Study of Microbes Having Potentiality for Biodegradation of Plastics

REVIEWARTICLE

Study of microbes having potentiality for biodegradationof plastics

Swapan Kumar Ghosh & Sujoy Pal & Sumanta Ray

Received: 1 December 2012 /Accepted: 1 April 2013 /Published online: 24 April 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Plastic is a broad name given to the differenttypes of organic polymers having high molecular weightand is commonly derived from different petrochemicals.Plastics are generally not biodegradable or few are degrad-able but in a very slow rate. Day by day, the global demand ofthese polymers is sharply increasing; however, consideringtheir abundance and potentiality in causing different environ-mental hazards, there is a great concern in the possiblemethods of degradation of plastics. Recently, there have beensome debates at the world stage about the potential degrada-tion procedures of these synthetic polymers and microbialdegradation has emerged as one of the potential alternativeways of degradation of plastics. Alternatively, some scientistshave also reported many adverse effects of these polymers inhuman health, and thus, there is an immediate need of apotential screening of some potential microbes to degradethese synthetic polymers. In this review, we have taken anattempt to accumulate all information regarding the chemicalnature along with some potential microbes and their enzymat-ic nature of biodegradation of plastics along with some keyfactors that affect their biodegradability.

Keywords Plastics . Environment . Pollution . Microbes .

Biodegradation . Enzymes

Introduction

Plastic is one of the most vital man-made product that hasbeen produced in huge quantity and is used widely for

different purposes in our everyday life, and gradually, theglobal demand of this synthetic product is rapidly growingday by day. The word plastic is derived from the Greekwork “plastikos” meaning the substance which can bemolded into any shape (Joel 1995). Chemically, plastic is along hydrocarbon chain polymer having high molecularweight. Plastics are mainly derived from petrochemicalswhich are further synthetically arranged by some chemicalprocesses to produce these long chain polymers (Shimao2001). Now, plastics are mainly categorized into twogroups: one group is nonbiodegradable and another is bio-degradable. Question may arise why nonbiodegradable plas-tics are not degradable and why biodegradables aredegradables. Basically, plastics are usually long chains ofcarbon and hydrogen atoms. The enzymes found in livingthings can perform many chemical reactions, but they gen-erally exploit some sort of imbalance of electric chargewithin a molecule to do their job. A long chain of carbonsand hydrogens contains very balanced charges along itslength, making the molecule stable and difficult to changewith enzymes. Most biodegradable substances contain somemixture of carbon and atoms like oxygen, nitrogen, sulfur,and phosphorus, which create charge imbalances that en-zymes can exploit. There are some bacteria that can breakdown plastics. These bacteria usually contain enzymescalled oxygenases, which can add oxygen to a long carbonchain. This destabilizes the local electric charge, and theplastic can then be broken down. The oxygenase enzymestoo often are not found, however, because they can easilydestroy the molecules in the bacteria that carry them.

In India, plastic consumption grew exponentially in the1990s. During the last decade, the total consumption ofplastics grew twice as fast as 12 % per annum (Shimao2001) as the gross domestic product growth rate based onpurchasing power parities is 6 % per annum. The currentgrowth rate in Indian polymer consumption which is 16 %per annum is clearly higher than that in China which has10 % per annum and many other key Asian countries. The

Responsible editor: Philippe Garrigues

S. K. Ghosh (*) : S. Pal : S. RayMycopathology Laboratory, Department of Botany, RamakrishnaMission Vivekananda Centenary College, P.O. Rahara,Kolkata 700118 West Bengal, Indiae-mail: [email protected]

Environ Sci Pollut Res (2013) 20:4339–4355DOI 10.1007/s11356-013-1706-x

average Indian consumption of virgin plastics per capitareached 3.2 kg in 2000/2001 (5 kg if recycled material isincluded) from a mere 0.8 kg in 1990/1991. However, this isonly one-fourth of the consumption as compared to that ofChina (12 kg/capita, 1998) and one-sixth of the worldaverage (18 kg/capita). This consumption led to more than5,400 tonnes of plastic waste being generated per day in2000/2001 (totaling about two million tonnes per annum)(en.wikipedia.org).

The Middle East countries alone produce about 35 milliontons of polyethylene and polypropylene, and the capacity ofproduction of this man-made product is about to cross the 7million tons per annum mark (data obtained fromCIEPT/Government of India). Asia alone is the world’s largestplastic consumers, accounting about 35 % of the global con-sumption which is followed by North America with about26 % andWestern Europe about 23 %. India (5 %) along withJapan (6 %) comprises 11 % of the world plastic consumptionand about 40 % of the total consumption in Asia (EuropeanCommission DG ENV 2011). According to the U.S. Environ-mental Protection Agency (www.epa.gov), about 31 milliontons of plastic waste was produced in 2010, which representsabout 12.4 % of the total Mean Solid Waste, and out of that,only 8 % waste was recycled. In contrast to that, to meet theincreasing demand of plastic, the production of these poly-mers is rapidly increasing, and consequently, this man-madechemical is causing high environmental pollution and alsonotable human health hazards. Thus, almost every country inthis world is suffering from these problems.

India has witnessed a substantial growth in the consump-tion of plastics and an increased production of plastic waste.Polyolefins account for the major share of 60 % in the totalplastic consumption in India. Packaging is the major plastic-consuming sector, with 42 % of the total consumption,followed by consumer products and the construction indus-try. The relationship observed between plastic consumptionand the gross domestic product for several countries wasused to estimate future plastic consumption (master curve).Elasticities of the individual material growth with respect toGDP were established for the past and for the next threedecades estimated for India, thereby assuming a develop-ment comparable with that of Western Europe. On this basis,the total plastic consumption is projected to grow by a factorof 6 between 2000 and 2030. The consumption of variousend products is combined with their corresponding lifetimesto calculate the total waste quantities. The weighted averagelifetime of plastic products was calculated as 8 years. Of thetotal plastic waste generated, 47 % is currently recycled inIndia; this is much higher than the share of recycling in mostof the other countries. The recycling sector alone employs asmany people as the plastic-processing sector, which em-ploys about eight times more people than the plastic-manufacturing sector. Due to the increasing share of long-

life products in the economy, and consequently in the vol-ume of waste generated, the share of recycling will decreaseto 35 % over the next three decades. The total waste avail-able for disposal (excluding recycling) will increase at least10-fold up to the year 2030 from its current level of 1.3million tonnes (Wikipedia.org).

The increasing quantities of plastic waste and their effec-tive and safe disposal have become a matter of public concern.The increasingly visible consequences of indiscriminatelittering of plastic wastes (in particular, plastic packagingwastes and discarded bags) have stimulated public outcryand shaped policy. Littering also results in secondary prob-lems such as drains becoming clogged and animal healthproblems (both domesticated and wild). As a consequence,many big cities, e.g., Kolkata, Mumbai, Bangalore, and Delhi,are facing big threat of solid waste accumulation. Along withthat, it is becoming a serious threat as a soil and air pollutantfor if heated/burned, they produce some high amount of toxicnoxious gas (also some greenhouse gases) which are danger-ous to human health, thus leading to severe air pollution.When plastics are dumped in a field or in dumping areas,there is evidential proof that they are causing a great change inthe pH of the soil followed by disturbance in the leaching ofthe rain water and moisture, making the land bare andunfertile. The biological degradation time of is very high,and it takes thousands of years to degrade these long chainpolymers into simple hydrocarbons. Latest reports confirmedthat some plastic products are mimicking human hormones(e.g., thyroxin and sex hormones), causing human healthhazards (Soto et al. 1991; Hao et al. 2011). It is also creatinga major problem in marine ecosystem.

