Environmental Toxicity, Human Hazards and Bacterial Degradation of Polyethylene

Plastics are the most rapidly growing materials in terms of production and consumption. The durability, inertness, light weight, flexibility, and low cost are the key characteristics that make plastic suitable for application in various fields, including the construction, automotive, electronics, and packaging industries. Due to widespread usage in daily life and many industrial processes and operations, more than 300 million tons of plastic waste are produced globally annually. Indiscriminate use of plastics such as polyethylene causes environmental pollution and impacts human health due to irreversible changes in the ecological cycle. Due to its low biodegradability, polyethylene accumulation has recently emerged as a momentous environmental concern. The conventional methods, such as recycling or disposing of polyethylene, are exorbitant, and incineration results in the emission of toxic chemical compounds. Therefore, the most recent research progressively focused on the biodegradation of polyethylene with the application of bacteria as novel approaches to counteract plastic waste. This review summarizes the type of polyethylene and the environmental issues. It also briefly discussed the genes and enzymes of bacteria involved in the degradation of polyethylene. In addition, it attempts to address factors influencing degradation and techniques used for monitoring degradation.


INTRODUCTION
The etymology of plastic is derived from the Greek word "plastikós," which defines materials as being able to be molded into different desired shapes and sizes due to their chemical composition of carbon, chlorine, hydrogen, nitrogen, oxygen, and silicon. Plastics are polymer macromolecules with long chains, and other compounds are added to alter properties such as stability and processability (Bardají et al. 2020). The introduction of plastics into archaeological and geologic history may serve as the defining characteristic of anthropogenic pollution, and the twentieth century is referred to as the "Plastic Age" (Mytum & Meek 2021). Plastics have generally substituted paper and packaging materials due to superior tensile properties, lightweight nature, and low susceptibility to microbial degradation (Muhonja et al. 2018).
Around 80% of all plastics used globally are petrochemical plastics, including polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polyethylene terephthalate esters (PET) (Urbanek et al. 2018). Although plastic materials are indispensable to the world economy, serious complications associated with their widespread use must not be omitted (Chu et al. 2023). The everyday use of plastic has increased global plastic production exponentially, reaching 367 metric tons in 2020 (Plastics Europe 2021), and the amount is significantly increasing annually due to extremely efficient applications. It is estimated that approximately 710 million metric tons of plastic will be accumulated in the environment or landfills by 2040 if the current management practices, use, and production endure (Lau et al. 2020). Hefty amounts of plastic waste are produced primarily due to the short product lifecycle. In 40% of cases, the lifespan of thermoplastic plastic products is anticipated to be less than one month (Hahladakis et al. 2018). Furthermore, plastic waste management has lagged significant manufacturing output, resulting in environmental pollution (Geyer et al. 2017).
Furthermore, toxic chemical compounds such as Bisphenol A (BPA), phthalates, antiminitroxide, flame retardants, and polyfluorinated compounds are also found in plastics. These substances pose environmental and public health risks (Proshad et al. 2018). Hence, it is necessary to reduce plastic pollution through environmentally friendly methods. Conventional methods such as recycling, dumping, and incineration are not feasible and generates toxic substances as a by-product (Venkatesan et al. 2022).
Actinomycetes, bacteria, and fungi are among the microorganisms capable of degrading PE (Dang et al. 2018, Saritha et al. 2021. Bacteria degrading PE include Phormidium lucidum, Oscillatoria subbrevis (Sarmah & Rout 2018), Bacillus wudalianchiensis, and Pseudomonas aeruginosa (Bakht et al. 2020). Those potential microorganisms are isolated from different soil types, including landfill soil (Montazer et al. 2018), to water bodies (Dhanraj et al. 2022). This indicates that the bacteria exist in most places and in sufficient numbers to cause PE biodegradation.
Biodegradation is defined as the capability of microbes to degrade plastic materials into simpler molecules with the help of enzymes by altering the chemical structures of the plastics into an easily degradable property (Gaur et al. 2022). Temperature, crystallinity, hydrophobicity, structure, and enzymes influence the mechanism of PE's biodegradation. Therefore, this review focuses on the toxicity, bacterial degradation, and factors affecting the degradation mechanism of PE. Moreover, it summarizes the technique used to investigate the biodegradation and bacterial enzymes responsible for the degradation of PE and suggests future research scopes.

