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Biodiversity International Journal
Diversity and Production of Extracellular Polysaccharide
by Halophilic Microorganisms
Submit Manuscript | http://medcraveonline.com
Volume 1 Issue 2 - 2017
Department of Botany, University of Calcutta, India
*Corresponding author: Paul AK, Department of Botany,
University of Calcutta, West Bengal, Kolkata, India, Tel:
91-033-2461-5445/4959/5277/4711; Fax: 0332461-4849;
Email:
Received: February 19, 2017 | Published: August 02, 2017
Review Article
Biodiversity Int J 2017, 1(2): 00006
Abstract
Halophilic microorganisms derived from diverse thalassohaline and athalassohaline
environments including marine estuaries, saline and soda lakes, inland solar salterns
and acidic habitats are categorized as slight-, moderate- and extreme halophiles
according to their NaCl requirements. Taxonomic studies with culturable diversities
of halophiles revealed that they belong to both Archaea and Bacteria representing the
families Halobacteriaceae, Methanosarcinaceae and the class Gammaproteobacteria. As
adaptive strategies against harsh salt stresses, majority of halophiles often synthesize
of their physical, chemical and material properties. So far the novelty in structure and
functions of exopolysaccharides are concerned, producer strains belonging to the
genera Halomonas and Haloferax have attracted the main attention. However, EPS
producing strains belonging to the genera Idiomarina, Salipiger and Alteromonas are
not uncommon. Through process optimization and metabolic regulation a number
of potent halophilic strains have been found to produce copious amount of EPS
chemical and biological properties along with the possible applications of halophilic
EPS in industry and biotechnology have also been highlighted.
Keywords: Halophiles; Archaea; Bacteria; Thalassohaline environment;
Athalassohaline environment; Solar salterns; Compatible solutes; Extracellular
polysaccharides
Introduction
Halophiles, the salt-loving microorganisms are distinguished
by their characteristic requirement of high salt concentration
for growth and have evolved physiological and genetic features
to survive in hypersaline environments. In addition, factors like
temperature, pH, availability of oxygen and nutrients, as well as
solar radiations plays an important role in determining the growth
of halophiles. Ever since the time of Larsen [1] and Kushner
[2], these organisms, depending on their salt dependence and
tolerance have been distinguished as slight halophiles, moderate
halophiles, and extreme halophiles [3].
Most of the halophiles are inhabitants of hypersaline waters
and soils, salt or salt deposits and salted products [4,5] Multi-
pond solar salterns, representing typical thalassohaline water
systems with salinities ranging from seawater salinity to halite
saturation have been correlated with the changes in microbial
community densities [6]. Likewise, aerobic, anaerobic and
facultative anaerobic microbes belonging to domains Archaea and
Bacteria have been recovered from the athalassohaline waters of
the Dead Sea, Great Salt Lake, hypersaline lakes in Antarctica,
Lake Magadi etc. [7].
The halophiles follow two different strategies to cope with
the osmotic stress exerted by the saline environment. Halophilic
Archaea maintain an osmotic balance of their cytoplasm with the
hypersaline environment by accumulating high concentrations
of inorganic ions in the cytoplasm (salt in cytoplasm strategy).
In contrast, halophilic or halotolerant bacteria adapt themselves
by accumulating high concentrations of various organic osmotic
solutes (compatible solutes) [4]. Apart from these, the deleterious
hypersaline environment forced the halophiles to produce a
variety of biomolecules, pigments, biosurfactants, proteins,
extracellular polysaccharides (EPS) and intracellular polyester
polyhydroxyalkanoates, which have attracted the attention of
microbial biotechnologists [8].
Production of extracellular polysaccharides by halophilic
Archaea and Bacteria has been reported by several workers and
the members of the genus Halomonas
most potential producers [9-11]. While the chemistry, structure
and functions of microbial extracellular polymeric substances in
general have been highlighted with special emphasis on microbial
environmental biotechnology; their role in the remediation of
heavy metals, toxic compounds and dyes from the anthropogenic
environments could not be ruled out [12]. Survey of literature
have clearly indicated that the information pertaining to the
production and characterization of extracellular polysaccharides
by the wide variety of halophilic microorganisms isolated from
hypersaline environments are inadequate [9,13]. However, there
is an increasing demand for the production of extracellular
polysaccharides by the halophiles with properties better than
those of the existing ones. The present review is aimed at to
Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms 2/9
Copyright:
©2017 Biswas et al.