For the last 30 years, scientists are trying to develop somealternative ways other than the natural destruction to de-grade these high molecular synthetic polymers, but yet now,very few evidences are available where scientists are able todevelop some alternative ways to enhance the mode ofdegradation and make it faster. Recent research suggeststhat there have been a notable number of microorganisms(especially some bacteria and fungi) which have the capac-ity to degrade these synthetic polymers in much faster wayin comparison to the natural method by using some exo-enzymes under stress conditions. The enzyme lipases fromRhizopus arrhizus, Rhizopus delemar, Achromobacter sp.,and Candida cylindracea and esterase from hog livershowed activities on polyethylene adipate (PEA) andpoly(ε-caprolactone) (PCL) (Tokiwa and Suzuki 1977a, b).

Enzymatic degradation of PCL by Aspergillus flavus andPenicillium funiculosum showed that faster degradation wasobserved in the amorphous region (Cook et al. 1981). More-over, a novel poly(3-hydroxybutyrate) (PHB) depolymerasefrom a thermophilic Streptomyces sp. was also capable ofdegrading poly(β-propiolactone) (PPL) (Calabia andTokiwa 2006). In comparative studies, the biodegradability

4340 Environ Sci Pollut Res (2013) 20:4339–4355

http://en.wikipedia.org

http://www.epa.gov

of three polyalkylene succinate [polyethylene succinate(PES), polybutylene succinate (PBS), and polypropylenesuccinate (PPS)] with the same molecular weight was in-vestigated using Rhizopus delemar lipase. PPS with lowmelting temperature (Tm, 43–52 °C) had the highest biodeg-radation rate followed by PES, owing to the lower crystallinityof PPS compared to PES and PBS (Bikiaris et al. 2006).

The objectives of this review literature are to study thenature and classification of plastic and its effect on theecosystem including the human health and to categorizethe potential microbes and their mechanisms of biodegrada-tion of different plastic products. This work will provide aready reference for further scientific exploration of thesepotential microbes.

Categories and classification of plastics

Chemically, there are various types of plastics which areclassified according to their chemical structure and theirproperties.

Classification of plastic according to their thermal properties

Based on their thermal properties, plastics are classified intotwo groups: thermoplastics and thermosetting polymers.

Thermoplastics

Thermoplastics are those types of plastics which cannotundergo chemical changes in their composition when heat-ed, and thus, they can undergo molding for several times.Polyethylene (PE), polypropylene (PP), polystyrene (PS),polyvinyl chloride (PVC), and polytetrafluoroethylene aresuch examples. They are also known as common plasticswhich range from 20,000 to 500,000 amu in molecularweight. They have different numbers of repeating unit derivedfrom a simple monomer unit.

Thermosetting polymers

Thermosetting polymers are other types of plastics, whichwhen melt and been casted into a particular shape, remainsolid and after that they cannot be melt and modified again.In the thermosetting polymers, the chemical change is irre-versible and hence they are not recyclable too. Examplesinclude phenol–formaldehyde, polyurethanes, etc.

Classification of plastic according to their designingproperties

The other way of classification is based upon their relevanceof manufacturing process and designing. It is classified in

different parameters like electrical conductivity, durability,tensile strength, degradability, and thermal stability.

Classification of plastic according to their degradabilityproperties

The chemical properties of the plastics are also importantcriteria for differentiating them into degradable andnondegradable polymers. Usually, nonbiodegradable plas-tics are known as synthetic plastics and they are derivedfrom petrochemicals. They have an unusual repeat of smallmonomer units and thus have very high molecular weight.In comparison, degradable plastics are made of starch andthus are not of very high molecular weight. They usuallybreak down in interaction with UV, water, enzymes, andgradual changes in pH. Biopol is an expensive biodegrad-able plastic (which comprises polyhydroxybutyrate) avail-able in market. Ecoflex, which is fully biodegradable, isanother biodegradable plastic polymer which is producedby the German company BASF.

Types of plastics we use every day

Plastics are relatively very low cost, durable, and very easyto manufacture. In Table 1, we have tried to briefly describesome of the regular usable plastics and their application.

Effect of plastic on the environment and human health

With the rapid modernization and development of humansociety, there has been a very sharp increase in the demandof plastic-based products. Approximately 30–40 % of plas-tics produced worldwide are used for different packagingapplications, and it is steeply increasing at the rate of about12 % per annum (Sabir 2004). The reason behind such anincrease in the demand of plastic products lies in two im-portant things: the durability and cost. For example, if wecompare a wooden chair with a plastic chair, the latter israther very cheap and highly durable. In this way, all plastic-based products have very rapidly replaced almost everynatural product in the market, but due to their massiveproduction worldwide, there has been a sharp increase inthe generation of nondegradable solid plastic wastes. Thus,slowly within the last 50 years, this has become one of thegreat concerns for the modern human society. Many studiesalso showed the striking effect of plastic waste on theaquatic and marine ecosystem, and thus, it has become oneof the major problems for the modern environmentalist. Toget rid of such a menace, people usually put them in landfillsor burn it, but both these practices cause very serious threatsto the environment and the ecosystem. Burning plasticsusually produce some noxious gases like furans and dioxins

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which are some dangerous greenhouse gases and play animportant role in ozone layer depletion. In fact, dioxinscause serious problems in the human endocrine hormoneactivity, thus becoming a major concern for the humanhealth too (NoPE 2002; Pilz et al. 2010). Dioxins also causevery serious soil pollution, causing a great concern for thescientific community worldwide.

Some plastics which are produced for packing foods aremade with the assistance of a substance called bisphenol A(BPA), which is a synthetic chemical compound[(CH3)2C(C6H4OH)2]. This plastic has an excellent propertyof heat stability, making it tremendously useful in manyapplications. Studies show that BPA can interfere with theregulation of both development and reproduction, through its

Table 1 List of various types of plastic we use everyday

Types Use/application

Polyester (PES) Fibers, textiles

Polyethylene terephthalate (PET) Carbonated drinks bottles, peanut butter jars, plastic film, microwavable packaging

Polyethylene (PE) Wide range of inexpensive uses including supermarket bags, plastic bottles

High-density polyethylene (HDPE) Detergent bottles and milk jugs

Polyvinyl chloride (PVC) Plumbing pipes and guttering, shower curtains, window frames, flooring

Polyvinylidene chloride (PVDC)(Saran)

Food packaging

Low-density polyethylene (LDPE) Outdoor furniture, siding, floor tiles, shower curtains, clamshell packaging

Polypropylene (PP) Bottle caps, drinking straws, yogurt containers, appliances, car fenders (bumpers), plastic pressure pipesystems

Polystyrene (PS) Packaging foam, food containers, plastic tableware, disposable cups, plates, cutlery, CD and cassetteboxes

High impact polystyrene (HIPS) Refrigerator liners, food packaging, vending cups

Polyamides (PA) (nylons) Fibers, toothbrush bristles, fishing line, under-the-hood car engine moldings

Acrylonitrile butadiene styrene (ABS) Electronic equipment cases (e.g., computer monitors, printers, keyboards), drainage pipe

Polycarbonate (PC) Compact discs, eyeglasses, riot shields, security windows, traffic lights, lenses

Polycarbonate/acrylonitrile butadienestyrene (PC/ABS)

A blend of PC and ABS that creates a stronger plastic. Used in car interior and exterior parts and mobilephone bodies

Polyurethanes (PU) Cushioning foams, thermal insulation foams, surface coatings, printing rollers (currently the sixth orseventh most commonly used plastic material, for instance, the most commonly used plastic found incars)

