Microbial biodegradation of polymeric materials

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J Biochem Tech (2010) 2(4):210-215
ISSN: 0974-2328

Microbial deterioration and degradation of polymeric materials

S Krishna Mohan*, Tanu Srivastava

Received: 27 October 2010 / Received in revised form: 14 December 2010, Accepted: 24 February 2011, Published online: 2 March 2011,
© Sevas Educational Society 2008-2011

Abstract
Polymeric materials due to its structural versatility are widely used
in aerospace applications, aviation and space industries. As they are
potential source of carbon and energy for heterotrophic
microorganisms including bacteria and fungi in several ways their
biodegradation affect these industries. The information on
degradability can provide fundamental information facilitating
design and life-time analysis of materials. Literature survey shows
that polymers which are susceptible to biofilm formation includes
paints, adhesives, plastics, rubbers, sealants, FRPCMs, lubricating
materials, fuels etc. Even though the understanding of polymer
degradation has been advanced in recent years the subject is still
inadequately addressed because of the lack of information available.
The review focuses on polymer biodeterioration and biodegradation
and its mechanisms, the types of microorganisms involved the
reactions of enzymes of importance in the biodegradation of
polymers, consequences of biodegradation, the factors involved in
biodegradation of polymers and its prevention and the tests used to
evaluate it.

Keywords:

Introduction
Polymeric materials have gained a wide influence due to their
excellent mechanical and thermal properties and high stability. They
are very unique in chemical composition, physical forms,
mechanical properties and applications. Because of this structural
versatility, polymeric materials are widely used in aerospace
applications, aviation and space industries as paints, adhesives,
sealants, plastics, composites, rubbers, lubricants, fuels, matrix
materials for fiber reinforced polymeric composites (FRPCMs) etc.
Polymeric materials are potential source of carbon and energy for
heterotrophic microorganisms including bacteria and fungi in
several ways. The actions of microorganisms on polymers are
influenced by two different processes:
1. Direct action: The deterioration of plastics which serve as
a nutritive substance for the growth of the
microorganisms
2. Indirect action: The influence of metabolic products of
the microorganisms, e.g., discoloration or further
deterioration.
Biodegradation of a polymeric material is chemical degradation
brought by the action of naturally occurring microorganisms such as
bacteria and fungi via enzymatic action into metabolic products of
microorganisms (e.g., H2O, CO2, CH4, biomass etc.) (David et al.
1994; Chandra et al. 1998; Lenz 1993; Mohanty et al. 2000). In
contrast to the biodegradation of polymers, where a near complete
conversion of the material components takes place only a change in
the polymer structure or the plastic composition is observed in many
cases in polymer biodeterioration or biocorrosion (Gu et al. 2003).
The ultimate result in the both the cases are a complete loss of
structural integrity as a result of drastic decrease in molecular
weight. In current times the term ‘biodegradable’ is considered an
essential property of many manufactured materials to ensure that the
particular material with stand its effect or not. Although
biodegradation may be seen as a direct opposite to biodeterioration,
they are usually the same processes, changed in meaning and
significance solely by human need. Hence, in this review the term
biodegradation also implicitly include biocorrosion or
biodeterioration or microbially influenced corrosion (MIC).
Microorganisms are involved in the degradation and deterioration of
both synthetic and natural polymers forming biofilms (Gu et al.
2000). Polymers which are susceptible to biofilm formation include
paints, adhesives, plastics, sealants, composites, lubricating
materials, fuels etc (Gross et al. 1995; Gross et al. 1993; Gu et al.
1993; Gu et al. 1993; Gu et al. 1994; Gu et al. 1996; Albinas et al.
2003). This review article focuses on mode of biodegradation of
polymeric materials, its mechanisms, consequences, factors

S Krishna Mohan
Scientist D, PJ-10(MS), Defence Research & Development
Laboratory (DRDL) Hyderabad-500 058, India
* Tel: 0091-9849910832, Fax: 0091-40-24087087
Tanu Srivastava
Scientist C, PJ-10(MS), Defence Research & Development
Laboratory (DRDL) Hyderabad-500 058, India

involved, prevention and test methods to be followed for its
evaluation.