GLOBAL PLASTIC AND POLYETHYLENE PRODUCTION
Every year, the world witnesses an unprecedented production of plastics. 335 million tons of plastic were produced worldwide in 2016 and 367 million tons in 2020. Europe produced 55 million tons of plastics in 2020 (Plastics Europe 2021). It was estimated that Asia produced 49% of the global plastics, with China as the leading manufacturer (28%). Furthermore, Europe and North America contributed approximately 19% of global plastics production in 2015, while the rest of the country contributed negligible production, but not necessarily of plastic usage (Worm et al. 2017). According to Hahladakis et al. (2018), European nations primarily used plastic for wrapping (38%), infrastructure tools (21%), motorized (7%), electrical applications (6%), as well as other segments (28%). In India, PE had the highest demand (33%) in 2020, followed by PP (32%), and worldwide consumption of PE was growing at a rate of 12% per year (Venkatesan et al. 2022). Bhutan generates over 170 metric tons of waste daily, encompassing various types. Plastics, including HDPE, soft plastics, and PET bottles, contribute to approximately 17% of this overall waste production (Namgay 2020).

CLASSIFICATION OF POLYETHYLENE
The polymerization of ethylene monomers yields polyethylene, also known as polyethylene, a thermoplastic polymer. The chemical formula for ethylene is C 2 H 4 , while PE has the formula (C 2 H 4 ) n . The Ziegler-Natta and metallocene catalysts are used for the polymerization of polyethylene. Polyethylene is a polyolefin resin family that is the most commonly used plastic worldwide for various This publication is licensed under a Creative Commons Attribution 4.0 International License purposes, such as wrapping, films, packaging, and nursery bags. There are mainly three types of PE: LDPE, LLDPE, and HDPE (Fig. 1). The fundamental distinction between HDPE, LDPE, and LLDPE is the branching degree at the microstructural level. High-density polyethylene has the lowest or no branching, LLDPE incorporates a high degree of short chain, and LDPE contains an irregular distribution of short and long branches (Rani et al. 2020, Varyan et al. 2022.

THE EFFECT OF PE POLLUTION
The effect of polyethylene on aquatic, terrestrial, and human health are illustrated in Fig. 2.

THE EFFECT OF PE ON AQUATIC LIFE
The main source of plastics in the water is anticipated to be from the terrestrial environment. Microplastics are diluted as transport from the deposition hotspot downstream. Therefore, ineffective waste management practices lead to environmental contamination. Several plastic materials, encompassing PP, PVC, PE, PET, and nylon, as well as particle shapes such as fragments, sheets, and threads, have been found in the Megaptera novaeangliae (humpback whales) intestinal tract (Guzzetti et al. 2018). Microplastic accumulation in the hindguts of Lysianassoidea amphipod populations is found at depths ranging from 7,000 m to 10,890 m in the Pacific Ocean (Cózar et al. 2017). This suggests that MP contamination can occur in the ocean beds as well.
Histological fluctuations were detected in the intestine and liver of Dicentrarchus labrax (European sea bass) exposed to PE-MPs. Polyethylene microplastics suppress immunity and antioxidant enzyme activity, signifying the oxidative response/stress, and longer exposure resulted in irreversible damage to fish health (Espinosa et al. 2019). Antioxidants, biochemistry, cholinesterase activity, erythron profile, hematological, histological, and immune parameters were altered in young Cyprinus carpio (common carps), divulging to PE particles (Hamed et al. 2022).
Polyethylene microplastic ingestion alters morphology, erythrocyte mutagenicity, and cytotoxicity in Physalaemus cuvieri tadpoles, affecting their development and health (da Costa Araújo et al. 2020). The gut examination of Phalaropus fulicarius carcasses in Canada found that most commonly ingested plastics such as PE and PP likely contributed to mortality (Teboul et al. 2021). Polyethylene microplastics induce oxidative damage and alter the functioning of antioxidant enzymes in Mytilus galloprovincialis (mussels) at environmentally relevant concentrations (Abidli et al. 2021). Apart from disruption of metabolic activity in an organism, plastic waste in the rivers also contributes to the spatial distribution of invasive species leading to competition with native species (Hasnat & Rahman 2018). Thus, through ingestion, PE-MPs and NPs can enter organisms' bodies and continue to be transferred along food chains.

