30
Bacterial Cellulose
compared.to.VHb−.bacteria.(no.VHb.expression).with.a.lag.time.of.6.hours..
This.is.probably.due.to.an.increased.efficiency.of.oxygen.uptake.in.the.bacteria.caused.by.the.hemoglobin..Differences.have.also.been.observed.between.
VHb+.and.VHb−.in.static.cultures.under.normal.oxygen.conditions..Under.
microaerobic. conditions,. VHb. expression. was. shown. to. enhance. . cellulose.
production.two-fold..It.is.speculated.that.the.increased.production.of.BNC.
is.dependent.on.increased.production.of.ATP.and.that.the.glucose.could.be.
used.for.BNC.production.instead.of.ATP.synthesis.(Chien.et.al..2006)..This.
assumption. is. based. on. the. findings,. demonstrated. in. E. coli,. where. VHb.
expression.enhances.respiratory.activity.and.increases.ATP.production.
Spontaneous Mutation in G. xylinus Can Lead to a
Loss of the Ability to Produce Cellulose
G. xylinus.is.prone.to.spontaneous.mutations.sometimes.resulting.in.a.reduced.
or. lost. ability. to. produce. cellulose. (Cel − ).. This. type. of. mutation. was. first.
described.by.Schramm.and.Hestrin.in.1954.and.is.more.common.in.agitated.
and.aerated.cultures.than.in.static.cultures..Schramm.and.Hestrin.found.three.
different.types.of.G. xylinus:.the.first.type.was.the.G. xylinus.wild-type.strain.
that.produced.cellulose.(Cel+).and.the.other.two.were.low.producers.of.cellulose. (Cel − ).. To. prevent. mutation. of. G. xylinus. in. agitated. cultures,. ethanol.
can. be. added. to. the. cultivation. medium. (Son. et. al.. 2001).. There. are. several.
other.studies.of.G. xylinus.mutations.and.different.properties.of.the.mutants.
have.been.discovered..Valla.and.Kjosbakken.(1982).reported.the.probability.of.
an.indirect.connection.between.cellulose.production.and.the.production.of.a.
soluble.extracellular.polysaccharide.in.Cel − mutants,.since.these.produce.large.
amounts.of.extracellular.polysaccharides,.such.as.acetan..In.the.same.study.it.
was.also.demonstrated.that.Cel − .mutants.grow.faster.than.cellulose-producing.
cells.. Furthermore,. according. to. Valla. et. al.. (1987),. the. plasmids. by. themselves.can.also.be.involved.in.cellulose.production,.since.many.mutants.have.
. demonstrated.changes.in.their.plasmid.DNA.profile.in.comparison.with.the.
wild-type.G. xylinus.strain..The.mechanism.is.difficult.to.understand.clearly.
since. G. xylinus. has. a. complex. system. of. plasmids. (Rezaee. et. al.. 2005).. One.
explanation.may.be.that.the.Cel − .bacteria.lack.two.enzymes,.phosphoglucomutase. and. glucose-1-phosphate. uridylyltransferase. (Krystynowicz. et. al.. 2005),.
which.are.both.key.enzymes.in.cellulose.production.in.G. xylinus..Moreover,.
spontaneous. mutations. of. G. xylinus. into. Cel − . can. be. caused. by. insertion.
sequences.(ISs)..Movement.of.the.IS.can.lead.to.inactivation.of.genes.involved.
in.the.cellulose.synthesis..Cel − .mutants.have.been.shown.to.have.two.or.more.
IS.elements.and.their.DNA.appears.to.be.rearranged.within.the.IS.element..
This.was.not.the.case.for.Cel + .G. xylinus (Coucheron.1991).
In.Cel − .mutants.three.enzymes.were.shown.to.be.upregulated.while.two.
were. downregulated,. as. demonstrated. by. Nguyen. et. al.. (2010).. One. of. the.
upregulated.enzymes.was.deoxythymidine.diphosphate.(dTDP)-4-dehydrorhamnose.3,5-epimerase,.which.can.be.associated.with.a.decrease.in.cellulose.