Mode of biodegradation

The biological environment includes the biological agents such as
bacteria, fungi and their enzymes responsible for the deterioration of
polymeric substances. They consume a substance as a food source
so that its original form disappears.
Microorganisms
Microorganisms are highly adaptive to environment and secrete
both endoenzymes and exoenzymes that attack the substrate and
cleave the molecular chains into segments (Albinas et al. 2003;
Huang et al. 1990). The secreted enzymes are proteins of
complicated chemical structure with high molecular weights
possessing hydrophilic groups such as -COOH, –OH, and -NH2
(Potts 1978) which can attack and eventually destroy almost
anything. Several factors including the availability of water,
temperature, oxygen usage, minerals, pH, redox potential, carbon
and energy source influence the growth of microorganisms (Holmes
1988; Sand 2003) (Table 1). The degradative action of fungi and
Table 1: Spectrum of conditions under which microbial life is
observed

S.No Parameter Conditions
1 pH 0 to 13
2 Temperature -5oC to +116oC
3 Pressure To 1.000 bar
4 Redox
potential
-500 mV to +850 mV
5 Salinity Ultrapure water to almost saturated
water
6 Radiation Biofilms on Ultra violet lamps,
irradiation units and nuclear power
plants
7 Nutrient
concentration
From 10 μgL-1 (drinking and purified
water) Up to life in and on carbon
sources
bacteria on the polymeric material is a result of enzyme production
and resultant breakdowns to the non living substrate in order to
supply nutrient materials. Differences between fungi and bacteria
are illustrated in Table 2.

Table 2: Differences between fungi and bacteria
Fungi Bacteria
Singular – fungus Singular – Bacterium
Fungi kingdom Monera kingdom
Multi cellular except yeast Single celled
Heterotrophs Heterotrophs or autotrophs
Aerobic Aerobic or anaerobic
Eukaryotes prokaryotes
Prefers slightly acidic (most
cases)
Prefers neutral to slightly
alkaline (most cases)

Enzymes

Wide range of microorganisms utilizes enzymes which are
structurally high specialized proteins with complex three
dimensional structures as carbon and energy sources. In the
presence of enzymes, a rise in reaction rate of 106–1020 without
creating undesirable products can be often observed (Lenz 1993). In
general many different types of enzymes (over 2000) exist in a

biological system and each enzyme performs one chemical function.
Susceptibility of the polymers to microbial attack generally depends
on enzyme availability, availability of a site in the polymers for
enzyme attack and enzyme specificity for that polymer and the
presence of coenzyme if required. The type of reaction which is
probably of most importance in the enzymatic degradation of
polymers is the bimolecular reaction in which the enzyme catalyzes
the interaction of the polymer and a low molecular reagent. These
reactions can occur by either single displacement mechanism or
double displacement mechanism. In the single displacement
mechanism both substrates, A and B are bound to the free enzyme
En by consecutive reversible reactions. After that the final complex
EnAB dissociates into the products C, D and the free enzyme En (Fig
1). Whereas, in the double displacement mechanism only one
substrate at a time is bound to the enzyme but the complex of the
first bound substrate, EnAY undergoes unimolecular dissociation at
an appropriate functional group to form a new complex EnY
between the enzyme and a fragment Y of AY. This intermediate
complex combines with the second substrate B and transfers the Y
fragment to B (Fig 2).
+B
En + A ———- EnA ——–EnAB———-C+D+E

Single displacement mechanism
Factors affecting polymer biodegradation
All polymers are more or less biodegradable to some extent due to
the organic nature of their principle elements like resin and
hardener. Polymeric materials complexicity, structures and
compositions is one of the important aspects which govern polymer
biodegradation. Polymer biodegradation is a heterogeneous process.
Polymers do not consist simply of only one chemical homogenous
component, but contain different polymers (blends) or low MW
additives (plasticizers) which can serve as good nutrients for the
ambient microorganisms developed on polymer surfaces (Kyrikou et
al. 2007). Moreover, within one polymer itself different structural
elements can be present (copolymers), and these may either be
distributed statistically along the polymer chains (random
copolymers) which or alternatively (alternating copolymers).
Another structural characteristic of a polymer is the possible
branching of chains or the formation of networks (cross linked
polymers). Despite having the same overall composition these
different structures of a polymer can directly influence accessibility
of the material to the enzyme catalyzed polymer chain cleavage, and
also have a crucial impact on the higher ordered structures of the
polymers. In recent years a considerable amount of qualitative and
semi-quantitative information has been accumulated to draw some
conclusions about the following important factors (Chandra et al.
1998; Baljit et al. 2008; Gijpferich 1996) which are affecting the
rate of degradation of synthetic polymers in a biological
environment.
• Polymer structure and morphology
• Molecular weight
• Hydrophobic and hydrophilic characteristics
• Additives
• Methods of synthesis
• Environmental conditions