Citation: Biswas J, Paul AK (2017) Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms. Biodiversity Int J 1(2):
00006. DOI: 10.15406/bij.2017.01.00006
explore the diversity of halophilic microorganisms from saline and
hypersaline ecosystems and also to evaluate their potential for
production of extracellular polysaccharides with special emphasis
Diversity of Halophiles
The culturable diversity of halophilic microorganisms from
hypersaline environments includes both the extremely halophilic
Archaea and moderately halophilic Bacteria. Halophilic archaea
represented by the family Halobacteriaceae is currently comprised
of 47 genera and 165 species and Methanosarcinaceae which
includes 4 genera and 7 species [14,15]. While the members
of Halobacteriaceae are aerobic or facultatively anaerobic and
generally red-pigmented ones, the methanogens obtain their
energy form methylated amines under anaerobic conditions [14].
Moderate or extremely halophilic bacteria isolated from diverse
environments are currently represented by a large number of
species included under the phyla of Proteobacteria, Firmicutes,
Actinobacteria, Spirochaetes, Bacteroidetes, Thermotogae,
Cyanobacteria and Tenericutes [15]. Members belonging to these
phyla constitute a heterogeneous assemblage of microorganisms
with diverse physio-biochemical activities and morphological
variations [15]. The class Gammaproteobacteria of Proteobacteria
contains the largest number of moderately halophilic genera and
the members of the family Halomonadaceae represented the best
studied and most important genera [15,16].
Quesada et al. [17] following the conventional plate count
method, analyzed the hypersaline soils of multi-pond salterns
near the Mediterranean coast in Alicante, Spain and reported
the predominance of Gram-negative halophiles belonging to the
genera of Pseudomonas, Alcaligenes, Vibrio, Flavobacterium and
Acinetobacter. Gram-positive rods and cocci were assigned to
the genera Bacillus, Nesterenkonia, Arthrobacter, Marinococcus,
Staphylococcus, Corynebacterium, Brevibacterium, Nocardia
and Actinomyces. Subsequently, species assigned to the genera
Planococcus and Sporosarcina were also added to the list [18].
Garabito et al. [19] also isolated 71 halotolerant Gram-positive
endospore forming rods from saline soils and sediments of
salterns located in different parts of Spain and tentatively assigned
them to the genus Bacillus. Numerical taxonomic studies have
been conducted on inland, athalassohaline salterns near Granada,
Spain [20] and Chile [21] and the isolated strains were assigned
to moderately halophilic genera Halomonas, Vibrio, Alteromonas
and Acinetobacter. In a more selective diversity study, Ghozlan et
al. [22] isolated 90 Gram-positive and Gram-negative moderately
halophilic bacterial strains from coastal solar salterns, salt
marshes and salt lakes of Alexandria, Egypt. The Gram-negative
isolates belonged to the genera Psudoalteromonas, Flavobacterium,
Chromohalobacter, Halomonas and Salegenibacter while, the
Gram-positive strains were included in the genera Halobacillus,
Salinicoccus, Staphylococcus and Tetragenococcus. They concluded
that greater diversity occurred in the inlet and lower salinity
ponds.
Yeon et al. [23] studied the culturable diversity of moderately
halophilic, organotrophic bacteria from solar salterns of Taean-
Gun, Chungnam Province, Korea following RFLP analysis of PCR
rDNA sequences. Based on these, 64 strains were segregated into
genera like Vibrio, Pseudoalteromonas, Halomonas, Alteromonas
and Idiomarina
of halophilic community present in saltern at Eilat, Israel, as
well as Cargill Solar Salt Plant in Newark, California by using
traditional and molecular techniques. Recently, Mutlu and Guven
[25,26] employed a combination of denaturing gradient gel
extracted from the water samples of the saltern and 16S rRNA
gene library analysis to identify the bacterial diversity of Camalti
solar saltern in Turkey and explored a total of 42 isolates, which
belonged to the genera Halobacillus, Virgibacillus and Halomonas.
