Civil Engineering Reference
In-Depth Information
synthase to conserve the high energy between glucose and fructose in sucrose, and that
sucrose is used for UDP-glucose synthesis and can therefore increase cellulose synthe-
sis, a sucrose synthase gene was introduced into
G. xylinus
[110] . h is gene, isolated
from mung bean (
Vigna radiata
), was under the control of a
lac
promoter, and resulted
in carbon directly from sucrose being incorporated into cellulose (via UDP-glucose)
and also prevented UDP accumulation.
Considering
Gluconacetobacter
's dependence on oxygen concentration, Chien
et
al.
[111] attempted to introduced the
Vitreoscilla
haemoglobin gene, which allows
Vitreoscilla
to grow in oxygen-poor environments.
G. xylinus
was transformed with
a plasmid containing the haemoglobin gene under the control of a
bla
promoter, and
demonstrated increased cellulose production in static culture under microaerophillic
conditions. It is believed that lowered oxygen tensions limited the production of glu-
conic acid, and subsequently increased cellulose production.
Many of the mutants described here were isolated via natural means rather than
by genetic manipulation techniques. h ey were selected specii cally based on previ-
ous observation of the factors that enhance cellulose production. It may be of interest
to create a transposon library and determine if any randomly created mutants lead to
changes, either in structure or yield, in cellulose.
From a review of the literature, it is apparent that large changes in cellulose yield can
be obtained from varying the bacterial species or strain (whether it be a mutant or a
naturally occurring strain), media composition or cultivation conditions. Determining
an appropriate combination of these factors to produce high amounts of cellulose at
a reasonable cost is a necessary step in the development of composite materials using
bacterial cellulose. Further considerations of the properties must also be taken into
account and are discussed in Section 4.3.
4.3
Tailor-Designing Bacterial Cellulose
4.3.1
Modifying the Properties of Bacterial Cellulose
Bacterial cellulose exhibits properties such as nanosized i bers and high crystallinity
that confers high stif ness and makes it suitable as a reinforcement material [112], how-
ever it also has some signii cant disadvantages. While bacterial cellulose would have
a natural ai nity to hydrophilic matrices due to its hydrophilic nature, it would have
an inherent incompatibility to hydrophobic matrices. h is is a very important factor
when determining the overall success of a composite material. Interaction between
the two materials is important as it leads to the determination of the properties, as
good mechanical properties result from good adhesion between the two materials in a
composite, and weak interfacial adhesions result in poor mechanical properties [113].
However it is possible to obtain a variety of modii cations to bacterial cellulose due to
its inherent nature of being biological and its chemical structure. h ese qualities pro-
vide the opportunity to alter its properties in favor of achieving specii c characteristics
using a variety of techniques. h is cellulose is cultivated by bacteria, which allows for
samples to be produced quickly. h ere is also a huge number of ways to change the
growth conditions, from changing the media by varying the carbon and/or nitrogen