En + AY ======== EnAY ===== ENY+A

EnYB =========== YB+En

Double displacement mechanism

Generalization concerning biodegradation
Biodegradability is primarily dependent on hydrolyzable and
oxidizable chemical structures, balance of hydrophobicity, and
molecular weights. Physical properties such as crystallinity,
orientation, Tm or Tg, and morphological properties such as surface
area or thickness affect the rate of degradation. The following
guidelines were made for the relationship between polymer structure
and biodegradation (Swift 1993; Kawai 1995).
1. Naturally occurring polymers are biodegradable. Chemically
modified natural polymers may biodegrade depending on the
extent of modification and the kind of modifying group.
2. Synthetic addition polymers except polyvinyl alcohol with
carbon-chain backbones do not biodegrade at molecular
weights greater than 1000.
3. Synthetic addition polymers include polyacetals and polyesters
with hetero-atoms in their backbones may biodegrade.
4. Synthetic condensation polymers are generally biodegradable
to a greater or lesser extent depending on the following factors
• Chain coupling (ester > ether > amide > urethane)
• Molecular weight (lower is faster than higher)
• Morphology (Tm) (amorphous is faster than crystalline)
• Hardness (Tg) (softer is faster than harder) and
• Hydrophilicity vs hydrophobicity (hydrophilic is faster
than hydrophobic).
5. Water solubility does not guarantee biodegradability.
General mechanism of biodeterioration and biodegradation
Biodeterioration
Biodeterioration of polymers involves primarily enzyme-catalyzed
chemical reactions which can occur due to endoenzymes and
exoenzymes (Lenz 1993; Ranjith et al. 2005). The former results in
random chain cleavage with a substantial decrease in molecular
weight where as the later in which the immediate effect on
molecular weight of the residual polymer will be much less results
in removal of only terminal units, which are generally either
monomers, dimers or trimers (A1, A2, A3) (Fig 3). Here, A
represents both the internal repeating units and the terminal units,
the subscript represent the number of such units in the product
formed.
Biodegradation
Microorganisms are not able to transport the polymers directly
through their outer cell membranes into the cells where most of the
biochemical processes take place due to the lack of water-solubility
and the length of the polymer molecules. Inorder to use such
materials as a carbon and energy source, microorganisms have
developed a special strategy. The microbes excrete extracellular
enzymes which depolymerize the polymers outside the cells.
Extracellular and intracellular depolymerases enzymes are actively
involved in biological degradation of polymers. Anaerobic and
aerobic biodegradation mechanism pathways are given in Fig 4 (Gu
2003). During degradation, exoenzymes from microorganisms break
down complex polymers yielding short chains or smaller molecules,
e.g., oligomers, dimers, and monomers, that are smaller enough
(water soluble) to pass the semi-permeable outer bacterial
membranes and then to be utilized as carbon and energy sources
(Gu 2003). This initial process of polymer breaking down is called
depolymerization. When the end products are inorganic species,
e.g., CO2, H2O, or CH4, the degradation is called mineralization.

When O2 is available, aerobic microorganisms are mostly
responsible for destruction of complex materials with microbial
biomass, CO2, and H2O as the final products. In contrast, in the
absence of O2 i.e under anoxic conditions, anaerobic consortia of

microorganisms are responsible for polymer deterioration. In this
case the primary products will be microbial biomass, CO2, CH4 and
H2O under methanogenic conditions (Barlaz et al. 1989; Barlaz et
al. 1989; Gu et al. 2001) or H2S, CO2 and H2O under sulfidogenic
conditions. Since thermodynamically O2 is a more efficient electron
acceptor than SO2
4− and CO2, aerobic processes yield much more
energy and are capable of supporting a greater population of
microorganisms than anaerobic processes. It is important to
understand that biodeterioration and degradation of polymer
substrate can rarely reach 100% becuase a small portion of the
polymer will be always incorporated into microbial biomass, humus
and other natural products (Alexander 1977; Atlas et al. 1997;
Narayan 1993; Gu et al. 2006).

Properties deterioration

Biodegradation of polymers results in the deterioration of some of
the mechanical, optical and electrical properties given in the
Table 3. Since usually mechanical properties of polymers are
predominantly determined by the length of the polymer chains in the
material, scission of polymer chains is one major reason for changes
in mechanical properties. Even just a single endo cleavage in a
polymer chain can reduce the molar mass to 50%, and hence cause
significant changes in mechanical properties. The small variations in
the chemical structures will result in large differences in term of
biodegradability. When plasticizers are removed from the plastic
materials by micro organisms, embrittlement can also occur.
The changes in electrical properties often are due to principally
surface growth, its associated moisture and to pH changes caused by
excreted metabolic products. Removal of susceptible plasticizers,
modifiers, and lubricants, results in increased modulus, changes in
weight, dimensions, other physical properties, and deterioration of
electrical properties such as insulation resistance, dielectric constant,
power factor, and dielectric strength.