The diversity analysis of bacteria and archaea present in El
Golea’s sebkha of Algerian Sahara [27] has lead to the isolation of
471 strains belonging to 31 different genera of halophilic bacteria
and archaea. The bacterial genera include Vibrio, Pseudomonas,
Staphylococcus, Pasteurella, Streptococcus, Salmonella, Shigella,
and Escherichia. However, only 52 isolates belonged to the
halophilic aerobic archaea which were placed in the genera
Halobacterium, Halococcus, Natronobacterium, Haloferax,
Natronoccoccus, Haloarcula and Natrinema.
The occurrence of red pigmented haloarchaeal communities
has been documented from the north arm of the Great Salt Lake
[28], the Dead Sea [29] and hypersaline alkaline lake, Lake Magadi
[30]. Halo archaea isolated from coastal salt-marsh sediments able
to grow at lower salinities have also been reported [31]. Birbir &
Sesal [32] studied the extreme halophilic bacterial communities
biochemical features, while Birbir et al. [33] characterized
extremophilic communities in Tuzkoy salt mine and the adjacent
and denaturing gradient gel electrophoresis (DGGE) analysis of
DNA sequences which revealed the phylogenetic inclusion of the
isolated strains within the genera Halobacterium, Haloarcula,
Natrinema and Halorubrum. Elvi et al. [34] isolated extremely
halophilic strains from the Ayvalik saltern in the north-eastern
part of Turkey. Similarly, Enache et al. [35] isolated extreme
as members of the genus Haloferax.
Bacterial and archaeal aerobic communities were recovered
from sediments of El-Djerid salt lake in Tunisia [36] by using
phenotypic and phylogenetic approaches, the authors found
that the members of the domain Bacteria belonged to Salicola,
Pontibacillus, Halomonas, Marinococcus and Halobacillus,
whereas, the only member of domain Archaea was represented
by Halorubrum. Recently, Kim et al. [37] analyzed the hypersaline
sediment of Death Valley National Park and documented the
availability of the genera Halorubrum, Halococcus, Haloarcula,
Halorhabdus and Halobacterium. Studies on halophilic diversity
from hypersaline environments of India are not uncommon.
Dave & Desai [38] isolated halophiles from marine salterns
of Bhavnagar, Gujarat, India and majority of them belonged
to Archaeal domain. Presence of Gram-positive alkalitolerant
Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms 3/9
Copyright:
©2017 Biswas et al.
Citation: Biswas J, Paul AK (2017) Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms. Biodiversity Int J 1(2):
00006. DOI: 10.15406/bij.2017.01.00006
moderate halophiles of the genera Bacillus, Micrococcus,
Planococcus, Vagococcus as well as Gram-negative representatives
of Paracoccus, Halomonas and Providencia were recorded from
Alkaline Lonar Lake of Maharashtra, India [39]. An elaborate
account of the community structure of the halophilic archaea at
initial and crystallization stage of salt production in solar salterns
of Goa was documented by Mani et al. [40]. The isolates obtained
during the pre-salt harvesting phase, belonged to Halococcus spp.
while, at salt harvesting phase, Halorubrum, Haloarcula, Haloferax
and Halococcus were predominant.
Surve et al. [41,42] in their survey of moderate halophiles
reported the presence of Virgibacillus pantothenticus, Bacillus
atrophaeus, Corynebacterium diphtheria and Idiomarina zobellii
from salt pans of Goa using FAME and 16S rDNA sequence
analysis. Recently, Sardar & Pathak [43] investigated the halophilic
microbiota of solar salterns of Mumbai, India and recorded
Halorubrum, Haloferax and Halobacterium as the extremely
halophilic genera while, moderate halophilic members belonged
to Halomonas, Halobacillus, Pseudomonas, Salicola and Halovibrio.
Diversity of halophiles in solar salterns of Tamilnadu,
India revealed the presence of representatives of the family
Halobacteriaceae which were dominated by members of
genera Haloferax, Halorubrum, Haloarcula, Halobacterium
and Halogeometricum [44] while, that of Kovalam saltpans in
Kanyakumari belonged to Staphylococcus, Halobacillus, Halococcus,
Natronobacterium and Halobacterium [45]. The halophilic
bacterial diversity along the coastal regions of Karnataka,
India showed the predominance of the genera Virgibacillus,
Halobacillus, Salinibacillus, Nesterenkonia, Pontibacillus and
Staphylococcus [46]. Similarly, moderately halophilic aerobic
bacteria belonging to the genera Halomonas, Salinicoccus, Bacillus,
Aidingimonas, Alteromonas, and Chromohalobacter were isolated
by Biswas & Paul [47] from multi-pond solar salterns along the
coasts of Gujarat (Figure 1), Orissa, and West Bengal, India. Colony
morphology of some of the representative members are shown in
Figure 2.
