Polyester-degrading thermophilic actinomycetes isolated from different environment in Taiwan

Lack of functional polyester-biodegrading potential in marine versus terrestrial environments evidenced by an innovative airbrushing technique

Biodegradable plastics, primarily aliphatic polyesters, degrade to varying extents in different environments. However, the absence of easily implementable techniques for screening microbial biodegradation potential -coupled with the limitations of non-functional omics analyses- has restricted comparative studies across diverse polymer types and ecosystems. In this study, we optimized a novel airbrushing method that facilitates functional analyses by simplifying the preparation of polyester-coated plates for biodegradation screening. By repurposing an airbrush kit, polyester microparticles (MPs) could be evenly sprayed onto solid media, enabling rapid detection of extracellular depolymerizing activity via clearing zone halos. This technique was effective in screening both isolated microbial cultures and natural environmental samples, demonstrating its versatility. The method was successfully applied across multiple environments, ranking the biodegradability of six polyesters, from most to least biodegradable: poly[(R)-3-hydroxybutyrate] (PHB), polycaprolactone (PCL), poly(ethylene succinate) (PES), poly(butylene succinate) (PBS), poly(lactic acid) (PLA), and poly(butylene adipate-co-terephthalate) (PBAT). Most notably, it revealed a consistent 1,000-fold higher biodegradation potential in terrestrial compared to marine environments. This approach offers a valuable tool for isolating novel polyester-degrading microbes with significant biotechnological potential, paving the way for improved plastic waste management solutions.

Biological Degradation of Plastics and Microplastics: A Recent Perspective on Associated Mechanisms and Influencing Factors

Plastic and microplastic pollution has caused a great deal of ecological problems because of its persistence and potential adverse effects on human health. The degradation of plastics through biological processes is of great significance for ecological health, therefore, the feasibility of plastic degradation by microorganisms has attracted a lot of attention. This study comprises a preliminary discussion on the biodegradation mechanism and the advantages and roles of different bacterial enzymes, such as PET hydrolase and PCL-cutinase, in the degradation of different polymers, such as PET and PCL, respectively. With a particular focus on their modes of action and potential enzymatic mechanisms, this review sums up studies on the biological degradation of plastics and microplastics related to mechanisms and influencing factors, along with their enzymes in enhancing the degradation of synthetic plastics in the process. In addition, biodegradation of plastic is also affected by plastic additives and plasticizers. Plasticizers and additives in the composition of plastics can cause harmful impacts. To further improve the degradation efficiency of polymers, various pretreatments to improve the efficiency of biodegradation, which can cause a significant reduction in toxic plastic pollution, were also preliminarily discussed here. The existing research and data show a large number of microorganisms involved in plastic biodegradation, though their specific mechanisms have not been thoroughly explored yet. Therefore, there is a significant potential for employing various bacterial strains for efficient degradation of plastics to improve human health and safety.

Microplastics and their interactions with microbiota

As a new pollutant, Microplastics (MPs) are globally known for their negative impacts on different ecosystems and living organisms. MPs are easily taken up by the ecosystem in a variety of organisms due to their small size, and cause immunological, neurological, and respiratory diseases in the impacted organism. Moreover, in the impacted environments, MPs can release toxic additives and act as a vector and scaffold for colonization and transportation of specific microbes and lead to imbalances in microbiota and the biogeochemical and nutrients dynamic. To address the concerns on controlling the MPs pollution on the microbiota and ecosystem, the microbial biodegradation of MPs can be potentially considered as an effective environment friendly approach. The objectives of the presented paper are to provide information on the toxicological effects of MPs on microbiota, to discuss the negative impacts of microbial colonization of MPs, and to introduce the microbes with biodegradation ability of MPs.