Table 3: Deteriorated mechanical, optical and electrical properties
Mechanical
properties
Electrical
properties
Optical properties
Tensile strength
Stiffness
Hardness
Weight loss
Embrittlement
Impact resistance
Gloss
Dielectric strength
Dielectric
constant
Insulation
resistance
Arc resistance
Optical
transmission
Haze
Water vapour
transmission
Biofilms

Deterioration of polymeric materials is caused by adhering
microorganisms that colonize their surfaces, forming biofilm
(Mitchell et al. 1996; Viktorov et al. 1992). The formation of a
biofilm is a prerequisite for substantial corrosion and deterioration
of these materials to take place. Biofilm is a slimy layer where
bacterial cells can encase themselves in a hydrated matrix of
polysaccharides and protein which is composed of water (80- 95%),
extracellular polymer substances (EPS) that contribute 85-98% of
the organic matter, the microorganisms, entrapped organic and
inorganic particles (e.g. humic substances, debris, clay, silica,
gypsum, etc.), substances sorbed to EPS, cells or particles and
substances dissolved in the interstitial water (Flemming 1998). The
process of establishment of complex community of microorganisms
on surface attachment as biofilm is known as biofouling or
microfouling. The major five damaging mechanisms (both direct
and indirect) through which the structure and function of synthetic
polymeric materials can be damaged by biofilms can be very high.
This includes (Flemming 1998) (1) Coating the surface, masking
surface properties and contaminating adjacent media such as water,
(2) Increasing the leaching additives and monomers out of the
polymer matrix, (3) Attack by enzymes or radicals of biological
origin to polymer and additives; leading to both embrittlement and
loss of mechanical stability, (4) Accumulating water and penetrating
the polymer matrix with microbial filaments, causing swelling and
increased conductivity and (5) Excretion of lipophilic microbial
pigments that lead to unwanted colours in the polymer.
Biocides
Polymer biodegradation can be treated by Physical-mechanical,
biological, electrochemical and chemical methods (Videla 2002).
Among which chemical methods are most frequently employed in
which using biocides is the most profound characteristic of chemical
treatment. The biocides are single compounds (or a mixture of
compounds) capable of killing microorganisms or inhibiting
microbial growth. They are also employed to remove populations
from either within the matrix of a material or on the surfaces of a
material and are divided into Oxidising biocides and Non-oxidising
biocides where the latter being more effective because of their
overall control of bacteria, algae and fungi. Moreover, they have
greater persistence, as many of them are pH independent.
Frequently, a combination of oxidizing biocides and non-oxidizing
biocides is used to optimize the microbiological control. The action
of a biocide used to disinfect any system should be bactericidal,
fungicidal and algicidal, thus requiring the application of broadspectrum
biocide compounds. Although the intrinsic activity of
biocidal agents is important the more concern is the interaction
between them and the material/system which they are designed to
protect. The spectrum of activity must be appropriate to the
challenge the materials and the biocide must be compatible with the
material as well as be able to provide protection for a suitable period
of service.
Test methods for biodegradable polymers
While biodeterioaration test aims to characterize changes in the
material properties (which can even be caused by minor chemical
changes in the polymers such as extraction of plasticizer or
oxidation, etc.), biodegradable tests for plastics material is finally
transformed into natural biological products. Due to the following
facts multiple test procedures are necessary in evaluating the
biodegradability of a material:
• Observed weight loss may result not from polymer
degradation, but from the leaching of additives, including
plasticizers.
• Carbon dioxide production might result from the degradation
of low molecular weight fraction of the polymer, with no
degradation of longer chains.
• A large loss of material strength might come from a very small
change in its chemical makeup.
• Strength is often disproportionately affected by the loss of
additives and 90% decrease of strength can result from as little
as 5% mineralization.
The current available methods are not adequate to address the wide
array of polymers utilized under different environmental conditions
( Gu 2003; Gu et al. 2005). In addition, available test methods offer
very little flexibility because the objectives of these methods are in
the development stage. The extent of degradation of the advanced
polymers used in aviation and electronics cannot be determined
quantitatively with the existing techniques. When testing
degradation phenomena of plastics in the environment one has to
face a general problem concerning the kind of tests applied and the
conclusions which can be drawn. Besides reproducibility, the
shortening of test durations and minimization of the material needed
is a crucial point when performing extended systematic
investigation for biodegradation testing. In principle the
degradation tests can be classified into three categories.