Figure 1: Typical multi-pond solar salterns at Jogrinar located in
Kachchh districts of Gujarat, India.
Figure 2: Variations in colony morphology of halophilic bacteria
isolated from soil and water samples of multi-pond solar salterns
distributed along the coasts of India.
Production of Extracellular Polysaccharides (EPS) by
Halophiles
polysaccharide in 1880, continued search for novel polysaccharides
from microbial resources have resulted in the discovery of a
number of extracellular polymers. Some of them have been
commercially accepted, while others are at various stages of
development. Production of extracellular polysaccharides by
halophilic bacteria highlighting their properties, distribution
and possible applications has been done over the last couple of
years. The main EPS producers so far reported are represented by
members of the family Halomonadaceae and Alteromonadaceae.
Members of the genus Halomonas, the most common moderately
producers synthesizing polymers of diverse physico-chemical
properties [9-11].
to the genus Halomonas was made by Quesada et al. [48] They
have optimized the cultural parameters of EPS production by
H. eurihalina and recorded a maximum production of 2.8 g/L.
showed an unusual property to jellify at acidic pH. Moreover, the
sulfate content of the EPS and cations affected the rheology of the
EPS. Bejar et al. [49] also established similar rheological behavior
of EPS isolated from strains of H. eurihalina. However, substrate
hydrocarbon and oil was established by Calvo et al. [50]. EPS
produced by H. eurihalina in hydrocarbon supplemented medium
showed enhanced emulsifying activity, but reduced viscosity. This
was possibly due to a change in chemical composition of the EPS
produced in hydrocarbon and oil supplemented media [40-51].
Halomonas maura
by Bouchotroch et al. [9,52] during isolation and screening of EPS
producing moderate halophiles. Later, Arias et al. [53] isolated an
anionic, sulfated heteropolysaccharide, mauran from H. maura
S-30, which under optimized cultural condition produced 3.8
g/L of highly viscous EPS. Mauran displayed pseudoplastic and
thixotropic rheological properties.
Under optimum cultural conditions EPS production by H.
ventosae and H. anticariensis [54] was 0.28 g/L and 0.49 g/L
respectively. Though, the production was comparatively low,
Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms 4/9
Copyright:
©2017 Biswas et al.
Citation: Biswas J, Paul AK (2017) Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms. Biodiversity Int J 1(2):
00006. DOI: 10.15406/bij.2017.01.00006
the polysaccharides exhibited high capacity of metal binding.
Moreover, these exopolymers showed emulsifying activity
to many hydrophobic substrates possibly due to their high
protein content and low viscosity. Extracellular polysaccharide
production by H. almeriensis [11, 55] have been optimized and
revealed a growth associated production of 1.7 g/L of EPS. The
sulfated EPS so produced was capable of emulsifying several
hydrophobic substrates and binding of metal ions. More recently,
Amjres et al. [56] characterized an extracellular polysaccharide,
haloglycan produced by H. stenophila HK30. Under optimized
cultural conditions, the strain produced 3.89 g/L of haloglycan
which was highly viscous and capable of emulsifying different
hydrocarbons.
Halomonas strain CRSS isolated from salt sediments of
Antarctica produced 2.9 g of EPS per g dry cells [57]. Mannan and
xylomannan were obtained when cells were grown on complex
media. However, in presence of acetate, a fructo-glucan was
produced. Halomonas spp. AAD6 isolated from Camalti saltern
area in Turkey was found to produce high levels of levan in the
sucrose containing medium and yielded 1.84 g/L of levan [58].
