24
Bacterial Cellulose
Factors Involved during Nanocellulose
Network Formation by G. xylinus
There.are.different.theories.about.the.role.of.cellulose.production.by.G. xylinus,.
which.results.in.a.nanocellulose.network..The.cellulose.is.believed.to.prevent.
the.bacteria.from.dehydrating.and.also.helps.the.bacteria.to.become.floatable,.
via.entrapment.of.carbon.dioxide.(produced.from.the.tricarboxylic.acid.[TCA].
cycle)..This.helps.the.bacteria.to.reach.the.air–liquid.interface,.an.important.
issue. since. the. bacteria. is. an. obligate. aerobe. (Schramm. and. Hestrin. 1954)..
The.cellulose.biofilm.is.also.a.natural.way.of.immobilizing.the.bacteria.and.
helps.it.attach.to.the.surface.(Cook.and.Colvin.1980)..Moreover,.the.biofilm.has.
been.shown.to.protect.the.bacteria.from.ultraviolet.(UV).light.to.some.extent.
(Williams.and.Cannon.1989)..Many.external.factors.affecting.the.morphology.
of.the.nanocellulose.network.and.the.production.rate.of.. cellulose.exist..Oxygen.
is.one.important.factor,.demonstrated.by.Watanabe.and.Yamanaka.(1985).in.
their.study,.where.they.show.that.oxygen.tension.has.a.great.influence.on.the.
cellulose. network. produced.. Oxygen. tension. greater. than. 40%. results. in. a.
higher.growth.rate.of.G. xylinus.as.compared.to.when.the.bacteria.are.cultured.
at.oxygen.tensions.of.less.than.15%.(Watanabe.and.Yamanaka.1985).
The.branching.of.the.nanocellulose.network.is.affected.by.the.proliferation.rate.of.G. xylinus.(Figure 2.3A).(Yamanaka.et.al..1989),.which.is.in.turn.
dependent. on. the. oxygen. tension. (Figure  2.3B). (Watanabe. and. Yamanaka.
1985).and.affects.the.density.of.the.cellulose.network.produced.
Prior.to.cell.division,.the.catalytic.sites.for.cellulose.production.are.doubled.
and. activated. directly. after. division.. No. breakage. of. the. cellulose. . ribbon.
occurs. after. division,. just. the. creation. of. a. branching. point. (Figure  2.3A).
(Watanabe. and. Yamanaka. 1985).. When. the. dissolved. oxygen. tension. is.
around.50%.of.atmospheric.pressure,.a.denser.nanocellulose.network.is.produced;.if.a.lower.oxygen.tension.is.applied,.the.cellulose.network.becomes.
less. dense. (Figure  2.3B).. Watanabe. and. Yamanaka. found. that. the. cellulose. content. increased. as. the. oxygen. tension. increased.. The. opposite. was.
found.for.lower.oxygen.tension,.causing.a.decrease.in.the.cellulose.content..
This. phenomenon. is. probably. due. to. the. cell. division. rate. of. the. bacteria,.
as.a.correlation.between.oxygen.tension.and.cellulose.network.density.was.
observed..Therefore.the.thickness.of.the.cellulose.membrane.is.only.affected.
by.the.branching.of.the.network,.not.by.an.increased.width.of.the.cellulose.
fibrils.produced.(Watanabe.and.Yamanaka.1985).
The.degree.of.polymerization.(DP).is.the.number.of.repeated.units.in.a.polymer,.which.in.cellulose.is.D-glucose..The.mechanism.for.development.of.the.
DP.has.been.suggested.to.follow.a.Poisson.distribution,.which.means.that.all.
molecules.have.the.same.probability.to.be.added.to.the.growing.glucan.chain..
The.DP.in.BNC.is.relatively.high,.around.2000–4000.(Marx-Figini.and.Pion.
1974),.whereas.the.cultivation.conditions.can.have.an.impact.on.the.DP.of.the.
produced.cellulose..In.an.agitated.culture.of.G. xylinus,.the.DP.of.the.cellulose.