1. Field tests: Complex environment and variable conditions

2. Simulation tests: Complex environment and defined
conditions

3. Laboratory tests: Synthetic environment and defined
conditions

Field tests

Field tests such as burying plastic samples in soil, river or full-scale
composting performed represent ideal practical environmental
conditions, but there are some serious disadvantages of such kinds
of tests. One is that environmental conditions such as temperature,
pH, or humidity, cannot be efficiently controlled in nature and
secondly, analytical methods for monitoring the degradation process
are very limited. In most cases it is only possible to evaluate visible
changes of the polymer samples or to determine the disintegration
by measuring the weight loss. Analysis of residues and
intermediates is complicated due to the complex and undefined
environment. Since a pure physical disintegration of a plastic
material is not regarded as biodegradation these tests alone are not
suitable to prove whether a material is biodegradable or not.
Simulation tests
Various simulation tests have been developed to overcome the
problems at least partially with field tests. Here, the degradation
takes place in a real environment (e.g. compost, soil or sea water),
but the exposure to the environment is performed in a laboratory
reactor. The important external parameters which can affect the
degradation process (e.g. temperature, pH, humidity, etc.) can here
be controlled and adjusted. Examples for such tests include the soil
burial test (Pantke 1990), the so-called controlled composting test
(Pagga et al. 1995; Tosin et al. 1996; Ohtaki et al. 1998; Tuominen
et al. 2002; Degli-Innocenti et al. 1998), test simulating landfills
(McCartin et al. 1990; Smith et al. 1990; McCarthy et al. 1992), or
aqueous “aquarium tests (Püchner et al. 1995). Nutrients are
sometimes added in these tests with the aim to accelerate
degradation and to reduce the duration of the degradation tests.
Laboratory tests
The most reproducible biodegradation tests are laboratory tests,
where defined media are used which then are inoculated with a
mixed microbial population. In some cases individual microbial
strains or mixtures of some strains are used for inoculation. Such
tests often take place under conditions optimized for the activity of
the particular microorganisms (e.g. temperature, pH, humidity, etc.)
with the effect, that polymers often exhibit a much higher
degradation rate in laboratory tests than observed under natural
conditions.
The most reproducible degradation tests directly use the isolated
extracellular enzymes of the microorganisms which are responsible
for the first step of the degradation process, the molar mass
reduction of the polymers by depolymerization (Tokiwa et al.
1977; Marten et al. 2003; Marten et al. 2005; Walter et al. 1995;
Vikman et al. 1995). Even though with laboratory tests it is not
possible to prove biodegradation in terms of metabolization by
microorganisms, the shorter test durations and the reproducible test
conditions makes them especially useful for systematic investigation
when studying basic mechanisms of polymer biodegradation.
However, conclusions on the absolute degradation rate in a natural
environment can only be drawn to a limited extent. Examples for
such tests include Rapid detection method, Closed bottle test,
Environmental chamber method, Petri dish screen, Gravimetry,
Respirometry, Measurement of Biogas and Surface hydrolysis.
Conclusions
In recent years the issue of degradable polymers and plastics with
particular emphasis on biodegradation has received a great deal of
attention. Naturally occurring polymers are readily biodegradable in
the environment but most of the synthetic high polymers biodegrade
only very slowly under comparable exposure conditions. There are,
however, exceptions to this observation and several classes of
synthetic polymers that undergo ready environmental
biodegradation are known. Protection of the materials from
biodegradation can be achieved to some extent through surface
engineering and control of the physical, chemical and biological
environments. Biocides have been widely used for the protection but
the development of resistant bacteria and fungi is becoming a
serious problem than expected. Assessment of biodegradability is a
key consideration in the development of biodegradable polymers.
Strict definitions do not exist about the constituent of abiotic
environment to carry out such testing and criteria to be followed to
establish biodegradability of a polymer in the laboratory. The test
results are sensitive to a variety of factors, particularly the consortia
of micro-organisms used. It is, therefore, often difficult to appreciate
the full significance of the reported data and to test results relate to
each other. Material biodeterioration is highly undesirable to
material integrity as these are used mostly in structural designs of
aerospace vehicles. Damage to the structure may result in premature
weakening which is often translated to system failure and enormous
economic losses. This review thus stresses for the need of microbial
susceptibility studies for all types of synthetic polymeric materials
before using in the actual hardware.

Acknowledgements

The authors sincerely thank DRDO for supporting this work and
encouraging in the preparation of the manuscript.

Nomenclature
FRPCM – Fiber reinforced polymeric composites
Tm – Melting point temperature
Tg – Glass transition temperature
EPS – Extracelluar polymer substances
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