Melamine formaldehyde (MF) One of the aminoplasts and used as a multicolorable alternative to phenolics, for instance, in moldings(e.g., break-resistance alternatives to ceramic cups, plates, and bowls for children) and the decoratedtop surface layer of the paper laminates (e.g., Formica)

Plastarch material Biodegradable and heat-resistant thermoplastic composed of modified cornstarch

Phenolics (PF) or (phenolformaldehydes)

High-modulus, relatively heat-resistant, and excellent fire-resistant polymer; used for insulating parts inelectrical fixtures, paper laminated products (e.g., Formica), and thermally insulation foams. It is athermosetting plastic, with the familiar trade name Bakelite that can be molded by heat and pressurewhen mixed with filler-like wood flour or can be cast in its unfilled liquid form or cast as foam (e.g.,Oasis). Problems include the probability of moldings naturally being dark colors (red, green, brown),and as thermoset, it is difficult to recycle

Polyetheretherketone (PEEK) Strong, chemical, and heat-resistant thermoplastic; biocompatibility allows for use in implantapplications and aerospace moldings. One of the most expensive commercial polymers

Polyetherimide (PEI) (Ultem) A high-temperature, chemically stable polymer that does not crystallize

Polylactic acid (PLA) A biodegradable thermoplastic that can be converted into a variety of aliphatic polyesters derived fromlactic acid which in turn can be made by fermentation of various agricultural products such ascornstarch, once made from dairy products

Polymethyl methacrylate (PMMA) Contact lenses, glazing (best known in this form by its various trade names around the world, e.g.,Perspex, Oroglas, PLEXIGLAS), aglets, fluorescent light diffusers, rear light covers for vehicles. Itforms the basis of artistic and commercial acrylic when suspended in water with the use of otheragents

Polytetrafluoroethylene (PTFE) Heat-resistant, low-friction coatings, and used in things like nonstick surfaces for frying pans, plumber’stape, and water slides. It is more commonly known as Teflon

Urea–formaldehyde (UF) One of the aminoplasts and used as a multicolorable alternative to phenolics. Used as a wood adhesive(for plywood, chipboard, hardboard) and electrical switch housings


interaction with estrogen. In 1993, some scientific groupsreported on the interference estrogenic activity of BPA thatwas released from polycarbonate flasks during the autoclavingof media. There was also evidence that BPA could act onMCF-7 human breast cancer cells as an estrogen, stimulatingcellular proliferation and inducing progesterone receptors.BPA could bind to estrogen receptors, and the estrogeniceffects induced by BPA were blocked by the estrogen antag-onist tamoxifen, thus supporting the notion that the estrogenicactivity of BPA was mediated via the estrogen receptor(Krishnan et al. 1993). Soto and colleagues (1991) at TuftsUniversity showed very similar results with another compo-nent of plastics, p-nonylphenol. This alkylphenolic substance,which is used as a plastic additive and surfactant, could bereleased from polyvinyl chloride and polystyrene plasticseven without autoclaving. Soto et al. (1995) also developedan “E-SCREEN” to assay the estrogenic activity of unknownsubstances or mixtures based upon their ability to stimulateMCF-7 cell proliferation. A link between chemicals calledphthalates and thyroid hormone levels was confirmed by theUniversity of Michigan in the first large-scale and nationallyrepresentative study of phthalates and BPA in relation tothyroid function in humans (Science Daily: July 11, 2011). Ithas also been noted that long exposure to BPA shows asignificant effect on the sex hormones (progesterone) in fe-males (Hao et al. 2011).

BPA is now declared a controversial plastic product becauseit exerts weak, but detectable, hormone-like properties, raisinggreat concerns about its presence in consumer products andfoods contained in such products. In 2008, several govern-ments questioned about its safety, prompting some retailersto withdraw polycarbonate products. In 2010, reports from theU.S. Food and Drug Administration raised further concernsregarding exposure of fetuses, infants, and young children(U.S. FDA 2010). In September 2010, Canada became thefirst country to declare BPA as a toxic substance (Mittelstaedt2010; Canada Gazette Part II 2010). In the European Unionand Canada, BPA use is banned in baby bottles. There has notbeen any suchmajor report on other types of plastics, but thereis a great potentiality that these polymersmay have some othertypes of effects on the human health which is yet to bediscovered.

There is an evidential proof that every year, thousands ofaquatic and marine life suffer due to dumping of solidplastic wastes in the sea and river. According to the WorldWide Fund for Nature (formally known as the World Wild-life Fund) organization, it is estimated that about 100,000marine mammals are killed due to plastic debris around theworld. It occurs because the floating plastic debris is oftenmistaken as food, causing blockage in the digestive systemof the animals. Recently, in August 2000, an autopsy of adead Bryde’s whale near Cairns, Australia revealed that thestomach of the whale was filled with plastic rubbishes which

are thrown in the sea (WWF-Australia 2010). Sometimes, theendocrine- and sex hormone-disrupter chemicals follow thefood chain (as sea food and sea fish is consumed in very highamount), thus showing an indirect effect on human health.

Potential degradation procedures of plastics

Degradation in broad terms is defined as any process whichresults into breaking of large complex molecules into smallermolecules. Any type of physical or chemical changes can resultinto degradation of these long polymers into their monomers.There are several ways implemented in degrading the plastic.

Thermal degradation

It is a process where heat is used to degrade the plasticpolymers into their simpler form. It is also known as athermo-oxidative reaction as it requires oxygen. In thisprocess, the long polymers are broken into smaller mono-meric units (radicals) which further react with oxygen toproduce peroxide radicals.

Photoreactive degradation

It is another type of process where high-intensity photonparticles are used to degrade the large polymers into smallerones. In this process, high-energy radiations like UV areused to react with the photoreactive groups to break theselong chain polymers.

Biochemical/microbial degradation (biodegradation)

It is a new concept in which some live microorganisms areused, which produce some biologically active enzymes todegrade the long polymers, and further, these microorgan-isms use these polymers as carbon and energy sources.

What modern scientists are thinking about degradabilityof plastic polymers?

There is an evidential proof that the current techniques ofplastic polymer degradation are not so much effective;hence, scientists are looking for some alternative wayswhere they can use microbes to degrade these long chainsynthetic polymers into their respective monomers. Thisprocess may also be termed as reverse flow to producesimple hydrocarbons because these polymers are usuallyproduced from different petrochemical products. Thus, wecan be able to go back to those simple monomers which canbe an alternative source of energy and may even be the nextgeneration fuel. Alternatively, the demand for petrochemical


products will surely be reduced as we can again recyclethese monomers to reproduce these polymers. Hence, thereis a huge demand in exploring these microbes which cangrow in different conditions and, under specific stress con-dition, may be directed to grow and use those carbon poly-mers as their energy source, thus degrading these syntheticpolymers. As our world is a great natural source of thesevaried types of microbes, scientists are trying to explore andutilize them for such activities. Some scientists are nowengaged in research in these directions, and an attempt hasbeen made to focus their works in this review article.

Some of the well-known microbes which have beenutilized and have the capacity to degrade plastic polymersinto their respective simple monomeric units are shown inTable 2.

