Growth kinetics and biodeterioration of polypropylene microplastics by Bacillus sp. and Rhodococcus sp. isolated from mangrove sediment.

Microbial degradation and other environmental aspects of microplastics/plastics.

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.

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Microbial and Enzymatic Degradation of Synthetic Plastics

Synthetic plastics, which are widely present in materials of everyday use, are ubiquitous and slowly-degrading polymers in environmental wastes. Of special interest are the capabilities of microorganisms to accelerate their degradation. Members of the metabolically diverse genus Pseudomonas are of particular interest due to their capabilities to degrade and metabolize synthetic plastics. Pseudomonas species isolated from environmental matrices have been identified to degrade polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyurethane, polyethylene terephthalate, polyethylene succinate, polyethylene glycol and polyvinyl alcohol at varying degrees of efficiency. Here, we present a review of the current knowledge on the factors that control the ability of Pseudomonas sp. to process these different plastic polymers and their by-products. These factors include cell surface attachment within biofilms, catalytic enzymes involved in oxidation or hydrolysis of the plastic polymer, metabolic pathways responsible for uptake and assimilation of plastic fragments and chemical factors that are advantageous or inhibitory to the biodegradation process. We also highlight future research directions required in order to harness fully the capabilities of Pseudomonas sp. in bioremediation strategies towards eliminating plastic wastes.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/jam.13472

Degradation and metabolism of synthetic plastics and associated products by Pseudomonas sp.: capabilities and challenges

Screening of Bacillus strains isolated from mangrove ecosystems in Peninsular Malaysia for microplastic degradation.

Microbial assisted High Impact Polystyrene (HIPS) degradation.

Microbial degradation of high impact polystyrene (HIPS), an e-plastic with decabromodiphenyl oxide and antimony trioxide

The concept of biodegradable plastics is receiving significant attention with respect to solid waste accumulation. Enormous efforts have been made in revolutionizing degradable biological materials without any environmental pollution to substitute for oil-based traditional plastics. With the overuse of plastics and increasing pressure being placed on capacities available for plastic waste disposal, the requirement for biodegradable plastics and biodegradation of plastic wastes has obtained immense importance in recent years. Awareness of the impact on the environmental waste problem has awakened research in the area of degradable polymers. Considering the emerging environmental issues, the demand to develop a material that does not burden nature has increased. It is essential to study the microbial degradation of biopolymers to understand what is necessary for biodegradation and the mechanisms involved. Widespread investigations on the biodegradation of plastics have been carried out to overcome the environmental problems associated with synthetic plastic waste. This chapter provides an overview of the degradation mechanisms of biodegradable polymers with particular emphasis on the main parameters affecting the degradation of polymeric biomaterials.

Biodegradation of Biopolymers

Bacopa monnieri, commonly known as Brahmi, has been extensively used as a neuromedicine for various disorders such as anxiety, depression and memory loss. Chemical characterization studies revealed the major active constituents of the herb as the triterpenoid saponins, bacosides. Bacoside A, the vital neuroprotective constituent, is composed of four constituents viz., bacoside A3, bacopaside II, jujubogenin isomer of bacopasaponin C (bacopaside X) and bacopasaponin C. B. monnieri extracts as well as bacosides successfully establish a healthy antioxidant environment in various tissues especially in the liver and brain. Free radical scavenging, suppression of lipid peroxidation and activation of antioxidant enzymes by bacosides help to attain a physiological state of minimized oxidative stress. The molecular basis of neuroprotective activity of bacosides is attributed to the regulation of mRNA translation and surface expression of neuroreceptors such as AMPAR, NMDAR and GABAR in the various parts of the brain. Bioavailability as well as binding of neuroprotective agents (such as bacosides) to these receptors is controlled by the Blood Brain Barrier (BBB). However, nano conversion of these drug candidates easily resolves the BBB restriction and carries a promising role in future therapies. This review summarizes the neuroprotective functions of B. monnieri extracts as well as its active compounds (bacoside A, bacopaside I) and the molecular mechanisms responsible for these pharmacological activities.

Insights into the Molecular Aspects of Neuroprotective Bacoside A and Bacopaside I

Bioplastics are eco-friendly and derived from renewable biomass sources. Innovation in recycling methods will tackle some of the critical issues facing the acceptance of bioplastics. Polylactic acid (PLA) is the commonly used and well-studied bioplastic that is presumed to be biodegradable. Considering their demand and use in near future, exploration for microbes capable of bioplastic degradation has high potential. Four PLA degrading strains were isolated and identified as Penicillium chrysogenum, Cladosporium sphaerospermum, Serratia marcescens and Rhodotorula mucilaginosa. A consortium of above strains degraded 44 % (w/w) PLA in 30 days time in laboratory conditions. Subsequently, the microbial consortium employed effectively for PLA composting.

Vini C. Sekhar

Papers

Microbial assisted High Impact Polystyrene (HIPS) degradation.

Microbial degradation of high impact polystyrene (HIPS), an e-plastic with decabromodiphenyl oxide and antimony trioxide

Biodegradation of Biopolymers

Insights into the Molecular Aspects of Neuroprotective Bacoside A and Bacopaside I

Augmentation of a Microbial Consortium for Enhanced Polylactide (PLA) Degradation