Isolation and optimization of extracellular PHB depolymerase producer Aeromonas caviae Kuk1-(34) for sustainable solid waste management of biodegradable polymers

Bioplastics, synthesized by several microbes, accumulates inside cells under stress conditions as a storage material. Several microbial enzymes play a crucial role in their degradation. This research was carried to test the biodegradability of poly-β-hydroxybutyrate (PHB) utilizing PHB depolymerase, produced by bacteria isolated from sewage waste soil samples. Potent PHB degrader was screened based on the highest zone of hydrolysis followed by PHB depolymerase activity. Soil burial method was employed to check their degradation ability at different incubation periods of 15, 30, and 45 days at 37±2°C, pH 7.0 at 60% moisture with 1% microbial inoculum of Aeromonas caviae Kuk1-(34) (MN414252). Without optimized conditions, 85.76% of the total weight of the PHB film was degraded after 45 days. This degradation was confirmed with Fourier-transform infrared spectroscopy (FTIR) and Scanning electron microscope (SEM) analysis. The presence of bacterial colonies on the surface of the degraded film, along with crest, holes, surface erosion, and roughness, were visible. Media optimization was carried out in statistical mode using Plackett Burman (PB) and Central Composite Design (CCD) of Response Surface Methodology (RSM) by considering ten different factors. Analysis of Variance (ANOVA), Pareto chart, response surface plots, and F-value of 3.82 implies that the above statistical model was significant. The best production of PHB depolymerase enzyme (14.98 U/mL) was observed when strain Kuk1-(34) was grown in a media containing 0.1% PHB, K₂HPO₄ (1.6 gm/L) at 27 ℃ for seven days. Exploiting these statistically optimized conditions, the culture was found to be a suitable candidate for the management of solid waste, where 94.4% of the total weight of the PHB film was degraded after 45 days of incubation.

Biodegradation of polymers in managing plastic waste - A review

The modern economy that is fast-moving and convenience centric has led to excessive consumption of plastic. This has unwittingly led to egregious accumulation of plastic waste polluting the environment. Unfortunately, present means of plastic waste management have all been proven as less than adequate; namely recycling, landfill and incineration. Recent focus on plastic waste management has seen the confluence of the developments in biodegradable polymers and microbial engineering strategy for more expedient decomposition of plastic waste at composting facilities. This review paper is an assimilation of current developments in the areas of biodegradable polymer as well as microbial strategy towards management of polymer waste. Advents in biodegradable polymers have been promising, especially with aliphatic polyesters and starch in blends or co-polymers of these. Microbial strategies have been pursued for the identification of microbial strains and understanding of their enzymatic degradation process on polymers. New insights in these two areas have been focused in improving the rate of degradation of plastic waste at composting facilities. Recent alignment of testing and certification standards is outlined to give intimate insights into the mechanisms and factors influencing biodegradation. Despite recent milestones, economic viability of composting plastic waste in mainstream waste facilities is still a distance away. As it remains that a polymer that is biodegradable is functionally inferior to conventional polymers. Rather, it requires a shift in consumer behaviour to accept less durable biodegradable plastic products, this will then lower the threshold for biodegradable polymers to become a commercial reality.

Two Extracellular Poly(ε-caprolactone)-Degrading Enzymes From Pseudomonas hydrolytica sp. DSWY01T: Purification, Characterization, and Gene Analysis

Poly(ε-caprolactone) (PCL) is an artificial polyester with commercially promising application. In this study, two novel PCL-degrading enzymes named PCLase I and PCLase II were purified to homogeneity from the culture supernatant of an effective polyester-degrading bacterium, Pseudomonas hydrolytica sp. DSWY01^T. The molecular masses of PCLase I and PCLase II were determined to be 27.5 and 30.0 kDa, respectively. The optimum temperatures for the enzyme activities were 50 and 40°C, and the optimum pH values were 9.0 and 10.0, respectively. The two enzymes exhibited different physical and chemical properties, but both enzymes could degrade PCL substrates into monomers and oligomers. Weight loss detection and scanning electron microscopy revealed that PCLase I had more effective degradation ability than PCLase II. The genes of the two enzymes were cloned on the basis of the peptide fingerprint analysis results. The sequence analysis and substrate specificity analysis results showed that PCLase I and PCLase II were cutinase and lipase, respectively. Interface activation experiment also confirmed this conclusion. Structural analysis and modeling were further performed to obtain possible insights on the mechanism.