The information in Table 2 showed that many fungi arecapable of degrading various categories of plastic. PHB andpolyesters are degraded by many fungi genera such asAcremonium, Cladosporium, Debaryomyces, Emericellopsis,Eupenicillium, Fusarium, Mucor, Paecilomyces, Penicillium,Pullularia, Rhodosporidium, and Verticillium. Similarly, PCLis degraded by Aspergillus, Aureobasidium, Chaetomium,Cryptococcus, Fusarium, Rhizopus, Penicillium, andThermoascus . PEA is degraded by Aspergi l lus ,Aureobasidium, Penicillium, Pullularia. On the other hand,polylactic acid (PLA) is subjected to degradation by only twogenera of fungi (Penicillium roqueforti and Tritirachiumalbum). Mogil'nitskii et al. (1987) reported that Aspergillusniger van Tieghem F-1119 had the ability to degrade PVCplastic. Table 2 also indicated that there is a maximumnumber of plastic-degrading fungal genera belonging toDeuteromycota, whereas there is only a minimum number ofgenera belonging to Zygomycota (Mucor sp. and Rhizopus sp.).

Different kinds of plastics and microbeswhich can degrade these plastics

Aliphatic polyesters from fossil resources

Polyethylene adipate (PEA)

PEA ([−OCH2CH2OOC(CH2)4CO–]n) is a prepolymer ofpolyurethane. PEA-degrading microorganisms werescreened and isolated using PEA (number average molecu-lar weight (Mn) 3,000) as the sole source of carbon. Amongthe isolated PEA-degrading microorganisms, Penicillium sp.strain 14–3 exhibited the strongest activity. PEA was de-graded in 120 h at high cell concentrations. This strain candegrade not only PEA but also aliphatic polyesters such asPES, PBS, and polybutylene adipate (PBA) (Tokiwa andSuzuki 1974). The enzyme responsible for the degradationof PEA has been purified and is considered to be a kind of

lipase with broad substrate specificity. The purified enzymehas a molecular weight of 25 kDa and could degrade variouskinds of aliphatic polyesters, such as PPL and PCL, butnot poly(DL-3-methylpropiolactone) or poly(DL-3-hydroxybutyrate) (Tokiwa and Suzuki 1977a, b). This enzymecan also hydrolyze plant oils, triglycerides, and methyl estersof fatty acids. Given that the purified enzyme of Penicilliumsp. strain 14–3 has properties that are similar to lipase, somecommercially available lipases and esterases were used toconfirm if it was capable of degrading PEA. Results showedthat lipases from Rhizopus arrhizus, Rhizopus delemar,Achromobacter sp., and Candida cylindracea and esterasefrom hog liver showed activities on PEA and PCL (Tokiwaand Suzuki 1977a, b).

Poly(ε-caprolactone) (PCL)

PCL ([−OCH2CH2CH2CH2CH2CO–]n) is a biodegradablesynthetic partially crystalline polyester with low meltingpoint (60 °C) and with a glass transition temperature (Tg)of −60 °C. It is prepared by ring-opening polymerization ofε-caprolactone. PCL has been shown to be degraded by theaction of aerobic and anaerobic microorganisms that arewidely distributed in various ecosystems. Furthermore, thedegradation of high molecular weight PCL was investigatedusing Penicillium sp. strain 26–1 (ATCC 36507) isolatedfrom soil. PCL was almost completely degraded in 12 days.This strain can also assimilate unsaturated aliphatic andalicyclic polyesters, but not aromatic polyesters (Tokiwa etal. 1976). A thermotolerant PCL-degrading microorganismwhich was identified as Aspergillus sp. strain ST-01 wasisolated from soil. PCL was completely degraded by thisstrain after 6 days of incubation at 50 °C (Sanchez et al.2000). PCL and PHB were degraded under anaerobic con-dition by new species of microorganisms belonging to thegenus Clostridium (Abou-Zeid et al. 2001). PCL can bedegraded by lipases and esterases (Tokiwa and Suzuki1977a, b). The degradation rate of PCL is dependent on itsmolecular weight and degree of crystallinity. Enzymaticdegradation of PCL by Aspergillus flavus and Penicilliumfuniculosum showed that faster degradation was observed inthe amorphous region (Cook et al. 1981).

Poly(β-propiolactone) (PPL)

PPL ([−OCH2CH2CO–]n) is a chemosynthetic biodegrad-able aliphatic polyester with good mechanical properties.Many PPL-degrading microorganisms are widely distribut-ed in various environments, and majority of these microor-ganisms belong to Bacillus sp. (Nishida et al. 1998). PPL-degrading microorganisms were isolated from different eco-systems, and out of 13 isolates, nine of these strains wereidentified as Acidovorax sp., Variovorax paradoxus, and


Table 2 List of fungal strains and the types of plastic which they degrade

Fungal strain Polyesters hydrolyzed Group Reference

Acremonium sp. PHB, poly[3HB-co-(10 mol%) 3HV] Deuteromycota Mergaert et al. (1993)

Aspergillus fischeri PCL Deuteromycota Benedict et al. (1983a, b)

A. flavus ATCC9643

PCL Deuteromycota Benedict et al. (1983a, b)

A. flavus QM380 PEA, PPA, PBA Deuteromycota Darby and Kaplan (1968)

Aspergillus nigervan Tieghem F-1119

PVC Deuteromycota Mogil'nitskii et al. (1987)

A. fumigatus M2A PHB, poly[3HB-co-(7–77 mol%) 3HV], PHV, poly[3HB-co-(13–61 mol%) 4HB], PES, PEA, PBA, PES/A, PES, PBS/A

Deuteromycota Scherer et al. (1999)

A. fumigatus PHB, poly[3HB-co-(10 mol%) 3HV] Deuteromycota Mergaert et al. (1993, 1994)

A. fumigatus LAR 9 PHB, Sky-Green Deuteromycota Kim et al. (2000a, b, c)

A. fumigatus ST-01 PHB, PCL, PBS, PBS/A Deuteromycota Sanchez et al. (2000)

A. fumigatus Pdf1 PHB, poly(3HB-co-3HV), PHV Deuteromycota Iyer et al. (2000)

A. niger ATCC 9642 Sky-Green Deuteromycota Kim et al. (2000a, b, c)

A. niger QM386 PEA, PPA, PBA Deuteromycota Darby and Kaplan (1968)

A. penicilloides PHB Deuteromycota Mergaert et al. (1992)

A. ustus T-221 PHB Deuteromycota Gonda et al. (2000)

A. ustus M-224 PHB Deuteromycota Gonda et al. (2000)

A. ustus LAR 25 Sky-Green Deuteromycota Kim et al. (2000a, b, c)

A. versicolorQM432

PEA, PPA, PBA Deuteromycota Darby and Kaplan (1968)

Aureobasidiumpullulans

PCL Deuteromycota Fields et al. (1974)

A. pullulansQM279c


Aureobasidiumpullulans

PCL, PU Deuteromycota Fields et al. (1974); Howard(2002)

A. pullulansQM279c


Candidaguilliermondii

PHB Deuteromycota Gonda et al. (2000)

Cephalosporium sp. PHB Deuteromycota Matavulj and Molitoris (1992)

Chaetomiumglobosum ATCC6205

PCL Ascomycota Benedict et al. (1983a, b)

C. globosumQM459

PEA, PPA, PBA Ascomycota Darby and Kaplan (1968)

Cladosporium sp. PHB Deuteromycota Matavulj and Molitoris (1992)

Curvulariasenegalensis

PE Deuteromycota Howard (2002)

Cryptococcuslaurentii

PCL Basidiomycota Benedict et al. (1983a, b)

Curvulariaprotuberate LAR12

Sky-Green Deuteromycota Kim et al. (2000a, b, c)

Debaryomyceshansenii

PHB Ascomycota Gonda et al. (2000)

Emericellopsisminima W2

PHB, poly[3HB-co-(30 mol%) 3HV] Ascomycota Kim et al. (2002a, b, c)

Eupenicillium sp.IMI 300465

PHB Ascomycota McLellan and Halling (1988)