Besides these, Halomonas spp. V3a’ [59] was able to produce
an EPS as potential biosurfactant, while Halomonas spp. strain
TG39 [60] produced an EPS with high uronic acid content and
EPS derived from members of the family Alteromonadaceae were
low in viscosity and was capable of emulsifying hydrocarbons and
binding of heavy metals [61]. Salipiger mucosus A3T belonging
to Alphaproteobacteria, produced a fucose containing EPS (1.7
g/L) which showed solution properties similar to EPS of most
halophilic strains [62].
Optimization, isolation, and characterization of EPS produced
by H. xianhensis SUR308 were studied elaborately [63,64]. Under
optimum cultural conditions of 2.5% NaCl, 3% glucose, 0.5%
casein hydrolysate, the strain produced 7.87 g/L of EPS (Figure
3) [65] which showed antioxidant and emulsifying activity
against hydrocarbons as well as oils. However, a higher yield of
EPS (12.98 g/L) was successfully obtained with mutants of this
strain [66]. Massive amounts of EPS are also excreted by members
of the haloarchaeal genera Haloferax, Haloarcula, Halococcus,
Natronococcus and Halobacterium
to report the production of EPS by an archaebacterium, Haloferax
mediterranei (ATCC 33500). The structure of the neutral
extracellular polysaccharides of Haloferax gibbonsii (ATCC 33959)
has been determined by Paramonov et al. [68] while, Parolis et al.
[69] elucidated the structure of a linear, acidic exopolysaccharide
from In a screening program, Nicolaus et
al. [70] isolated three obligatory halophilic microorganisms (T5,
T6, and T7) from an unexplored site in Tunisia. These strains
produced sulfated extracellular polysaccharides in a minimal
medium containing glucose as sole carbon source with EPS yields
ranging from 35 to 370 mg/L. A comparative account of EPS
production by moderate and extreme halophiles so far reported
is presented in Table 1.
Table 1: Production of extracellular polymeric substances by moderate and extremely halophilic microorganisms.
Microorganism Medium Carbon Source and
NaCl Conc. (%)
Phase of Maximum
EPS Production
EPS
Production References
Moderate Halophile
Alteromonas hispanica F32TMY Medium Galactose; 7.5% NaCl Stationary phase 1.0 g/L Mata et al. [61]
Halomonas alkaliantarctica
strain CRSS Medium B Maltose; 7.5% NaCl ND 2.9 g/g Poli et al. [57]
H. alkaliphila Medium 2 1% Glucose; 10% NaCl ND ND Romano et al. [71]
H. almeriensis MSTMY Medium 1% Glucose; 7.5% NaCl Stationary phase 1.7 g/L Llamas et al. [11]
H. anticariensis MY Medium 1% Glucose; 7.5% NaCl Stationary phase 0.3-0.5 g/L Mata et al. [54]
H. eurihalina F2-7
MY Medium 1% Glucose; 7.5% NaCl Stationary phase
1.4 g/L
Quesada et al. [48]
H. eurihalina Al-12T2.8 g/L
H. maura S-30 MY Medium 1% Glucose; 2.5% NaCl Stationary phase 3.8 g/L Arias et al. [53]
H. rifensis MY Medium 1% Glucose; 7.5% NaCl ND ND Amjres et al. [72]
Figure 3: Time course of growth (O.D. at 540 nm (□), Dry weight in g/L
(°), glucose utilization (■) and production of EPS (▲) by H. xianhensis
SUR308 [63].
0246810 12 14
0
2
4
Growth, O. D. at 540 nm
Incubation, day
0
1
2
3
Gro wth, dry we ight , g/ L
0
2
4
6
8
10
Residual glucose, g/L
0
1
2
3
Production of EPS, g/L
Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms 5/9
Copyright:
©2017 Biswas et al.