POLYETHYLENE IMPACT ON THE TERRESTRIAL ENVIRONMENT
Plastic mulching and shading materials protect crops from pests, and harsh weather suppresses weeds (Maraveas 2020). However, plastic mulch significantly impacts soil properties such as pH, electrical conductivity, infiltration, nutrient exchange, and microbial community (Sintim et al. 2019). Polyethylene microplastics escalate the movement of pollutants while decreasing the retention capacities of the soil. For instance, the holding capacity of cadmium was reduced in MP-contained soil, thereby increasing the possibilities of lethal heavy metal accumulation in agriculture 4

THE EFFECT OF PE POLLUTION
The effect of polyethylene on aquatic, terrestrial, and human health are illustrated in Fig. 2.

THE EFFECT OF PE ON AQUATIC LIFE
The main source of plastics in the water is anticipated to be from the terrestrial environment. Microplastics are diluted as transport from the deposition hotspot downstream. Therefore, ineffective waste management practices lead to environmental contamination. Several plastic materials, encompassing PP, PVC, PE, PET, and nylon, as well as particle shapes such as fragments, sheets, and threads, have been found in the Megaptera novaeangliae (humpback whales) intestinal tract (Guzzetti et al. 2018). Microplastic accumulation in the hindguts of Lysianassoidea amphipod populations is found at depths ranging from 7,000 m to 10,890 m in the Pacific Ocean (Cózar et al. 2017). This suggests that MP contamination can occur in the ocean beds as well.
Histological fluctuations were detected in the intestine and liver of Dicentrarchus labrax (European sea bass) exposed to PE-MPs. Polyethylene microplastics suppress immunity and antioxidant enzyme activity, signifying the oxidative response/stress, and longer exposure resulted in irreversible damage to fish health (Espinosa et al. 2019).

Polyethylene microplastic ingestion alters morphology, erythrocyte mutagenicity, and cytotoxicity in
Physalaemus cuvieri tadpoles, affecting their development and health (da Costa Araújo et al. 2020). The gut examination of Phalaropus fulicarius carcasses in Canada found that most commonly ingested plastics such as PE and PP likely contributed to mortality (Teboul et al. 2021). Polyethylene microplastics induce oxidative damage and alter the functioning of antioxidant enzymes in Mytilus galloprovincialis (mussels) at environmentally relevant concentrations (Abidli et al. 2021). Apart from disruption of metabolic activity in an organism, plastic waste in the rivers also contributes to the spatial distribution of invasive species leading to competition with native species (Hasnat & Rahman 2018). Thus, through ingestion, PE-MPs and NPs can enter organisms' bodies and continue to be transferred along food chains.  products and groundwater and bestowing additional risks on the environment ). Plants exposed to PE particles had lower biomass, slow photosynthetic rates, and abnormal mineral nutrient metabolism. This indicates that various PE particles with different molecular weights could adversely affect the soil-plant physiology (Fu et al. 2022).
Low-density polyethylene fragments affect the microarthropod and nematode but slightly influence the biomass and soil microorganism (Lamichhane et al. 2022). Polymers such as LDPE and HDPE have been found in terrestrial species Armadillo, Porcellio, Lumbricus terrestris, Scolopendra, Eobania vermiculata, and Rumina decollata causing adverse effects on metabolic function and survival (Al Malki et al. 2021). Low-density polyethylene microplastics affect manure worms' nervous system, morphology, and oxidative response. The result illustrates that the MPs may have adverse biochemical effects on earthworms . The intestinal tract of terrestrial birds also contains cellulose, PE, and PET-MPs (Carlin et al. 2020). This indicates the abundance and variety of plastics on terrestrial land.