Fusarium sp. PCL Deuteromycota Benedict et al. (1983a, b)

F. moniliforme PCL, cutin Deuteromycota Murphy et al. (1996)

F. oxysporum F1-3 Poly[3HB-co-(12 mol%) 3HV] Deuteromycota Sang et al. (2002)

F. solani LAR 11 PHB Deuteromycota Kim et al. (2000a, b, c)


Table 2 (continued)


F. solani strain 77-2-3

PCL, cutin Deuteromycota Murphy et al. (1996)

F. solani ATCC38136

PCL, cutin Deuteromycota Murphy et al. (1996)

Mucor sp. PHB Zygomycota Matavulj and Molitoris (1992)

Paecilomycesfarinosus F4-7

Poly[3HB-co-(12 mol%) 3HV] Deuteromycota Sang et al. (2002)

P. farinosus LAR 10 PHB, Sky-Green Deuteromycota Kim et al. (2000a, b, c)

P. lilacinus D218 PHB, PCL Deuteromycota Oda et al. (1995)

P. lilacinus F4-5 Poly[3HB-co-(12 mol%) 3HV] Deuteromycota Sang et al. (2002)

P. marquandii PHB Deuteromycota Mergaert et al. (1992)

P. simplicissimumYK

PE Ascomycota Yamada-Onodera et al. (2001)

Penicilliumadametzii

PHB Deuteromycota Mergaert et al. (1992)

P. argillaceum IFO31071

PCL Deuteromycota Sanchez et al. (2000)

P. chermisinum PHB Deuteromycota Mergaert et al., 1995

P. chrysosporium Poly[3HB-co-(7 mol%) 3HV] Deuteromycota Renstad et al. (1999)

P. daleae PHB Deuteromycota Mergaert et al. (1992)

P. dupontii IFO31798

PCL Deuteromycota Sanchez et al. (2000)

P. funiculosumATCC 9644

PHB, PCL Deuteromycota Brucato and Wong (1991); Oda etal. (1995)

P. funiculosum IFO6345

PHB, PHV, poly[3HB-co-(7, 14 %) 4HB], poly[3HB-co-(7, 27,45, 71 %) 3HV]

Deuteromycota Miyazaki et al. (2000)

P. funiculosumQM301


P. funiculosumATCC 11797

PCL Deuteromycota Benedict et al. (1983a, b)

P. funiculosum LAR18

PHB Deuteromycota Kim et al. (2000a, b, c)

P. janthinellum PHB Deuteromycota Mergaert et al. (1995)

P. minioluteum LAR14


P. orchrochloron PHB Deuteromycota Mergaert et al. (1992)

P. pinophiliumATCC 9644

PHB Deuteromycota Han et al. (1998)

P. pinophilium LAR15


P. restricum PHB Deuteromycota Mergaert et al. (1992)

P. roqueforti PLA Deuteromycota Torres et al. (1996); Pranamudaet al. (1997)

P. simplicissimumIMI 300465

PHB Deuteromycota McLellan and Halling (1988)

P. simplicissimumLAR 13

PHB, Sky-Green Deuteromycota Kim et al. (2000a, b, c)

P. simplicissimum PHB Deuteromycota Mergaert et al. (1995)

P. simplicissimum Poly[3HB-co-(7 mol%) 3HV] Deuteromycota Renstad et al. (1999)

Penicillium sp. strain14-3

PEA, PCL, polyalkylene dicarboxylic acids Deuteromycota Tokiwa and Suzuki (1977a, b)

Penicillium sp. strain26-1

PHB, PCL, polyalkylene dicarboxylic acids Deuteromycota Tokiwa et al. (1976)

P. verruculosumLAR 17

Mater-Bi Deuteromycota Kim et al. (2000a, b, c)

Phanerochaetechrysosporium

Poly[3HB-co-(7 mol%) 3HV] Basidiomycota Renstad et al. (1999); Lee et al.(1991)


Sphingomonas paucimobilis. PHB was also degraded bythese isolates (Kobayashi et al. 1999). Rhizopus delemarcan also degrade PPL (Tokiwa and Suzuki 1977a, b). More-over, a novel PHB depolymerase from a thermophilic Strep-tomyces sp. was also capable of degrading PPL (Calabia andTokiwa 2006).

Polybutylene succinate (PBS) and polyethylene succinate(PES)

PBS ( [−O(CH2 ) 4OOC(CH2 ) 2CO– ] n ) a n d PES([−O(CH2)2OOC(CH2)2CO–]n) are aliphatic synthetic polyes-ters with high melting points of 112–114 and 103–106 °C,respectively. They are synthesized from dicarboxylic acids(e.g., succinic and adipic acid) and glycols (e.g., ethyleneglycol and 1,4-butanediol). Their mechanical properties arecomparable to polypropylene and low-density polyethylene(LDPE). PBS-degrading microorganisms are widely distribut-ed in the environment, but their ratio to the total microorgan-isms is lower than PCL degraders. The degradation of PBS byAmycolatopsis sp. HT-6 was investigated, and results showedthat this strain can degrade not only PBS but also PHB andPCL (Pranamuda et al. 1995). Several thermophilic actinomy-cetes from the Japan Collection of Microorganisms werescreened for their capability of degrading PBS. Microbisporarosea, Excellospora japonica, and Excellospora viridiluteaformed a clear zone on agar plates containing emulsifiedPBS. Microbispora rosea was able to degrade 50 % (w/v) ofPBS film after 8 days of cultivation in a liquid medium (Jareratand Tokiwa 2001a, b). PES is a chemosynthetically aliphaticpolyester which is prepared either by ring-opening polymeri-zation of succinic anhydride with ethylene oxide or by poly-condensation of succinic acid and ethylene glycol (Maeda et

al. 1993). In contrast with microbial polyesters which aresusceptible to degradation in various environments, the de-gradability of PES was found to be strongly dependent onenvironmental factors (Kasuya et al. 1997). Moreover, PES-degrading microorganisms have limited distribution in theenvironment in comparison with PHB- and PCL-degradingmicroorganisms. A thermophilic Bacillus sp. TT96, a PESdegrader, was isolated from soil. This bacterium can also formclear zones on PCL and PBS plates, but not on PHB(Tansengco and Tokiwa 1998). A number of mesophilicPES-degrading microorganisms were isolated from aquaticand soil environments. Phylogenetic analysis revealed thatthe isolates belong to the genera Bacillus and Paenibacillus.Among the isolates, strain KT102 which is related to Bacilluspumilus was chosen since it could degrade PES film at thefastest rate among the isolates. This strain can degrade PES,PCL, and olive oil, but not PBS, PHB, and PLA (Tezuka et al.2004). In addition, several fungi were isolated from variousecosystems and the isolates formed clear zones around thecolony on agar plates containing PES. A strain NKCM1003belonging to Aspergillus clavatus was selected, and it candegrade PES film at a rate of 21 μg/cm2/h (Ishii et al. 2007).Comparative studies on the biodegradability of threepolyalkylene succinate (PES, PBS, and PPS) with the samemolecular weight were investigated using Rhizopus delemarlipase. PPS with low Tm (43–52 °C) had the highest biodeg-radation rate followed by PES, owing to the lower crystallinityof PPS compared to PES and PBS (Bikiaris et al. 2006).

