Microbial Enzymatic Degradation of Biodegradable Plastics Current Pharmaceutical Biotechnology, 2017, Vol. 18, No. 5 3
direct access to polymer surface, which can be also catalyzed
with an acid, alkali or enzyme.
2.2.1. Alkali-catalyzed Hydrolysis of Polyesters
The alkali-catalyzed hydrolysis of polyesters involves the
reaction of hydroxide ion with the carbonyl carbon of the
ester group generating a tetrahedral intermediate, resulting in
the formation of an alcohol and carboxylic acid [22].
2.2.2. Acid-catalyzed Hydrolysis of Polyester
Under acidic conditions, the degradation of polyesters
begins with delocalization of proton from hydroxonium ion
(H
3
O
+
) to one of the lone pairs on the carbonyl oxygen of the
ester group, followed by hydrolysis of the carbonyl carbon,
resulting in the generation of tetrahedral intermediate. The
tetrahedral intermediate can then dissociate into a carboxylic
acid and an alcohol.
The chemical degradation leads to the release of mono-
mers and microbial polyester fragments which behaves as
organic pollutant, including polycyclic aromatic hydrocar-
bons and petroleum hydrocarbons into the environment.
These plastic derived contaminants are transferred to the
organisms by different stages most commonly ingestion,
inhalation and dermal sorption. These chemically degraded
monomers may be classified as mutagenic and/or carcino-
genic [23]. As chemical degradation causes release of toxic
monomers, biological degradation is, thus, a better and suit-
able alternate to chemical hydrolysis [24].
2.3. Degradation of Plastics by Biological Process
Both chemical and physical methods which have been
described above are associated with certain drawbacks;, bio-
logical degradation by microbial enzymes is therefore a fas-
cinating and environmental friendly option to discard plastics
[5]. This degradation process is very much different from
the normal biological degradation [25] where degradable
stages end with the disintegration of polymer due to the ac-
tion of high temperature, atmospheric moisture and sunlight
that condens the released product resulting in the production
of more stubborn persistent residues [26]. The biological
degradation of plastics by means of enzymes thus refers to
an attack by desired microflora on water immiscible plastic
polymers. Plastics are degraded by enzymatic activity of
those microbial flora that lead to a chain fragmentation of
polymer into monomers [21, 27-29].
Biodegradation of polymer involves following steps [30];
1) Adherence of the microorganisms to plastic surface.
Adherence leads to the formation of biofilms.
2) Growth of microorganisms by utilizing the degraded
polymer as an energy and food source (Assimilation).
3) Degradation of polymer (Fragmentation by hydrolysis).
4) Final disintegration of polymer (Mineralization).
Depending upon the polymer characteristics, whether it is
hydrophilic or hydrophobic, microorganism can attach to the
surface of the polymer. Microbes can degrade plastic by
aerobic or anaerobic oxidation. Under aerobic oxidation,
microorganisms biodegrade the plastic polymer to yield car-
bon dioxide and water molecule as the end products. Micro-
organisms utilize carbon as a substrate to derive chemical
energy. Aerobic biological degradation is an environmental
approach consisting of continuous degradation of synthetic
organic matter by a mixed microbial population in a moist
and aerobic environment.
Organic compounds + O
2
→ CO
2
+ H
2
O + Minerals
Anaerobic biological degradation is the fragmentation of
organic compounds without oxygen to generate carbon diox-
ide, water, methane, hydrogen sulphide, ammonia and com-
post product. The anaerobic degradation is a consequence of
series of metabolic interaction among various group of mi-
croorganism [31].
Several polymers degrading microbial species which
perform aerobic degradation have been isolated and charac-
terized [2-36]. The evaluation of the anaerobic breakdown of
plastics, is, however, still in a developing stage and only few
reliable investigations are available.
3. MICROORGANISMS AND THEIR ENZYMES RE-
SPONSIBLE FOR BIODEGRADATION OF PLASTICS
Several microorganisms like fungi and bacteria are re-
sponsible for the degradation of synthetic and natural plastics
[37-48]. Habitat of polymer/plastic degrading microorgan-
isms varies greatly amongst soil, compost, activated sludge
and sea water. As the microorganisms are present ubiqui-
tously, they have unique characteristics of forming an asso-
ciation with material surfaces [49-53]. The process of adher-
ence of complex microbial community on plastic surfaces is
known as “micro-fouling” or formation of biofilms, and in-
clude the activity of microorganisms and their extracellular
polysaccharides. Biofilms are widespread in aquatic as well
as terrestrial environments and show high diversity in terms
of space and time [26, 48, 54-56]. Microorganisms are re-
sponsible for degradation as they utilize hydrocarbons in the
polymer backbone as the primary carbon source [45]. Mo-
lecular weight and crystallinity are the two main parameters
which decide the ability of microorganisms to degrade the
plastic polymer [53]. Plastic polymers having high molecular
weight are not suitable for bacterial attack as it necessitates
the phagocytosis of the substrate through plasma membrane
and then it is degraded by intracellular enzymes [55].
Within living cell, esterase enzymes degrade the polyes-
ters which are linked with ester bond. Premraj et al. [27]
reported that in Pseudomonas sp. even lipase enzymes are
able to degrade the ester bond but only in low molecular
weight polyester. Several extracellular bacterial PHB and
PHBV depolymerase have been reported to metabolize PHB
and other PHA molecules. The resulting enzymatically
cleaved water soluble products are then absorbed by the mi-
crobial cells where they are further metabolized. The cleaved
products can be metabolized aerobically as well as an-
aerobically [31]. Carbon dioxide and water are the end prod-
ucts during aerobic biodegradation while in anaerobic bio-
degradation, carbon dioxide, water and methane is produced