Streptomyces as Potential Synthetic Polymer Degraders: A Systematic Review

The inherent resistance of synthetic plastics to degradation has led to an increasing challenge of waste accumulation problem and created a pollution issue that can only be addressed with novel complementary methods such as biodegradation. Since biocontrol is a promising eco-friendly option to address this challenge, the identification of suitable biological agents is a crucial requirement. Among the existing options, organisms of the Streptomyces genus have been reported to biodegrade several complex polymeric macromolecules such as chitin, lignin, and cellulose. Therefore, this systematic review aimed to evaluate the potential of Streptomyces strains for the biodegradation of synthetic plastics. The results showed that although Streptomyces strains are widely distributed in different ecosystems in nature, few studies have explored their capacity as degraders of synthetic polymers. Moreover, most of the research in this field has focused on Streptomyces strains with promising biotransforming potential against polyethylene-like polymers. Our findings suggest that this field of study is still in the early stages of development. Moreover, considering the diverse ecological niches associated with Streptomyces, these actinobacteria could serve as complementary agents for plastic waste management and thereby enhance carbon cycle dynamics.

Cellulosimicrobium composti sp. nov., a thermophilic bacterium isolated from compost

A Gram-stain-positive, yellow-pigmented, non-motile actinobacterial strain, designated as BIT-GX5^T, was isolated from a sesame husks compost collected in Beijing, PR China. This bacterium was found to be able to grow in the temperature range from 16 to 50 °C and had an optimal growth temperature at 45 °C. Its taxonomic position was analysed using a polyphasic approach. The 16S rRNA gene sequence (1482 bp) of strain BIT-GX5^T was most similar to Cellulosimicrobium funkei ATCC BAA-886^T (99.45%), Cellulosimicrobium cellulans LMG 16121^T (99.17%) and Cellulosimicrobium marinum RS-7-4^T (98.75%). The results of phylogenetic analyses, based on the 16S rRNA gene, concatenated sequences of five housekeeping genes (gyrB, rpoB, recA, atpD and trpB) and genome sequences, placed strain BIT-GX5^T in a separate lineage among the genus Cellulosimicrobium within the family Promicromonosporaceae. The major polar lipids of strain BIT-GX5^T were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, aminophospholipid and aminolipid. The major isoprenoid quinone was MK-9(H₄), while the cell-wall sugars were galactose, rhamnose, glucose and mannose. The peptidoglycan type was A4α l-Lys-d-Ser-d-Asp. The major fatty acids were anteiso-C_15:0 and iso-C_15: 0, which were similar to other members in the genus Cellulosimicrobium. Results of in silico DNA-DNA hybridization and average nucleotide identity calculations plus physiological and biochemical tests exhibited the genotypic and phenotypic differentiation of strain BIT-GX5^T from the other members of the genus Cellulosimicrobium. Therefore, strain BIT-GX5^T is considered to represent a novel species within the genus Cellulosimicrobium, for which the name Cellulosimicrobium composti sp. nov. is proposed. The type strain is BIT-GX5^T (= CGMCC 1.17687^T = KCTC 49391^T).

Marine Actinomycetes, New Sources of Biotechnological Products

The Actinomycetales order is one of great genetic and functional diversity, including diversity in the production of secondary metabolites which have uses in medical, environmental rehabilitation, and industrial applications. Secondary metabolites produced by actinomycete species are an abundant source of antibiotics, antitumor agents, anthelmintics, and antifungals. These actinomycete-derived medicines are in circulation as current treatments, but actinomycetes are also being explored as potential sources of new compounds to combat multidrug resistance in pathogenic bacteria. Actinomycetes as a potential to solve environmental concerns is another area of recent investigation, particularly their utility in the bioremediation of pesticides, toxic metals, radioactive wastes, and biofouling. Other applications include biofuels, detergents, and food preservatives/additives. Exploring other unique properties of actinomycetes will allow for a deeper understanding of this interesting taxonomic group. Combined with genetic engineering, microbial experimental evolution, and other enhancement techniques, it is reasonable to assume that the use of marine actinomycetes will continue to increase. Novel products will begin to be developed for diverse applied research purposes, including zymology and enology. This paper outlines the current knowledge of actinomycete usage in applied research, focusing on marine isolates and providing direction for future research.