Aliphatic–aromatic copolyesters (AACs)

It has been reported that aliphatic–aromatic copolyester(AAC), which consisted of PCL and aromatic polyester such

Table 2 (continued)


Physarumpolycephalum

PMA Myxomycota Korherr et al. (1995)

Polyporus circinatus PHB Basidiomycota Matavulj and Molitoris (1992)

Pullularia pullulansQM279c


Rhizopus delemar PPA, PET copolymers with dicarboxylic acids Zygomycota Walter et al. (1995); Nagata et al.(1997); Fukuzaki et al. (1989)

R. arrhizus PCL, polyalkylene dicarboxylic acids Zygomycota Tokiwa et al. (1986)

Rhodosporidiumsphaerocarpum

PHB Basidiomycota Gonda et al. (2000)

Thermoascusaurantiacus IFO31910

PHB, PCL, PBS Ascomycota Sanchez et al. (2000)

Tritirachium albumATCC 22563

PLA Deuteromycota Jarerat and Tokiwa (2001a, b)

Verticilliumleptobactrum

PHB Deuteromycota Mergaert et al. (1994)


as polyethylene terephthalate (PET), polybutylene tere-phthalate, and polyethylene isophthalate (PEIP), was hydro-lyzed by Rhizopus delemar lipase (Tokiwa and Suzuki1981). The susceptibility of these AACs to hydrolysis byRhizopus delemar lipase decreased rapidly with an increasein aromatic polyester content. The susceptibility to lipase ofAAC [which consisted of PCL and PEIP, and the latterbeing used as a low Tm (103 °C) aromatic polyester] wasgreater than those of other AAC. It was assumed that therigidity of the aromatic ring in the AAC chains influencedtheir biodegradability with this lipase. Another syntheticAAC-containing adipic acid and terephthalic acid can alsobe attacked by microorganisms (Witt et al. 1995). Kleeberget al. evaluated the biodegradation of AAC synthesized from1,4-butanediol, adipic acid, and terephthalic acid.Thermobifida fusca (known previously as Thermomonosporafusca) isolated from compost, showed 20-fold higher degra-dation rates than that usually observed in a common composttest (Kleeberg et al. 1998). A thermophilic hydrolase fromThermobifida fusca was found to be inducible not only byAAC but also by esters. This enzymewas classified as a serinehydrolase with high similarity to triacylglycerol lipase fromStreptomyces albus G and triacylglycerol acylhydrolase fromStreptomyces sp. M11 (Kleeberg et al. 2005).

Aliphatic polyesters from renewable resources

Poly(3-hydroxybutyrate) (PHB)

PHB ([−O(CH3)CHCH2CO–]n) is a natural polymer pro-duced by many bacteria as a means to store carbon andenergy. This polymer has attracted research and commercialinterest worldwide because it can be synthesized from re-newable low-cost feedstocks and the polymerizations areoperated under mild process conditions with minimal envi-ronmental impact. Furthermore, it can be biodegraded inboth aerobic and anaerobic environments, without formingany toxic products.

Chowdhury reported for the first time the PHB-degradingmicroorganisms from Bacillus, Pseudomonas, and Streptomy-ces species (Chowdhury 1963). From then on, several aerobicand anaerobic PHB-degrading microorganisms have beenisolated from soil (Pseudomonas lemoigne, Comamonas sp.,Acidovorax faecalis, Aspergillus fumigatus, and Variovoraxparadoxus), activated and anaerobic sludge (Alcaligenesfaecalis, Pseudomonas, Ilyobacter delafieldi), seawater, andlakewater (Comamonas testosterone, Pseudomonas stutzeri)(Lee 1996). The percentage of PHB-degrading microorgan-isms in the environment was estimated to be 0.5–9.6 % of thetotal colonies (Suyama et al. 1998a, b). Majority of the PHB-degrading microorganisms were isolated at ambient ormesophilic temperatures, and very few of them were capableof degrading PHB at higher temperature. Tokiwa et al.

emphasized that composting at high temperature is one ofthe most promising technologies for recycling biodegradableplastics and thermophilic microorganisms that could degradepolymers play an important role in the composting process(Tokiwa et al. 1992). Thus, microorganisms that are capable ofdegrading various kinds of polyesters at high temperatures areof interest. A thermophilic Streptomyces sp. isolated from soilcan degrade not only PHB but also PES, PBS, andpoly[oligo(tetramethylene succinate)-co-(tetramethylene car-bonate)]. This actinomycete has higher PHB-degrading activitythan thermotolerant and thermophilic Streptomyces strains fromculture collections (Calabia and Tokiwa 2004). Athermotolerant Aspergillus sp. was able to degrade 90 % ofPHB film after 5 days of cultivation at 50 °C (Sanchez et al.2000). Furthermore, several thermophilic polyester-degradingactinomycetes were isolated from different ecosystems.Out of 341 strains, 31 isolates were PHB, PCL, andPES degraders and these isolates were identified asmembers of the genera Actinomadura, Microbispora, Strep-tomyces, Thermoactinomyces, and Saccharomonospora(Tseng et al. 2007).

Polylactic acid (PLA)

PLA ([−O(CH3)CHCO–]n) is a biodegradable and biocom-patible thermoplastic which can be produced by fermenta-tion from renewable resources. It can also be synthesizedeither by condensation polymerization of lactic acid or byring-opening polymerization of lactide in the presence of acatalyst (Carothers and Hill 1932). Ecological studies on theabundance of PLA-degrading microorganisms in differentenvironments have confirmed that PLA degraders are notwidely distributed, and thus, it is less susceptible to micro-bial attack compared to other microbial and synthetic ali-phatic polymers (Suyama et al. 1998a, b; Pranamuda et al.1997; Tansengco and Tokiwa 1998). The degradation ofPLA in soil is slow and takes a long time to start (Uruyamaet al. 2002; Ohkita and Lee 2006). Microbial degradation ofPLA using Amycolatopsis sp. was first reported byPranamuda et al. (1997). Since then, a number of researchstudies dealing with microbial and enzymatic degradation ofPLA have been published (Tokiwa and Calabia 2006).Many strains of genus Amycolatopsis and Saccharothrixwere able to degrade both PLA and silk fibroin. Severalproteinous materials such as silk fibroin, elastin, gelatin, andsome peptides and amino acids were found to stimulate theproduction of enzymes from PLA-degrading microorgan-isms (Pranamuda et al. 2001; Jarerat and Tokiwa 2001a, b;Jarerat and Tokiwa 2003a, b; Jarerat et al. 2004). Williams(1981) investigated the enzymatic degradation of PLA usingproteinase K, bromelain, and pronase. Among these en-zymes, proteinase K from Tritirachium album was the mosteffective for PLA degradation. Proteinase K and other serine


proteases are capable of degrading L-PLA and DL-PLA, butnot D-PLA. Furthermore, proteinase K preferentially hy-drolyzes the amorphous part of L-PLA and the rate ofdegradation decreases with an increase in the crystallinepart (Reeve et al. 1994; McDonald et al. 1996). Fukuzakiet al. reported that the degradation of PLA oligomerswas accelerated by several esterase-type enzymes, espe-cially Rhizopus delemar lipase (Fukuzaki et al. 1989).The purified PLA depolymerase from Amycolatopsis sp.was also capable of degrading casein, silk fibroin,Suc–(Ala)3–pNA, but not PCL, PHB, and Suc–(Gly)3–pNA (Pranamuda et al. 2001). Their studies showed thatPLA depolymerase was a kind of protease and not alipase. It was reported that α-chymotrypsin can degradePLA and PEA with lower activity on poly[(butylenesuccinate)-co-adipate] (PBS/A). Moreover, several serineproteases such as trypsin, elastase, and subtilisin wereable to hydrolyze L-PLA (Lim et al. 2005).