EFFECT OF PE ON HUMAN HEALTH
Microplastics and NPs enter humans through ingestion, inhalation, and dermal contact with water, air, food, and consumer products containing plastics (Karami et al. 2018). Moreover, humans consume plastic directly through table salts (Renzi & Blašković 2018), seafood (De-la-Torre 2020), and canned sardines and sprats (Karami et al. 2018). The most common plastic materials in foodstuffs are PP, PE, PET, polyether (PES), PVC, PS, PA, and nylon . The various MPs of polycarbonate (PC), polyoxymethylene (POM), polyurethane (PUR), PA, PE, PET, PS, PP, and PVC were found in human stool. Among the nine plastic types, PP (62.8%), PET (17.0%), PS (11.2%), and PE (4.8%) were the most abundant (Schwabl et al. 2019). The inhalation and ingestion of high concentrations of PE-MPs with their rough structures increases the risk of cytotoxicity in epithelial cells and triggers the release of pro-inflammatory cytokines. Polyethylene microplastics also cause the production of reactive oxygen species and nitric oxide in cells , Gautam et al. 2022. Depending on the hydrophilic nature, dimensions, and surface energy, inhaled airborne MPs can enter the bloodstream with increased epithelial or endothelial diffusion. Most MP particles (PE) measured in abdominal lymph nodes were 1-50 µm (Zarus et al. 2021).
The presence of MPs and NPs causes disruption of molecular and cellular function in humans. Ingestion of such plastic causes various types of cancer, particularly in industrial workers, because of exposure to high intensities of air pollutants . Human dopaminergic neurons and neurospheres in culture can absorb PE-NPs, which modifies gene expression and elevates malondialdehyde levels, signaling the initiation of proposed NPs can significantly increase levels of DNA damage that causes mutagenic processes (Rubio et al. 2020).
All that evidence suggests that PE, especially micro and nano, may be prevalent in human foodstuffs, ultimately creating health problems.

MICROBIAL POLYMER DEGRADATION MECHANISM
The mechanism of PE degradation occurs in four stages, as shown in Fig. 3.   . Enzymes cleave polymer carbon chains or bonds, producing low molecular weight, such as monomers, oligomers, and dimers (Kalidas et al. 2021). Due to low molecular weight, the cell can assimilate. Therefore, biofragmentation of the process involves enzymatic activities to reduce low molecular weight and oxidize the polymer. Depolymerization is another name for this process.

Bioassimilation:
The low molecular weight polymer formed through the biofragmentation is absorbed into the cytoplasm of the microbes. The fragmented products are assimilated into cells through the cell membrane using specific membrane carriers. While unassimilated oligomers, dimers, and monomers have to undergo biotransformation reactions to be absorbed by microbial cells with the help of intracellular enzymes (Danso et al. 2019).

Mineralization:
It is the final phase in the biodegradation of polymers. The term "mineralization" denotes the complete degradation of molecules with the production of oxidized metabolites. Microorganisms can either aerobically or anaerobically mineralize monomers, dimmers, and oligomers. Water, carbon dioxide, and biomass are produced as end products during aerobic degradation. Under anaerobic biodegradation conditions, by-products are water, carbon dioxide, and methane (Tamoor et al. 2021).

FACTORS INFLUENCING THE BACTERIAL DEGRADATION OF PE
The several factors that affect the bacterial degradation of PE depend on polymer properties and exposure conditions (Fig. 4). Polymer properties include additives, crystallinity, functional groups, hydrophobicity, molecular weight, shape, and size. The exposure conditions include biosurfactants, oxidative metabolism (Windheim et al. 2022). In addition to potentially endangering the pulmonary and digestive systems, researchers have proposed NPs can significantly increase levels of DNA damage that causes mutagenic processes (Rubio et al. 2020). All that evidence suggests that PE, especially micro and nano, may be prevalent in human foodstuffs, ultimately creating health problems.

MICROBIAL POLYMER DEGRADATION MECHANISM
The mechanism of PE degradation occurs in four stages, as shown in Fig. 3.
Biodeterioration: This process encompasses biotic and abiotic factors that cause erosion of the polymer surface, altering the chemical, mechanical, and physical characteristics. The LDPE exposure to different physicochemical treatments and biotic conditions increases biodeterioration as treatment changes hydrophobicity and surface roughness. Chemical alteration, such as generating polar groups and crosslink formation, also occurs (Gómez-Méndez et al. 2018). The biofilm that develops on the plastic enlarges the aperture dimension and accentuates cracks, compromising the polymer's physical properties (physical deterioration) or releasing chemicals that alter the pH inside the hole to cause structural changes known as chemical deterioration (Jacquin et al. 2019).