Plastic Degradation by Extremophilic Bacteria

Intensive exploitation, poor recycling, low repeatable use, and unusual resistance of plastics to environmental and microbiological action result in accumulation of huge waste amounts in terrestrial and marine environments, causing enormous hazard for human and animal life. In the last decades, much scientific interest has been focused on plastic biodegradation. Due to the comparatively short evolutionary period of their appearance in nature, sufficiently effective enzymes for their biodegradation are not available. Plastics are designed for use in conditions typical for human activity, and their physicochemical properties roughly change at extreme environmental parameters like low temperatures, salt, or low or high pH that are typical for the life of extremophilic microorganisms and the activity of their enzymes. This review represents a first attempt to summarize the extraordinarily limited information on biodegradation of conventional synthetic plastics by thermophilic, alkaliphilic, halophilic, and psychrophilic bacteria in natural environments and laboratory conditions. Most of the available data was reported in the last several years and concerns moderate extremophiles. Two main questions are highlighted in it: which extremophilic bacteria and their enzymes are reported to be involved in the degradation of different synthetic plastics, and what could be the impact of extremophiles in future technologies for resolving of pollution problems.

The potential of cold-adapted microorganisms for biodegradation of bioplastics

Due to the extensive use of plastics, their quantity in the environment is constantly increasing, which creates a global problem. In the present study, we sought to isolate, test and identify Antarctic microorganisms which possess the ability to biodegrade bioplastics such as poly(ε-caprolactone) (PCL), poly(butylene succinate) (PBS) and poly(butylene succinate-co-butylene adipate) (PBSA) at low temperatures. 161 bacterial and 38 fungal isolates were isolated from 22 Antarctic soil samples. Among them, 92.16% of bacterial and 77.27% of fungal isolates formed a clear zone on emulsified PBSA, 98.04% and 81.82% on PBS and 100% and 77.27% on PCL as an additive to minimal medium at 20 °C. Based on the 16S and 18S rRNA sequences, bacterial strains were identified as species belonging to Pseudomonas and Bacillus and fungal strains as species belonging to Geomyces, Sclerotinia, Fusarium and Mortierella, while the yeast strain was identified as Hansenula anomala. In the biodegradation process conducted under laboratory conditions at 14, 20 and 28 °C, Sclerotinia sp. B11IV and Fusarium sp. B3'M strains showed the highest biodegradation activity at 20 °C (49.68% for PBSA and 33.7% for PCL, 45.99% for PBSA and 49.65% for PCL, respectively). The highest biodegradation rate for Geomyces sp. B10I was noted at 14 °C (25.67% for PBSA and 5.71% for PCL), which suggested a preference for lower temperatures (at 20 °C the biodegradation rate was only 11.34% for PBSA, and 4.46% for PCL). These data showed that microorganisms isolated from Antarctic regions are good candidates for effective plastic degradation at low temperatures.

A sensitive pH indicator-based spectrophotometric assay for PHB depolymerase activity on microtiter plates

A continuous spectrophotometric assay for the screening of PHB depolymerase activity in microtiter plates was developed. We evaluated crystalline PHB in the suspension and coated it with the addition of a pH indicator to detect the breakage of the ester bond by proton titration. The reaction rate and the concentration of the recombinant PhaZ1 from Paucimonas lemoignei PHB depolymerase presented a linear correlation. A comparison of the proposed method with the turbidimetric method adapted to the microtiter plates revealed that the use of indicators increases the response signal by at least 5-fold, resulting in increased sensitivity and better signal-to-noise ratio. Furthermore, the proposed method offers a wide range of pH from 5.0 to 9.2 by using different buffer-indicator pairs and was employed for the screening of PHB-depolymerase activity on 140 bacterial strains isolated from Lake Chapala. Eleven strains were positive for PHB-depolymerase activity, which were ACSLRF-27, ACPLRF-6, and ACPLRF-5 (16S rRNA sequence alignment revealed 99-100% similarity with Actinomadura geliboluensis strain A8036, Streptomyces cavourensis strain NRRL 2740, and Streptomyces coelicolor strain DSM 40233, respectively); these that showed the highest activities. In conclusion, the method was successfully applied for finding new strains and for quantifying the PHB depolymerases activity with crystalline PHB.

