionalization, and activity of plastic-degrading enzymes (Figure 2) [73]. Huang et al. [74] utilized Bacillus subtilis-168 as a chassis microorganism and regulated PETase secretion by constructing strains with different Tat proteins (TatAd, TatCd, TatAy, TatCy, and TatAc) deficient strains. They found that PETase secretion was maximized when all five Tat proteins were knocked out, resulting in a 3.8-fold increase compared with the original strain. Yan et al. [75] developed a thermotolerant degrading engineered bacterium using a strong promoter strategy with Clostridium thermocellum. This led to high levels of expression of thermophilic cutinase (LCC), resulting in a 62% weight loss of PET film at 60 °C for 14 days, significantly better than previously reported thermophilic or microalgae-engineered microbial systems. However, it has been suggested that building synthetic microbial communities holds more promise than engineering modifications of single bacteria, as it can help promote depolymerization and upcycling of plastics through the synergy of different microbial metabolic pathways [76]. Bao et al. [14] developed a two-bacterial system comprising two engineered soil Pseudomonas putida strains, Pp-T and Pp-E: Pp-T was created by deleting ped operon and expressing tpaAa, tpaAb, tpaB, tpaC, and tpaK genes constitutively to specifically degrade TPA. By contrast, Pp-E promotes gene expression of gcl, glxR, hyi, ttuD, and pykA by knocking out the gclR gene and specifically degrades EG by replacing the native promoter of the glcDEF operon with a strong constitutive promoter (Ptac). This two-bacterial system demonstrated its capability to completely consume PET hydrolyzing monomer while converting it into medium chain length polyhydroxyalkanoates (mcl-PHA) as well as cis-cis muconate (MA) through the introduction of two different biosynthetic pathways.

5. Conclusions

Plastic products bring convenience to our daily lives, but they also cause severe pollution to the environment. Microbial degradation is a kind of green and sustainable method for (micro)plastic waste treatment, which meets the needs of low-carbon development in current society. Novel plastic-degrading microorganisms have recently been found in a wide variety of environments. However, microbial degradation of (micro)plastics still suffers from the problems of low efficiency, slow rate, and the tendency to produce smaller plastics with stronger toxicity, such as nanoplastics. Hence, future research should improve the (micro)plastic removal and achieve the upgraded recycling of plastic wastes, such as screening efficient plastic-degrading microorganisms, developing genetic engineering modification strategies based on the in-depth elucidation of biodegradation mechanisms, constructing stable and controllable artificial microbial communities, and combining biodegradation with physicochemical technologies. In addition, most of the existing studies conducted (micro)plastic biodegradation and microorganism screening under aerobic conditions. However, based on the diversity of environments containing (micro)plastic pollution, researchers should pay more attention to the microorganisms degrading (micro)plastics under anaerobic conditions.

Funding

This work was financially supported by the National Key Research and Development Program of China (2021YFC2103600), the Natural Science Foundation of Jiangsu Province of China for Excellent Young Scholars (BK20211591), the National Natural Science Foundation of China (22478184), the Jiangsu Synergetic Innovation Center for Advanced Bio-Manufacture (XTD2209), and the National Students’ Platform for Innovation and Entrepreneurship Training Program (202410291043Z).

Conflicts of Interest

The authors declare no conflict of interest.

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Fermentation 10 00441 g001
Figure 1. Bacteria-mediated degradation pathways of PET and PVC.
Figure 1. Bacteria-mediated degradation pathways of PET and PVC.
Fermentation 10 00441 g001
Fermentation 10 00441 g002
Figure 2. Coupled enhanced technologies for biodegradation.
Figure 2. Coupled enhanced technologies for biodegradation.
Fermentation 10 00441 g002
Table 1. Types and properties of common plastics.
Table 1. Types and properties of common plastics.
Plastic
Category		Plastic NameStructural FormulaDensity (g/cm3)CrystallinityGlass Transition Temperature (Tg) (°C)Molecular PolarityReference
PolyesterPolyethylene terephthalate (PET)Fermentation 10 00441 i0011.33–1.35Semi-crystalline73–78Polar[22]
Polylactic acid (PLA)Fermentation 10 00441 i0021.25–1.28Semi-crystalline60–65Polar[23]
Polyurethane (PU)Fermentation 10 00441 i0031.15–1.3Semi-crystalline−62Polar[24]
PolyolefinPolyethylene (PE)Fermentation 10 00441 i0040.92–0.97Semi-crystalline−110Non-polar[25]
Polyvinyl chloride (PVC)Fermentation 10 00441 i0051.15–1.70Amorphous60–100Non-polar[26,27]
Polystyrene (PS)Fermentation 10 00441 i0061.04–1.06Amorphous90Non-polar[28,29]
Polypropylene (PP)Fermentation 10 00441 i0070.89–0.91Semi-crystalline(−20)–0Non-polar[30]
PolyamidePolyamides (PA)Fermentation 10 00441 i0081.02–1.05Semi-crystalline−60Polar[31]
Table 3. Natural mixed bacterial degradation of (micro)plastics.
Table 3. Natural mixed bacterial degradation of (micro)plastics.
Plastic Dominant BacteriaCulture SourceDegradation ConditionPlastic Weight LossReference
HDPEBacillus sp., Pseudomonas sp.Discarded refuse30 °C, aerobic23.14%/4 weeks[51]
PVCStenotrophomonas,
Enterococcus,
Acinetobacter		Larval gut30 °C, 180 rpm/min, aerobic6.13%/30 d[53]
PEBacillus sp.,
Paenibacillus sp.		Landfill30 °C, aerobic14.7%/60 d[33]
PEBetaproteobacteria,
Alphaproteobacteria,
Gamma-proteobacteria		Discarded refuse25 °C, 120 rpm/min, aerobic19%/6 months[52]
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Gao, W.; Xu, M.; Zhao, W.; Yang, X.; Xin, F.; Dong, W.; Jia, H.; Wu, X. Microbial Degradation of (Micro)plastics: Mechanisms, Enhancements, and Future Directions. Fermentation 2024, 10, 441. https://doi.org/10.3390/fermentation10090441

AMA Style

Gao W, Xu M, Zhao W, Yang X, Xin F, Dong W, Jia H, Wu X. Microbial Degradation of (Micro)plastics: Mechanisms, Enhancements, and Future Directions. Fermentation. 2024; 10(9):441. https://doi.org/10.3390/fermentation10090441

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Gao, Wei, Mingxuan Xu, Wanqi Zhao, Xiaorui Yang, Fengxue Xin, Weiliang Dong, Honghua Jia, and Xiayuan Wu. 2024. "Microbial Degradation of (Micro)plastics: Mechanisms, Enhancements, and Future Directions" Fermentation 10, no. 9: 441. https://doi.org/10.3390/fermentation10090441

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