Biofragmentation:
The lytic phenomenon crucial for reducing large molecules into subunits. The microorganisms cleave polymers using a variety of mechanisms, comprising secretion of particular extracellular enzymes such as oxidoreductases (monooxygenases and dioxygenases), hydrolases (cellulases, amylases, and cutinases), and free Bioassimilation: The low molecular weight polymer formed through the biofragmentation is absorbed into the cytoplasm of the microbes. The fragmented products are assimilated into cells through the cell membrane using specific membrane carriers. While unassimilated oligomers, dimers, and monomers have to undergo biotransformation reactions to be absorbed by microbial cells with the help of intracellular enzymes (Danso et al. 2019).
Mineralization: It is the final phase in the biodegradation of polymers. The term "mineralization" denotes the complete degradation of molecules with the production of oxidized metabolites. Microorganisms can either aerobically or anaerobically mineralize monomers, dimmers, and oligomers. Water, carbon dioxide, and biomass are produced as end products during aerobic degradation. Under anaerobic biodegradation conditions, by-products are water, carbon dioxide, and methane (Tamoor et al. 2021).

FACTORS INFLUENCING THE BACTERIAL DEGRADATION OF PE
The several factors that affect the bacterial degradation of PE depend on polymer properties and exposure conditions  enzymes, and microorganisms as biotic factors and moisture, pH, and temperature as abiotic factors.

Exposure Conditions
The moisture provides favorable conditions for respiration, increasing the population of microbes. Furthermore, the chain scission of the polymers takes place in the presence of moisture (Mistretta et al. 2020). In the modification of the PE chemical bond, Streptomyces sp. performed better at pH 8, and Arthrobacter sp. was more effective at pH 6 with incubation in Czapek-Dox medium and liquid carbon-free basal medium containing PE . Thermophile microorganisms can grow at high temperatures ranging from 45 to 122°C, while psychrophilic microorganisms need temperatures between −20 and 20°C, and temperatures lower than 15°C are considered optimal growth and produce numerous enzymes (Atanasova et al. 2021). However, denaturation of enzymes may occur in psychrophilic bacteria due to high temperature, decreasing the enzymatic degradation of PE (Chamas et al. 2020). The biosurfactant in biofilm growth significantly decreased hydrophobicity and increased hydrophilic functional groups, devising PE sensitive to microbial attack (Tu et al. 2020).

Polymer Properties
The prooxidant additives present in polymer weaken the microstructure, leading to dissociation and allowing microorganism consumption that produces humus, carbon dioxide, and water (Al-Salem et al. 2019). Furthermore, weight loss and structural change with the formation of the new functional group of polymer can occur due to additives (Zhang et al. 2022). Thus, additives and impurities are required to remove before the investigation of biodegradation. High molecular weight polymers are stable and less susceptible to degradation than low molecular weight (Priya et al. 2022). Semi-crystalline polymers (PP, PE, and PET) display greater toughness, strength, and resistance compared to amorphous (Issac & Kandasubramanian 2021), making them less susceptible to enzymatic degradation. The PE backbone consists of linear alkyl chains, and the absence of polar characteristics or hydrolyzable functional groups renders it inactive to degradation. The biotic and abiotic treatments increase PE's hydrophilicity, microbial colonization, and degradation rapidity (Taghavi et al. 2021). The increase in surface area, water, and microorganism availability promotes biodegradation as the high surface area provides space for the growth of organisms.

Role of Bacterial Gene and Emzyme in Biodegradation of PE
The specialized bacteria and their genes and enzymes are imperative to PE biodegradation. It was found that the alkane hydroxylase (alkB) genes in the Pseudomonas sp. The E4 strain degraded 28.6% of organic carbon in 80 days. The ability of the alkB gene was verified by selecting the Escherichia coli BL21 strain as a host for gene expression biosurfactant in biofilm growth significantly decreased hydrophobicity and increased hydrophilic functional groups, devising PE sensitive to microbial attack (Tu et al. 2020).