Polyester-based biodegradable plastics: an approach towards sustainable development

Non-degradability of conventional plastics, filling of landfill sites, raising water and land pollution and rapid depletion of fossil resources have raised the environmental issues and global concerns. The current demand and production of plastics is putting immense pressure on fossil resources, consuming about 6% of the global oil and is expected to grow up to 20%. The polyester-based biodegradable plastics (BPs) are considered as a remedy to the issue of plastics waste in the environment. BPs appear to manage the overflow of plastics by providing new means of waste management system and help in securing the non-renewable resources of nature. This review comprehensively presents the environmental burdens due to conventional plastics as well as production of polyester-based BPs as an alternative to conventional commodity plastics. The diversity of micro-organisms and their enzymes that degrade various polyester-based BPs (PLA, PCL, PHB/PHBV and PET) has also been described in detail. Moreover, the impact of plastics degradation products on soil ecology and ecosystem functions has critically been discussed. The report ends with special focus on future recommendations for the development of sustainable waste management strategies to control pollution due to plastics waste. SIGNIFICANCE AND IMPACT OF THE STUDY: Polyester-based BPs considered as a solution to current plastic waste problem as well as leading polymers in terms of biodegradability and sustainability has been critically discussed. The role of microorganisms and their enzymes involved in the biodegradation of these polymers and ecotoxicological impact of degradation products of BPs on soil microbial community and biogeochemical cycles has also been described. This report will provide an insight on the key research areas to bridge the gap for development of simulated systems as an effective and emerging strategy to divert the overflow of plastic in the environment as well as for the greener solution to the plastic waste management problems.

Beyond oil degradation: enzymatic potential of Alcanivorax to degrade natural and synthetic polyesters

Pristine marine environments are highly oligotrophic ecosystems populated by well‐established specialized microbial communities. Nevertheless, during oil spills, low‐abundant hydrocarbonoclastic bacteria bloom and rapidly prevail over the marine microbiota. The genus Alcanivorax is one of the most abundant and well‐studied organisms for oil degradation. While highly successful under polluted conditions due to its specialized oil‐degrading metabolism, it is unknown how they persist in these environments during pristine conditions. Here, we show that part of the Alcanivorax genus, as well as oils, has an enormous potential for biodegrading aliphatic polyesters thanks to a unique and abundantly secreted alpha/beta hydrolase. The heterologous overexpression of this esterase proved a remarkable ability to hydrolyse both natural and synthetic polyesters. Our findings contribute to (i) better understand the ecology of Alcanivorax in its natural environment, where natural polyesters such as polyhydroxyalkanoates (PHA) are produced by a large fraction of the community and, hence, an accessible source of carbon and energy used by the organism in order to persist, (ii) highlight the potential of Alcanivorax to clear marine environments from polyester materials of anthropogenic origin as well as oils, and (iii) the discovery of a new versatile esterase with a high biotechnological potential.

Microbial degradation of four biodegradable polymers in soil and compost demonstrating polycaprolactone as an ideal compostable plastic

Plastics are an indispensable material but also a major environmental pollutant. In contrast, biodegradable polymers have the potential to be compostable. The biodegradation of four polymers as discs, polycaprolactone (PCL), polyhydroxybutyrate (PHB), polylactic acid (PLA) and poly(1,4 butylene) succinate (PBS) was compared in soil and compost over a period of more than 10 months at 25 °C, 37 °C and 50 °C. Degradation rates varied between the polymers and incubation temperatures but PCL showed the fastest degradation rate under all conditions and was completely degraded when buried in compost and incubated at 50 °C after 91 days. Furthermore, PCL strips showed a significant reduction in tensile strength in just 2 weeks when incubated in compost >45 °C. Various fungal strains growing on the polymer surfaces were identified by sequence analysis. Aspergillus fumigatus was most commonly found at 25 °C and 37 °C, while Thermomyces lanuginosus, which was abundant at 50 °C, was associated with PCL degradation.

Biodegradation of bioplastics in natural environments

The extensive production of conventional plastics and their use in different commercial applications poses a significant threat to both the fossil fuels sources and the environment. Alternatives called bioplastics evolved during development of renewable resources. Utilizing renewable resources like agricultural wastes (instead of petroleum sources) and their biodegradability in different environments enabled these polymers to be more easily acceptable than the conventional plastics. The biodegradability of bioplastics is highly affected by their physical and chemical structure. On the other hand, the environment in which they are located, plays a crucial role in their biodegradation. This review highlights the recent findings attributed to the biodegradation of bioplastics in various environments, environmental conditions, degree of biodegradation, including the identified bioplastic-degrading microorganisms from different microbial communities.