Citation: Biswas J, Paul AK (2017) Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms. Biodiversity Int J 1(2):
00006. DOI: 10.15406/bij.2017.01.00006
H. smyrensis Medium B Glucose; 10% NaCl Stationary phase ND Poli et al. [73]
H. stenophila MY Medium 1% Glucose; 5% NaCl Stationary phase 3.89 g/L Llamas et al. [74]
H. ventosae Al-12T
MY Medium 1% Glucose; 7.5% NaCl Stationary phase
0.28 g/L
Mata et al. [54]
H. ventosae Al-16 0.30 g/L
Halomonas sp. AAD6 CD Medium 5% sucrose; 13.5%
NaCl Exponential phase 1.073 g/L Poli et al. [58]
H. xianhensis SUR308 MY Medium 3% glucose, 2.5% NaCl Stationary phase 7.87 g/L Biswas et al. [65]
Idiomarina fontislapidosi F-23TMY Medium Glucose; 7.5% NaCl Stationary phase 1.4 g/L Mata et al. [61]
I. ramblicola R-22TMY Medium Glucose; 7.5% NaCl Stationary phase 1.5 g/L Mata et al. [61]
Palleronia marisminoris MY Medium 1% Glucose; 5% NaCl Stationary phase ND Martinez-Checa et al. [75]
Salipiger mucosus A3TMY Medium 1% Glucose Stationary phase 1.2 g/L Llamas et al. [52]
Extreme Halophile
Haloarcula japonica Minimal
Medium Glucose Stationary phase 35-350 mg/L Nicolaus et al. [70]
Nd Glucose ND ND Parolis et al. [69]
H. gibbonsii Nd Glucose ND ND Paramonov et al. [68]
H. mediterranei Minimal
Medium Glucose/ yeast extract Stationary phase 3 mg/mL Anton et al. [67]
Halopiger aswanensis SG Medium Na-citrate ND ND Hezayen et al. [76]
documented.
Signicance of EPS Production
The extracellular polysaccharides in general play a wide
variety of biological functions including prevention of desiccation,
protection from environmental stresses, adherence to surfaces,
pathogenesis and symbiosis [77]. In addition EPS can sequester
nutrient materials from the surrounding environment [78],
EPS molecules strengthen the interactions between the
microorganisms and as a result they determine the cell aggregates
formation process on the solid surface [80]. Parker et al. [81]
established that removal of exopolysaccharides from the cells of
Bacillus spp. decreased their attachment to stainless steel surfaces.
Bacteria living within EPS are believed to be about 1000 times
more resistant to antibacterial compounds than planktonic cells
value of bacterial cell surface often restrict the penetration of
antimicrobial agents with hydrophobic character. The functional
groups of exopolysaccharides react with antimicrobial agents
and prevent the diffusion process to cytoplasm. However, it is
recognized that killing properties of antibiotics are increased
when all possible binding sites in the EPS matrix are saturated.
Mechanisms and Regulation of EPS Production
made in understanding the biochemical and genetic mechanisms
and regulation of different classes of EPS production by wide
diversity of bacterial species. With the exception of a few,
majority of bacterial EPS are synthesized intracellularly and
secreted to the extracellular environment. In general, regulation
of such intercellular biosynthesis of EPS is determined by
various physiological and metabolic parameters of the producer
cells. These include the availability of sugar precursors, energy
for building the repeating units, expression of enzymes for
polymerization and transportation of building units across the
membrane [84]. In addition, several factors such as medium
composition, culture age, type of carbon and nitrogen sources,
carbon to nitrogen ratio, pH, temperature and aeration have
limiting factors, such as stress conditions, osmolarity of the
EPS biosynthesis in a coordinated fashion. Recently, Ates [85]
has analyzed the application of omics technology and system
biology tools in understanding the microbial EPS biosynthesis
mechanism and regulation and pointed out that the general
mechanisms of bacterial polysaccharide production involve
Wzx/Wzy-dependent pathway, the ATP binding cassette (ABC)
transporter-dependent pathway and the synthase-dependent
pathway, while the extracellular synthesis is accomplished by
the use of a single sucrase protein. On the contrary, information
related to the genetics of microbial EPS biosynthesis particularly
polymerization, transportation and regulation are know only for
the biosynthesis of xanthan, levan and dextran, however, genetic
data pertaining to EPS biosynthesis by halophilic microorganisms
is scanty [86].
Mauran, the EPS produced by Halomonas maura S-30 is similar
to xanthan and has interesting functional properties that make it
Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms 6/9
Copyright:
©2017 Biswas et al.
Citation: Biswas J, Paul AK (2017) Diversity and Production of Extracellular Polysaccharide by Halophilic Microorganisms. Biodiversity Int J 1(2):
00006. DOI: 10.15406/bij.2017.01.00006
suitable for use in food and pharmaceutical industries and and
biotechnology. Analysis of genes involved in mauran production
three conserved genes, epsA, epsB and epsC, and demonstrated
their role in the assembly and translocation of mauran. A wzx
homologue, epsJ, was also found which indicates that mauran is
formed by a Wzy-dependent polymerization system. This EPS-
gene cluster reaches maximum activity during stationary phase,
in the presence of high salt concentrations (5% w/v).