Polymer Properties
The prooxidant additives present in polymer weaken the microstructure, leading to dissociation and allowing microorganism consumption that produces humus, carbon dioxide, and water (Al-Salem et al. 2019). Furthermore, weight loss and structural change with the formation of the new functional group of polymer can occur due to additives (Zhang et al. 2022). Thus, additives and impurities are required to remove before the investigation of biodegradation.
High molecular weight polymers are stable and less susceptible to degradation than low molecular weight (Priya et al. 2022). Semi-crystalline polymers (PP, PE, and PET) display greater toughness, strength, and resistance compared to amorphous (Issac & Kandasubramanian 2021), making them less susceptible to enzymatic degradation. The PE backbone consists of linear alkyl chains, and the absence of polar characteristics or hydrolyzable functional groups renders it inactive to degradation. The biotic and abiotic treatments increase PE's hydrophilicity, microbial colonization, and degradation rapidity (Taghavi et al. 2021). The increase in surface area, water, and microorganism availability promotes biodegradation as the high surface area provides space for the growth of organisms.

Role of Bacterial Gene and Emzyme in Biodegradation of Pe
The specialized bacteria and their genes and enzymes are imperative to PE biodegradation. It was found that the alkane  and achieving a degradation potential of 19.3% for the PE's organic carbon (Priya et al. 2022). Similarly, three putative PE degrading enzymes (esterase, hydrolase, and hydrolase) were expressed in an E. coli cell, and their effects on PE films showed significant degradation (Gao & Sun 2021). It was also found that P. aeruginosa E7 isolated from an oil-contaminated area possesses alkane hydroxylase. The rubredoxin and rubredoxin reductase transport electrons, and the alkB gene participate in the mineralization of low molecular weight PE .
Moreover, Pseudomonas knackmussii N1-2 and P. aeruginosa RD1-3 have 26 and 20 genes, respectively, in their genomes that encode monooxygenases, dioxygenases, and hydroxylases (Hou et al. 2022). Similarly, the genome of Streptomyces albogriseolus LBX-2 also embraces 53 oxygenase genes, including monooxygenases and dioxygenases. Thus, S. albogriseolus LBX-2 utilized unprocessed PE films as an exclusive carbon source, causing surface deterioration and indicating enzymeassisted degradation (Shao et al. 2019). The genome of Alcanivorax sp. revealed enzymes such as esterases, peroxidases, and laccase that could play an imperative role in biodegradation (Zadjelovic et al. 2020). There are potential genes in Brevibacillus borstelensis AK1 responsible for the breakdown of PE and hydrocarbons. The genes encode enzymes involved in plastic degradation, including cutinase, laccases, hydroxylases, lipases, proteases, and polyphenol oxidase (Khalil et al. 2018). The ligninolytic and hydrolyzing enzymes degrade different plastics, including PE, at 37°C after two days of incubation. Furthermore, the activities of laccase increase by 13 times with 20 µm of copper treatment (Jaiswal et al. 2020).

Analytical Tools for Determination of Polythene Degradation
Various analytical techniques are applied to determine the biodegradation of PE (Table 1). Fourier transform infrared spectroscopy (FTIR) is used to identify the occurrence of new functional groups. The changes in peak values and functional groups, which typically vary in the double bond and carbonyl group formation, supported the conformational change of the polymer surface (Montazer et al. 2018). Scanning electron microscopy (SEM) assessed the alteration of physical structure on the surface of polyethylene films due to the microorganism's degradative activities through biofilm development (Soleimani et al. 2021). Higher resolution analysis of surface modification can be obtained using Atomic Force Microscopy (AFM), although SEM provides evidence of the polymers' biodeterioration. Atomic Force Microscopy makes it possible to map the topography of a polymer surface (Sullivan et al. 2018). Gel permeation chromatography (GPC) analysis determines the number average molecular weight (Mn), and weight average molecular weight (Mw), and molecular weight distribution (MWD). These parameters are the primary indicator of modification, depolymerization, and polymer degradation (Peng et al. 2020). The major method for assessing polymer degradation is weight loss as microorganism growth and their activities cause the polymer to reduce weight (Waqas et al. 2021). Additional techniques for assessing PE biodegradation include Raman spectroscopy, Nuclear Magnetic Resonance spectroscopy (NMR), Thermogravimetric Analysis (TGA), Electrospray Ionization Mass Spectrometry Analysis (ESI-MS), Different Scanning Calorimetry Analysis (DSC), Carbon, Hydrogen, and Nitrogen (CHN) Analyzer, and Universal Testing Machine (UTM), Sturm test, and Bacterial Adherence to Hydrocarbons (BATH) assay.