Stenotrophomonas sp. RZS 7, a novel PHB degrader isolated from plastic contaminated soil in Shahada, Maharashtra, Western India

This paper reports an isolation and identification of novel poly-β-hydroxybutyrate (PHB) degrading bacterium Stenotrophomonas sp. RZS 7 and studies on its extracellular PHB degrading depolymerase enzyme. The bacterium isolated from soil samples of plastic contaminated sites of municipal area in Shahada, Maharashtra, Western India. It was identified as Stenotrophomonas sp. RZS 7 based on polyphasic approach. The bacterium grew well in minimal salt medium (MSM) and produced a zone (4.2 mm) of PHB hydrolysis on MSM containing PHB as the only source of nutrient. An optimum yield of enzyme was obtained on the fifth day of incubation at 37 °C and at pH 6.0. Further increase in enzyme production was recorded with Ca²⁺ ions, while other metal ions like Fe²⁺ (1 mM) and chemical viz. mercaptoethanol severally affected the production of enzyme.

Thermophilic and alkaliphilic Actinobacteria: biology and potential applications

Microbes belonging to the phylum Actinobacteria are prolific sources of antibiotics, clinically useful bioactive compounds and industrially important enzymes. The focus of the current review is on the diversity and potential applications of thermophilic and alkaliphilic actinobacteria, which are highly diverse in their taxonomy and morphology with a variety of adaptations for surviving and thriving in hostile environments. The specific metabolic pathways in these actinobacteria are activated for elaborating pharmaceutically, agriculturally, and biotechnologically relevant biomolecules/bioactive compounds, which find multifarious applications.

Degradation of poly(ε-caprolactone) (PCL) by a newly isolated Brevundimonas sp. strain MRL-AN1 from soil

A poly(ε-caprolactone) (PCL)-degrading bacterium designated as strain MRL-AN1 was isolated from soil. The bacterium was identified as Brevundimonas sp. strain MRL-AN1 through biochemical tests and 16S rRNA gene sequencing. Scanning electron microscopy and Fourier transform infrared spectroscopy results confirmed the degradation of PCL by strain MRL-AN1. An extracellular PCL depolymerase was purified to homogeneity by column chromatography and molecular weight was estimated to be approximately 63.49 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. PCL depolymerase could degrade not only PCL but also other aliphatic polyesters. The enzyme was stable at wide range of temperature (20-45°C) and pH (5-9) as well as stable in the presence of various metal ions, surfactants and organic solvents. Phenylmethylsulfonyl fluoride inhibited enzyme activity that indicates this enzyme belongs to the serine hydrolase family. It is concluded from the results that the enzymes from strain MRL-AN1 might be applied in the process of biochemical monomer recycling in the polyester-contaminated environments.

Synthetic polyester-hydrolyzing enzymes from thermophilic actinomycetes

Thermophilic actinomycetes produce enzymes capable of hydrolyzing synthetic polyesters such as polyethylene terephthalate (PET). In addition to carboxylesterases, which have hydrolytic activity predominantly against PET oligomers, esterases related to cutinases also hydrolyze synthetic polymers. The production of these enzymes by actinomycetes as well as their recombinant expression in heterologous hosts is described and their catalytic activity against polyester substrates is compared. Assays to analyze the enzymatic hydrolysis of synthetic polyesters are evaluated, and a kinetic model describing the enzymatic heterogeneous hydrolysis process is discussed. Structure-function and structure-stability relationships of actinomycete polyester hydrolases are compared based on molecular dynamics simulations and recently solved protein structures. In addition, recent progress in enhancing their activity and thermal stability by random or site-directed mutagenesis is presented.

Study of microbes having potentiality for biodegradation of plastics

Plastic is a broad name given to the different types of organic polymers having high molecular weight and is commonly derived from different petrochemicals. Plastics are generally not biodegradable or few are degradable but in a very slow rate. Day by day, the global demand of these polymers is sharply increasing; however, considering their abundance and potentiality in causing different environmental hazards, there is a great concern in the possible methods of degradation of plastics. Recently, there have been some debates at the world stage about the potential degradation procedures of these synthetic polymers and microbial degradation has emerged as one of the potential alternative ways of degradation of plastics. Alternatively, some scientists have also reported many adverse effects of these polymers in human health, and thus, there is an immediate need of a potential screening of some potential microbes to degrade these synthetic polymers. In this review, we have taken an attempt to accumulate all information regarding the chemical nature along with some potential microbes and their enzymatic nature of biodegradation of plastics along with some key factors that affect their biodegradability.