Levan, a long linear homopolymeric EPS of ß (2-6)-linked
fructose residues is produced from sucrose-based substrates by a
halophilic bacterium Halomonas smyrnensis AAD6T [58]. However,
there is very limited information available about the mechanisms
involved in the biosynthesis of levan [88] and there is no report
about a systematic approach to analyze levan production by H.
smyrnensis AAD6T. Following this, systems-based approaches
were applied to improve the levan production capacity of H.
smyrnensis AAD6T. Mannitol as an effective stimulatory factor
for levan production has also been analyzed systematically [89].
several genes related to EPS biosynthesis, including the genes for
levansucrase and ExoD. More recently, whole-genome analysis of
H. smyrnensis AAD6T by Diken et al. [91] revealed Hs_SacB gene
encoding the extracellular levan sucrase which catalyzes levan
synthesis from sucrose-based substrates by transfructosylation
[86] and bear striking similarities with levansucrases from
Pseudomonas strains.
Halophilic EPS: Properties and Applications
Among the halophilic bacteria, the main EPS producers belong
to members of Halomonas, Alteromonas and Idiomarina and the
polymers produced by them are characterized by distinct physical
and biological properties for exploitation in biotechnological,
industrial and environmental purposes. Most of the EPS produced
by halophiles characteristically form both high and low viscous
solutions. EPS produced by H. maura form highly viscous solution
(800 cP) while that of H. eurihalina jellifying at acidic pH have
food industries [48,53]. In general EPS obtained from most of
the species of Halomonas [9,49,92] also possess pseudoplastic,
thixotropic and shear thinning rheological properties. Moreover,
EPS produced by H. xianhensis SUR308 was stable over a
number of different stress conditions and the viscosity of the
polysaccharide solution remains unaltered at extreme pH,
temperature and high concentrations of salts [63].
EPS produced by H. maura, H. almeriensis, H. xianhensis and
Salipiger mucosus were able to emulsify hydrocarbons, crude oils,
mineral oils, hexadecane, tetradecane, octane and many others
[63,92]. Moreover, the highest anionic activity of EPS produced
by Halomonas spp. TG39 was correlated with 100% emulsifying
capacity of hexadecane [60].
Antioxidant activity of extracellular polysaccharides
derived from halophilic strains is not common but recently, the
extracellular polysaccharide of H. xianhensis SUR308 has been
shown to exert 43 to 72% DPPH radical scavenging activity at
concentrations ranging from 0.06 to 1 mg/mL [63]. The sulphated
EPS from H. eurihalina
of human lymphocytes in response to the presence of the anti-CD3
mononuclear antibody [93], while that of H. stenophila (B100 and
N12T) [74] blocked the growth of human T-lymphocyte tumours
[94]. The halophilic EPS has the property of removing toxic heavy
metals and synthetic dyes present in anthropogenic environment.
The, EPS from Halomonas spp. TG39 was capable of removing
methylene blue at the rate of 464 mg/g of EPS [60] while removal
of polycyclic aromatic hydrocarbons such as naphthalene,
producing strain H. eurihalina H-28 [95,96].
Conclusion
From the above survey it is apparent that studies on
the diversity of halophilic microorganisms from natural
environments have received a momentum in the recent past, but
the tremendous diversity of halophiles are far from being explored
and exploited. Production of exopolysaccharides by a number of
potent moderate and extremely halophilic Archaea and Bacteria
has been optimized under laboratory conditions and found to
generated interests for potential applications and exploration
in a commercially viable manner. It may also be mentioned that
mass cultivation of halophiles for EPS production and overcoming
constrains of process development and bioreactor designing and
construction for halophiles will help in the commercialization
of the process. Finally, as an outcome, this neglected group of
the world of biotechnology.
Acknowledgement
Grants Commission, India under the scheme of Rajiv Gandhi
National Fellowship (Sanction No. F.14-2(SC)/2008 (SA-III), 31
March, 2009).
Conict of Interest
Authors have declared that no competing interests exist.
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