specificities. The microbial strain producing its specific EPS has also enzymic tools
to digest it back to monomers.
It is rather easy to isolate CGTases from soil microbes, i.e., it is not a rare enzyme.
It is costly to a microbial cell to synthetize an enzyme, and thus the enzyme must
have an ecological meaning. There have been repeated questions about the benefits
to produce CDs. One answer is that CDs can act as complexing agents to smallmolecular hydrophobic compounds (like siderophore for metal ions). For example,
lipids or vitamins could be “wrapped” inside CDs. Protein side chains can complex
with CDs (a correlation to compatible solutes). CDs could be functional also in
microbial signaling between them and between other life forms. CDs in soil can
retard germination of seeds and distort growth of micropropagated plants [84]. Typical symptoms of imbalance of growth factors were found (e.g., callus formation)
suggesting that CDs interfere with them. The retardation of germination of seeds was
suggested to be due to disturbances in gibberellin signaling in grain and not being
due to the amylase inhibition as thought earlier (solid-bound CDs had similar effect
as free). For a bacterium, postponing of the germination of a seed and/or inducing
abnormal morphological changes would increase probability of decay of a seed or
plantlet and thus create access to nutrients. Thus, there exist various reasons why
CDs can be ecologically advantageous.
EPSs produced by alkaliphiles are far from explored. Those studied
exopolysaccharides-producing alkaliphiles belong mainly to haloalkaliphiles and
representatives of genus Bacillus. Like others, alkaliphilic producers of EPSs have
been found from different sources, some being exotic like a shallow hydrothermal
vent [85], radioactive hot spring [86], or saline lake Cape Russell in Antarctica [87],
or they were even scraped off from the deteriorating marble of Moscow Kremlin
masonry [88].
Optimization of the bioprocess plays a crucial role in reducing the production
cost of EPSs. The production depends on factors like physiological properties of
employed microbial species and cultural condition. A maximum yield of EPSs of
18.5 g/L was observed when a strain of Bacillus sp. isolated from sugarcane field in
Bhilai, India, was cultured at 37
C. The strain was able grow in a 4% NaCl, in a pH
range of 3–10 and at a temperature range of 27–70
C [89]. The very high pH range
of cultivation implies to a significant role of EPSs in protection of the microbe. EPS
production of 12.9 g/L was also achieved by Halobacillus trueperi when cultivated
at pH 9 and 35
C for 72 h [90]. When grown at 37
C and pH 8, a strain of
Exiguobacterium aurantiacum, a member of the Bacilli class, isolated from
Marchica lagoon in Morocco synthesized maximum of EPS, 2.6 g/L. Chemical
analysis showed that the temperature and pH have no effect on the protein content
of EPS, while the carbohydrate amount varied [91].
The EPSs of alkaliphiles represent a diverse group of biopolymers probably for
their benefits in adaptation to alkaline environments. An exopolysaccharide obtained
from an alkaliphilic Bacillus spp. was found along with D-galactopyranuronic
acid (GalpA), 2,4-diacetamido-2,4,6-trideoxy-D-glucopyranose (QuipNAc4NAc),
2-acetamido-2-deoxy D-mannopyranuronic acid (ManpNAcA), and one uncommon
unit of D-galactopyranuronic acid with the carboxyl group amide linked to
178
E. Khalikova et al.
to digest it back to monomers.
It is rather easy to isolate CGTases from soil microbes, i.e., it is not a rare enzyme.
It is costly to a microbial cell to synthetize an enzyme, and thus the enzyme must
have an ecological meaning. There have been repeated questions about the benefits
to produce CDs. One answer is that CDs can act as complexing agents to smallmolecular hydrophobic compounds (like siderophore for metal ions). For example,
lipids or vitamins could be “wrapped” inside CDs. Protein side chains can complex
with CDs (a correlation to compatible solutes). CDs could be functional also in
microbial signaling between them and between other life forms. CDs in soil can
retard germination of seeds and distort growth of micropropagated plants [84]. Typical symptoms of imbalance of growth factors were found (e.g., callus formation)
suggesting that CDs interfere with them. The retardation of germination of seeds was
suggested to be due to disturbances in gibberellin signaling in grain and not being
due to the amylase inhibition as thought earlier (solid-bound CDs had similar effect
as free). For a bacterium, postponing of the germination of a seed and/or inducing
abnormal morphological changes would increase probability of decay of a seed or
plantlet and thus create access to nutrients. Thus, there exist various reasons why
CDs can be ecologically advantageous.
EPSs produced by alkaliphiles are far from explored. Those studied
exopolysaccharides-producing alkaliphiles belong mainly to haloalkaliphiles and
representatives of genus Bacillus. Like others, alkaliphilic producers of EPSs have
been found from different sources, some being exotic like a shallow hydrothermal
vent [85], radioactive hot spring [86], or saline lake Cape Russell in Antarctica [87],
or they were even scraped off from the deteriorating marble of Moscow Kremlin
masonry [88].
Optimization of the bioprocess plays a crucial role in reducing the production
cost of EPSs. The production depends on factors like physiological properties of
employed microbial species and cultural condition. A maximum yield of EPSs of
18.5 g/L was observed when a strain of Bacillus sp. isolated from sugarcane field in
Bhilai, India, was cultured at 37
C. The strain was able grow in a 4% NaCl, in a pH
range of 3–10 and at a temperature range of 27–70
C [89]. The very high pH range
of cultivation implies to a significant role of EPSs in protection of the microbe. EPS
production of 12.9 g/L was also achieved by Halobacillus trueperi when cultivated
at pH 9 and 35
C for 72 h [90]. When grown at 37
C and pH 8, a strain of
Exiguobacterium aurantiacum, a member of the Bacilli class, isolated from
Marchica lagoon in Morocco synthesized maximum of EPS, 2.6 g/L. Chemical
analysis showed that the temperature and pH have no effect on the protein content
of EPS, while the carbohydrate amount varied [91].
The EPSs of alkaliphiles represent a diverse group of biopolymers probably for
their benefits in adaptation to alkaline environments. An exopolysaccharide obtained
from an alkaliphilic Bacillus spp. was found along with D-galactopyranuronic
acid (GalpA), 2,4-diacetamido-2,4,6-trideoxy-D-glucopyranose (QuipNAc4NAc),
2-acetamido-2-deoxy D-mannopyranuronic acid (ManpNAcA), and one uncommon
unit of D-galactopyranuronic acid with the carboxyl group amide linked to
178
E. Khalikova et al.