Civil Engineering Reference
In-Depth Information
outwards and this sinking is prevented, a decrease in the production of cellulose occurs.
Design of appropriate apparatus is therefore clearly of importance. In conical l asks, the
cellulose sinks into the medium rather than sliding down the walls of the l ask, thus
eliminating the wall ef ect and leading to improved cellulose yield [103]. Designing a
cultivation system, such as a bioreactor, with a dei ned medium that would allow the
cost-ef ective production of bacterial cellulose is the ultimate goal. Determining a set
of conditions by which to produce this material is necessary if bacterial cellulose is to
be used for other applications such as material science. h erefore further investiga-
tion into these aspects of bacterial cellulose growth is required. Changes that occur to
the structure and morphology of the cellulose as a result of growth in these reactors
are discussed with other
in-situ
modii cations that occur due to changes in media in
Section 4.3.2.
4.2.2.3.9 MutantStrains
In addition to the work on media and cultivation conditions, several studies examin-
ing mutant strains of bacteria, either naturally-occurring or specii cally created using
genetic modii cation techniques, with increased cellulose levels have been completed.
As production of gluconic acid leads to a decrease in pH and cellulose production (with
the cellulose production decreasing as a result of both the pH decrease and incorpo-
ration of the carbon source into gluconic acid instead of cellulose), an early study on
Gluconacetobacter
mutant strains focused on the isolation and cultivation of mutants
with restricted gluconic acid production [104]. Bacterial cellulose produced from a
non-gluconic acid-producing mutant was found to be increased over the wild type.
An increase in cellulose production was observed in a mutant strain with resistance
to sulfaguanidine [105, 106]. h is mutant was selected based on the observation that
p
-aminobenzoic acid increases cell growth and cellulose production. Resistance to sul-
faguanidine, an analogue of
p
-aminobenzoic acid, is thought to enhance high-energy
compounds such as ATP, which is required for cellulose production [105]. Similarly, a
5-l urouridine-resistant mutant was isolated with increased cellulose production [107].
h is mutant was shown to have increased intracellular levels of UDP-glucose, the direct
precursor of cellulose. A strain of
G. xylinus
subspecies
sucrofermentans
named BPR2001
has been reported as being used to breed mutant strains [108]. BPR2001 was isolated
from a natural source and was found to produce high levels of acetan, a water-solu-
ble polysaccharide. As UDP-glucose is a precursor of acetan (and cellulose), a mutant
lower in acetan production and high in cellulose production was the target. h is was
obtained by treatment of BPR2001 cells with
N-
methyl
-N'-
nitro-
N
-nitrosoguanidine,
and the new mutant was named BPR3001A. In another study examining the ef ects of
acetan production, wild type BPR2001 was used to make an
aceA
mutant [109] . h e
aceA
gene is believed to be involved in the synthesis of acetan, and as a result of its
disruption, the mutant strain, named EP1, could no longer produce acetan. However,
this strain also produced signii cantly less cellulose than the wild type. When acetan
was added to the culture medium, cellulose synthesis increased to wild type levels. h e
authors concluded that acetan and cellulose are not genetically related, but that acetan
has a physiochemical ef ect on the culture conditions that stimulates cellulose produc-
tion. Increased levels of cellulose have also been obtained by introducing genes from
other species into
Gluconacetobacter.
Based on the observation that plants use sucrose