Bacterial Biodegradation
The degradation rate of polythene was observed in various microorganisms (Table 1). The six Bacillus sp. isolated from landfills and dumping sites were identified as LDPEdegrading bacteria. The degradation potential rates in mineral agar and mineral broth were found that B. carboniphilus (34.55% and 25%), B. sporothermodurans (36.54% and 21%), B. coagulans (18.37% and 16%), B. neidei (36.07% and 14%), B. smithii (16.40% and 8%), and B. megaterium (34.48% and 21%), respectively. Those bacteria were incubated for two months at 30°C (Shrestha et al. 2019). Pretreated LLDPE particles were incubated with the marine bacteria Microbulbifer hydrolyticus IRE-3 for up to 30 days. SEM images revealed the creation of cracks at various locations on the polymer surface, which was also confirmed using FTIR analysis with the formation of hydroxyl and carbonyl functional groups . A consortium of Enterobacter and Pseudomonas reduced 64.25% of PE (Skariyachan et al. 2021), indicating the mixture has a higher degradation rate. The weight of LDPE and HDPE with consortia of Lysinibacillus sp. and Salinibacterium sp. decreased to higher than 20% after 60 days and11.1% at 6 months, respectively (Syranidou et al. 2017). The bacterial consortia of Enterobacter sp. (IS2), Enterobacter sp. (IS3), and Pantoea sp. (IS5) degrade 38% and 81% of LDPE pellets and strips, respectively, after 120 days. Further, consortia of P. putida (MTCC1), P. stutzeri (MTCC2), and B. subtilis (MTCC3) 20% LDPE pellets and 49% for LDPE strips after 120 days of incubation respectively  (Skariyachan et al. 2016). However, pure or consortia of bacteria degradation also depends on the polymer and incubation period.

CONCLUSIONS
The massive accretion of plastic has emerged as a main concern across the world. PE's toxicity to the environment and human health is obvious, but research is still in its infancy. It is significant to have sustainable and robust technologies to combat plastic pollution and its impacts. Microbial degradation of PE is an environmentally friendly technique to reduce plastic waste. The biodeterioration and biofragmentation mechanisms are explicitly illustrated, but few reports on the bioassimilation or mineralization of PE exist. Thus, most investigation in the field of microbial degradation of PE is superficial rather than intrinsic. The biotic and abiotic factors, as well as the PE properties, substantially influence biodegradation. Therefore, it is critical to thoroughly consider the role of various factors when evaluating PE biodegradation.
The study found that most potential polyethylenedegrading bacteria are conducted in pure culture. This clearly illustrates that the high diversity of bacteria in various habitats has not been fully utilized. Moreover, bacteria consortiums have superior proficiency in plastic degradation due to synergism between the bacteria and enzymes involved. The enzymes responsible for PE degradation have been identified in bacteria; however, enzyme properties have not been thoroughly investigated for enzyme engineering. Therefore, a deep understanding of the mechanism of enzyme action is valuable for improving degradation efficiency. Further research into the mechanism of enzymatic degradation will lead to the discovery of efficient, biodegradable plastic.
Since the bacteria constantly adapt to their surroundings, viable PE-degrading bacteria are also expected to be acquired and developed commercially. In many studies, a variety of techniques are used to evaluate biodegradation. However, the lack of a standard protocol creates discrepancies in assessing biodegradation. Therefore, it is fundamental to establish a standardized protocol to acquire reliable and consistent outcomes. It is also indispensable to eliminate the additives and impurities and examine the pretreatment for a better result in the biodegradation of PE.