Degradation of Poly(ε-caprolactone) by thermophilic Streptomyces thermoviolaceus subsp. thermoviolaceus 76T-2

A thermophilic Streptomyces thermoviolaceus subsp. thermoviolaceus isolate 76T-2 that can degrade poly(ε-caprolactone) (PCL) was isolated from soil in Taiwan. Isolate 76T-2 grew well in urea fructose oatmeal medium and exhibited clear zones on agar plates containing PCL, indicating the presence of extracellular PCL depolymerases. The PCL powder present in culture medium was completely degraded within 6 h of culture at 45°C. Two PCL-degrading enzymes were purified to homogeneity from the culture supernatant. The molecular weights of these two enzymes were estimated to be 25 kDa and 55 kDa, respectively. A portion of the N-terminal region of the 25-kDa protein was determined, and the sequence Ala-Asn-Phe-Val-Val-Ser-Glu-Ala thus obtained was identical to that of A₆₄-A₇₁ of the Chi25 chitinase of Streptomyces thermoviolaceus OPC-520. The 25-kDa protein was shown to also degrade chitin, suggesting that isolate 76T-2 has the ability to degrade both PCL and chitin.

Biodegradation of poly(ε-caprolactone) (PCL) by a new Penicillium oxalicum strain DSYD05-1

In this study, fungi isolated from soil were screened for their ability to form clear zones on agar plates with emulsified poly(ε-caprolactone) (PCL). The most active strain, designated as DSYD05, was identified as Penicillium oxalicum on the basis of morphological characteristics and phylogenetic analysis. Mutant DSYD05-1, obtained by ultraviolet-light mutagenesis from strain DSYD05, was more effective in PCL degradation. In liquid cultures of the mutant strain with PCL emulsion, DSYD05-1 showed the highest PCL-degrading activity after 4 days of cultivation. The products of PCL degradation were analysed by mass spectrometry; the results indicated that 6-hydroxyhexanoic acid was produced and assimilated during cultivation. The degradation of PCL film by DSYD05-1 was observed by scanning electron microscopy, and was indicative of a three-stage degradation process. The degradation of amorphous parts of the film preceded that of the crystalline center and then the peripheral crystalline regions. In addition, DSYD05-1 showed a wide range of substrate specificity, with capability to degrade PCL, poly(β-hydroxybutyrate), and poly(butylene succinate), but not poly(lactic acid), indicating that the strain could have potential for application in the treatment or recycling of bio-plastic wastes.

Biodegradability of plastics

Plastic is a broad name given to different polymers with high molecular weight, which can be degraded by various processes. However, considering their abundance in the environment and their specificity in attacking plastics, biodegradation of plastics by microorganisms and enzymes seems to be the most effective process. When plastics are used as substrates for microorganisms, evaluation of their biodegradability should not only be based on their chemical structure, but also on their physical properties (melting point, glass transition temperature, crystallinity, storage modulus etc.). In this review, microbial and enzymatic biodegradation of plastics and some factors that affect their biodegradability are discussed.

Actinomadura miaoliensis sp. nov., a thermotolerant polyester-degrading actinomycete

An actinomycete, strain BC 44T-5(T), with the ability to degrade poly(d-3-hydroxybutyrate) was isolated from a soil sample collected from Miaoli County, Taiwan. The isolate displayed substrate mycelia and short spore chains were borne on the aerial mycelia. Spores were non-motile, round, 1 mum in diameter and spiny. The aerial spore mass was blue. Strain BC 44T-5(T) had meso-diaminopimelic acid as the diagnostic diamino acid of the cell-wall peptidoglycan. Whole-cell sugars of the novel strain were identified as glucose, galactose and madurose. Diphosphatidylglycerol and phosphatidylinositol were detected. The predominant menaquinones were MK-9(H(4)) and MK-9(H(2)). Mycolic acids were not detected. Major cellular fatty acids were iso-C(16 : 0) (14.82 %), C(16 : 0) (14.63 %), C(17 : 0) (13.79 %) and 10-methyl-C(17 : 0) (23.77 %.) The DNA G+C content of strain BC 44T-5(T) was 70.6 mol%. On the basis of phenotypic and genotypic data, it is proposed that strain BC 44T-5(T) (=FIRDI 002(T)=BCRC 16873(T)=LMG 24335(T)) should be classified as the type strain of a novel species of the genus Actinomadura, Actinomadura miaoliensis sp. nov.