Different blends of L-PLA/PCL (75/25, 50/50, 25/75)were prepared, and enzymatic degradation was observedusing proteinase K or Pseudomonas lipase. Proteinase Kwas able to degrade the amorphous domain of PLA, butnot the crystalline part of L-PLA or PCL. On the contrary,Pseudomonas lipase can degrade both the amorphous andcrystalline part of PCL, but not L-PLA (Liu et al. 2000).Tribedi et al. have reported a new isolated strain of Pseudo-monas sp. (AKS2) from soil and observed its efficacy in thebiodegradation of PES and found that the degradation wasmediated by esterase activity (Tribedi et al. 2011).

Polymer blends

Blends of polyester with other polymers

The blending of biodegradable polymers is an approach ofreducing the overall cost of the material and modifying thedesired properties and degradation rates. Compared to thecopolymerization method, blending is a much easier andfaster way to achieve the desired properties. Iwamoto andTokiwa (1994a, b) developed blend plastics by combiningPCL with conventional plastics, such as LDPE, PP, PS,nylon 6 (NY), PET, and PHB, and evaluated their level ofenzymatic degradabilities. The blends of PCL/LDPE andPCL/PP retained the high biodegradability of PCL. In con-trast, the degradability of the PCL part in the blends of PCLand PS, PCL and PET, and PCL and PHB dropped offremarkably. In the case of blends of PCL and NY or PS,the biodegradability of PCL did not change so much. Thus,it seems that the higher the miscibility of PCL and conven-tional plastics, the harder the degradation of PCL on theirblends by Rhizopus arrhizus lipase (Iwamoto and Tokiwa1994a, b). Furthermore, it was found that degradabilities ofPCL/LDPE (Tokiwa et al. 1990) and PCL/PP (Iwamoto and

Tokiwa 1994a, b) blends by the lipase could be controlled,depending on their phase structure. Different blends ofPHB have been performed with biodegradable andnonbiodegradable polymers and polysaccharides. The mis-cibility, morphology, and biodegradability of PHB blendswith PCL, PBA, and polyvinyl acetate (PVAc) were inves-tigated. PHB/PCL and PHB/PBA blends were immiscible inthe amorphous state while PHB/PVAc are miscible. Enzy-matic degradation of these blends was carried out usingPHB depolymerase from Alcaligenes faecalis T1. Resultsshowed that the weight loss of the blends decreased linearlywith an increase in the amount of PBA, PVAc, or PCL(Kumagai and Doi 1992). Koyama and Doi 1997 studiedthe miscibility, morphology, and biodegradability ofPHB/PLA blend. The spherulites of the blends decreasedwith an increase in the content of the PLA, and the rate ofenzymatic surface erosion also decreased with increasingPLA content in the blend. It was evident that polymer blendscontaining PHB usually showed improved properties andbiodegradability when compared with pure PHB (Kumagaiand Doi 1992). Different blends of L-PLA/PCL (75/25,50/50, 25/75) were prepared, and enzymatic degradation wasobserved using proteinase K or Pseudomonas lipase. Protein-ase K was able to degrade the amorphous domain of PLA, butnot the crystalline part of L-PLA or PCL. On the contrary,Pseudomonas lipase can degrade both the amorphous andcrystalline part of PCL, but not L-PLA (Liu et al. 2000).

Blends of polyester with starch

Blends of synthetic polymers and starch offer cost–perfor-mance benefits because starch is renewable, cheap, andavailable. In this case, the starch blend can be in the formof granules or gelatinized starch or even starch which hasbeen modified chemically to a thermoplastic. It is generallyknown that blends of PCL and granular starch exhibit a highdegree of biodegradation (Tokiwa et al. 1990). Takagi et al.developed PCL/gelatinized starch blends using corn starchacetates and evaluated their biodegradabilities by an en-zyme, α-amylase. Their biodegradabilities rapidly de-creased with an increase in PCL content (Takagi et al. 1994).

Polycarbonates

Aliphatic polycarbonates are known to have greater resistanceto hydrolysis than aliphatic polyesters. The distribution ofPEC (Mn 50,000)-degrading microorganisms seems to belimited, although PPC (Mn 50,000) appears to benonbiodegradable. Suyama et al. isolated polyhexamethylenecarbonate (PHC, Mn 2000)-degrading microorganisms whichwere phylogenetically diverse. Roseateles depolymerans 61Aformed di(6-hydroxyhexyl) carbonate and adipic acid from


PHC, and di(4-hydroxybutyl) carbonate and succinic acidfrom polybutylene carbonate (PBC, Mn 2,000) (Suyama etal. 1998a, b). Pranamuda et al. found that Amycolatopsis sp.HT-6 degraded high molecular weight PBC (Mn 37,000). In aliquid culture containing 150 mg of PBC film, 83 mg of filmwas degraded after 7 days of cultivation (Pranamuda et al.1999). Suyama et al. (1998a, b) reported that a cholesterolesterase from Candida cylindracea, lipoprotein lipase fromPseudomonas sp., and lipase from Candida cylindracea,Chromobacterium viscosum, porcine pancreas, Pseudomonassp., and Rhizopus arrhizus degraded PBC (Mn 2000). Lipaseand lipoprotein lipase from Pseudomonas sp. could also de-grade high molecular weight PBC (Mn 30,000). Lipoproteinlipase from Pseudomonas sp. produced 1,4-butanediol, CO2,and di(4-hydroxybutyl) carbonate from PBC (Suyama andTokiwa 1997).

Crabbe et al. reported on the degradation of a polyester-type polyurethane (ES-PU) and the secretion of an enzyme-like factor with esterase properties by Curvulariasenegalensis, a fungus isolated from soil (Crabbe et al.1994). Subsequently, Nakajima-Kambe et al. showed thatComamonas acidovorans strain TB-35 was able to degradeES-PU made from polydiethylene adipate (Mn 2,500 and2,690) and TDI. A purified ES-PU-degrading enzyme fromComamonas acidovorans TB-35, a type of esterase, hydro-lyzed the ES-PU and released diethylene glycol and adipicacid (Nakajima-Kambe et al. 1995). However, it seems thatno microbe can degrade polyurethane completely, and there-fore, it is difficult to clarify the fate of residues after degra-dation of ES-PU by both microorganisms and enzymes.Furthermore, it is difficult to determine whether ET-PUitself was degraded by microbes to any significant extent.

Polyethylene (PE)

PE is a stable polymer and consists of long chains ofethylene monomers. PE cannot be easily degraded withmicroorganisms. However, it was reported that lower mo-lecular weight PE oligomers (MW=600–800) were partiallydegraded by Acinetobacter sp. 351 upon dispersion, whilehigh molecular weight PE could not be degraded (Tsuchii etal. 1980). Furthermore, the biodegradability of low-densityPE/starch blends was enhancedwith a compatibilizer (Bikiarisand Panayiotou 1998). Biodegradability of PE can also beimproved by blending it with biodegradable additives andphotoinitiators or by copolymerization (Griffin 2007;Hakkarainen and Albertsson 2004). The initial concept ofblending PE with starch was established in the UK to producepaper-like PE bag. A few years later, the idea to blend PE withstarch and photoinitiators was conceived in the USA as a wayof saving petroleum, though its biodegradability was alsotaken into account. Environmental degradation of PE pro-ceeds by synergistic action of photo- and thermo-oxidative

degradation and biological activity (i.e., microorgan-isms). When PE is subjected to thermo- and photo-oxidization, various products such as alkanes, alkenes,ketones, aldehydes, alcohols, carboxylic acid, keto acids,dicarboxylic acids, lactones, and esters are released. Blendingof PE with additives generally enhances auto-oxidation, re-duces the molecular weight of the polymer, and then makes iteasier for microorganisms to degrade the low molecularweight materials. It is worthy to note that despite all theseattempts to enhance the biodegradation of PE blends, thebiodegradability with microorganisms on the PE part of theblends is still very low.

Polypropylene (PP)

PP is a thermoplastic which is commonly used for plasticmoldings, stationary folders, packaging materials, plastictubs, nonabsorbable sutures, diapers, etc. PP can be degrad-ed when it is exposed to ultraviolet radiation from sunlight.Furthermore, at high temperatures, PP is oxidized. Thepossibility of degrading PP with microorganisms has beeninvestigated (Cacciari et al. 1993).

Polystyrene (PS)

PS is a synthetic hydrophobic polymer with high molecularweight. PS is recyclable but not biodegradable. Although itwas reported that PS film was biodegraded with an actino-mycete strain, the degree of biodegradation was very low(Mor and Silvan 2008). At room temperature, PS exists insolid state. When it is heated above its glass transitiontemperature, it flows and then turns back to solid uponcooling. PS being a transparent hard plastic is commonlyused as disposable cutleries, cups, plastic models, and pack-ing and insulation materials.

Table 3 revealed 15 bacterial genera which have the ca-pacity to degrade various types of plastics. Among themPseudomonas is dominant. It can degrade polythene, PVC,PHB, poly(3-hydroxybutyrate-co-3-mercaptopropionate), andpoly(3-droxypropionate). Bacillus brevis can degrade onlypolycaprolactone while Streptomyces can degrade PHB,poly(3-hydoxybutyarate-co-3-hydroxyvalerate), and starchor polyester. Ochrobactrum TD is also able to degrade PVC.

Majority of the strains that are able to degrade PHBbelong to different taxa such as Gram-positive and Gram-negative bacteria, Streptomyces, and fungi (Mergaert andSwings 1996). It has been reported that 39 bacterial strainsof the classes Firmicutes and Proteobacteria can degradePHB, PCL, and PBS, but not PLA (Suyama et al. 1998a,b). Other bacterial species identified having the properties ofdegrading plastics were Bacillus sp., Staphylococcus sp.,Streptococcus sp., Diplococcus sp., Micrococcus sp., Pseu-domonas sp., and Moraxella sp. (Kathiresan 2003).


Mechanism of biodegradation of plastics by microbes

The mechanisms of action of microorganisms on polymersare influenced by two different processes:

1. Direct action: In this case, the deterioration of plasticsserves as a nutritive substance for the growth of themicroorganisms.

2. Indirect action: The influence of the metabolic productsof the microorganisms, e.g., discoloration or furtherdeterioration.

Biodegradation of a polymeric material is a chemicaldegradation brought by the action of naturally occurringmicroorganisms such as bacteria and fungi via enzymat-ic action into metabolic products of microorganisms (e.g.,H2O, CO2, CH4, biomass, etc.) (David et al. 1994; Chandraand Rustgi 1998; Lenz 1993; Mohanty et al. 2000). Lipase ,proteinase K, pronase, hydrogenase, etc. are important

enzymes secreted by microbes for plastic biodegrada-tion. Among these enzymes, proteinase K fromTritirachium album was the most effective for PLAdegradation. Many strains of the genera Amycolatopsisand Saccharothrix were able to degrade PLA. Fukuzakiet al. reported that the degradation of PLA oligomerswas accelerated by several esterase-type enzymes, espe-cially Rhizopus delemar lipase (Fukuzaki et al. 1989).Several serine proteases such as trypsin, elastase, andsubtilisin were able to hydrolyze L-PLA (Lim et al.2005). In contrast to the biodegradation of polymers,where a near complete conversion of the material com-ponents takes place, only a change in the polymerstructure or the plastic composition is observed in manycases in polymer biodeterioration or biocorrosion (Gu2003). The ultimate result in the both the cases are a completeloss of structural integrity as a result of a drastic decrease inmolecular weight.

Table 3 List of microbial strains and the types of plastic which they degrade

Plastic Microorganism Reference

Polyethylene Brevibacillus borstelensis Hadad et al. (2005)

Rhodococcus rubber Sivan et al. (2006); Gilan et al. (2004)

Pseudomonas chlororaphis Zheng et al. (2005)

Comamonas acidovorans TB-35 Akutsu et al. (1998)

Polyvinyl chloride Pseudomonas putida AJ Anthony et al. (2004)

Ochrobactrum TD Mogil'nitskii et al. (1987)Pseudomonas fluorescens B – 22

BTA copolyester Thermomonospora fusca Kleeberg et al. (1998)

Some biodegradable/natural plastics and their degrading microorganisms

Poly(3-hydroxybutyrate-co-3-mercaptopropionate) Schlegelella thermodepolymerans Elbanna et al. (2004)

Poly(3-hydroxybutyrate) Pseudomonas lemoignei Jendrossek et al. (1995)

Poly(3-hydroxybutyrate-co-3-mercaptopropionate) Pseudomonas indica K2 Elbanna et al. (2004)

Poly(3-hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Streptomyces sp. SNG9 Mabrouk and Sabry (2001)

Poly(3-hydroxybutyrate-co-3-hydroxypropionate) Ralstonia pikettii T1 Wang et al. (2002)Acidovorax sp. TP4

Poly (3-hydroxybutyrate), poly(3-hydroxypropionate), poly(4-hydroxybutyrate),polyethylene succinate, polyethylene adipate

Alcaligenes faecalis Kasuya et al. (1999)Pseudomonas stutzeri

Comamonas acidovorans

Poly(3-hydroxybutyrate) Alcaligenes faecalis Kita et al. (1997)Schlegelella thermodepolymerans

Caenibacterium thermophilum Romen et al. (2004)

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Clostridium botulinum Abou-Zeid et al. (2001)Clostridium acetobutylicum

Polycaprolactone Clostridium botulinum Abou-Zeid et al. (2001)Clostridium acetobutylicum

Amycolatopsis sp.

Bacillus brevis

Polymer blends and its degrading microorganisms

Starch/polyester Streptomyces Lee et al. (1991)


Conclusion

As it is the plastic era, we cannot think beyond it, butplastics become one of the major problems for the modernenvironmentalist. To get rid of such a menace, people usu-ally put them in landfills or burn them, but both thesepractices cause very serious threats to the environment andthe ecosystem. Burning plastics usually produces some nox-ious gases like furans and dioxins, which are some danger-ous greenhouse gases and play an important role in ozonelayer depletion. In fact, dioxins cause serious problems inthe human endocrine hormone activity, thus becoming amajor concern for the human health too (NoPE 2002; Pilzet al. 2010). Dioxins also cause very serious soil pollution,causing a great concern for the scientific community world-wide. Hence, under such circumstances degradation of plasticby microbes is one of the eco-friendly and innovativemethods. Many fungal genera (e.g., Acremonium ,Cladospor ium , Debaryomyces , Emer ice l lops i s ,Eupenicillium, Fusarium, Mucor, Paecilomyces, Penicillium,Pullularia, Rhodosporidium, Verticillium, Aspergillus,Aureobasidium, Chaetomium, Cryptococcus, Fusarium, Rhi-zopus, Penicillium, Thermoascus, Penicillium roqueforti,Tritirachium album, etc.) and bacterial genera (Brevibacillus,Streptomyces, Amycolatopsis, Clostridium, Schlegelella,Pseudomonas, etc.) have been reported to degrade variouskinds of plastics (PEA, PPA, PBA, PCL, PVC, PHB, PU,etc.). This review may give brief information regardingthe nature and biodegradation of plastic by means ofdifferent microbes. It is expected that this review workwill encourage young scientists to find out one or moremicrobial strain(s) from nature for the potential biodeg-radation of plastic wastes.

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