4
Bacterial Cellulose
The.occurrence.of.these.mutants.is.not.necessarily.due.to.a.high.rate.of.mutation.as.much.as.their.ability.to.stay.free.and.divide.normally.without.being.
trapped.in.the.cellulose.product.where.cellulose-producing.cells.are.trapped.
(Valla.and.Kjosbakken.1982)..The.cellulose-producing.cells.have.a.selective.
advantage.in.a.static.culture,.as.these.cells.are.still.attached.to.the.cellulose.
product,. and. as. the. cellulose. pellicle. floats. on. the. surface,. these. cells. are.
brought. to. the. air–liquid. interface.. In. the. same. culture,. Cel − . cells. are. not.
attached. to. any. cellulose. microfibrils,. and. even. though. they. may. produce.
small.amounts.of.cellulose,.they.do.not.get.trapped.in.the.cellulose.pellicle.
and.settle.to.the.bottom.of.the.container.where.their.growth.is.constrained.
as.the.concentration.of.diffused.oxygen.becomes.limiting.
G. xylinus.strains.have.the.ability.to.utilize.a.variety.of.sugars.and.other.
compounds.for.the.synthesis.of.cellulose,.and.the.pathways.of.carbon.metabolism.in.this.bacterium.have.been.summarized.very.well.by.Ross.et al..(1991)..
In. . laboratory. cultures. of. G. xylinus,. glucose. is. commonly. provided. in. the.
medium,.and.essentially.four.enzymatic.steps.have.been.characterized.in.the.
pathway.from.glucose.to.cellulose..These.are.the.phosphorylation.of.glucose.by.
glucokinase,.the.isomerization.of.glucose-6-phosphate.to.glucose-1-phosphate.
by. phosphoglucomutase,. the. synthesis. of. uridine. diphosphate. glucose.
(UDP-glucose). from. glucose-1-phosphate. by. UDP-glucose. pyrophosphorylase,.and.the.synthesis.of.cellulose.from.UDP-glucose.by.cellulose. .synthase.
(Ross.et.al..1991)..Although.a.large.fraction.(almost.50%).of.the.supplied.glucose.ends.up.in.cellulose,.a.significant.fraction.of.glucose.in.the.medium.is.
converted.to.gluconic.acid.in.the.early.stages.of.culture.of.G. xylinum .cells..
Even.though.carbon.from.gluconic.acid.does.end.up.in.cellulose. .in.the.later.
stages.of.culture,.one.of.the.goals.in.the.commercial.production.of.BNC.has.
been. to. isolate. strains. where. the. production. of. gluconic. acid. is. reduced..
However,.such.efforts.have.resulted.in.only.a.marginal.increase.in.the.yield.of.
cellulose..A.much.more.intriguing.aspect.of.carbon.metabolism.that.is.linked.
to. the. aerobic. lifestyle. of. G. xylinus. and. its. inability. . to. metabolize. glucose.
anaerobically. is. the. lack. of. phosphofructokinase. activity. . in. this. bacterium.
(Ross.et.al..1991)..Whether.this.alone.is.the.reason.why.G. xylinus.is.unable.to.
metabolize.glucose.anaerobically.can.now.be.determined.through.. analysis.
of. the. genome. sequences. available. for. two. different. strains. of. G. xylinus.
(Iyer et al..2010;.Ogino.et.al..2011).
Structure of Cellulose I is Determined by the
Cell-Directed Assembly of Glucan Chains
Microscopic. studies. of. bacterial. cells. producing. cellulose. show. the. presence. of. cellulose. ribbons. attached. to. the. longitudinal. axis. of. the. cell,. and.
Bacterial Cellulose
The.occurrence.of.these.mutants.is.not.necessarily.due.to.a.high.rate.of.mutation.as.much.as.their.ability.to.stay.free.and.divide.normally.without.being.
trapped.in.the.cellulose.product.where.cellulose-producing.cells.are.trapped.
(Valla.and.Kjosbakken.1982)..The.cellulose-producing.cells.have.a.selective.
advantage.in.a.static.culture,.as.these.cells.are.still.attached.to.the.cellulose.
product,. and. as. the. cellulose. pellicle. floats. on. the. surface,. these. cells. are.
brought. to. the. air–liquid. interface.. In. the. same. culture,. Cel − . cells. are. not.
attached. to. any. cellulose. microfibrils,. and. even. though. they. may. produce.
small.amounts.of.cellulose,.they.do.not.get.trapped.in.the.cellulose.pellicle.
and.settle.to.the.bottom.of.the.container.where.their.growth.is.constrained.
as.the.concentration.of.diffused.oxygen.becomes.limiting.
G. xylinus.strains.have.the.ability.to.utilize.a.variety.of.sugars.and.other.
compounds.for.the.synthesis.of.cellulose,.and.the.pathways.of.carbon.metabolism.in.this.bacterium.have.been.summarized.very.well.by.Ross.et al..(1991)..
In. . laboratory. cultures. of. G. xylinus,. glucose. is. commonly. provided. in. the.
medium,.and.essentially.four.enzymatic.steps.have.been.characterized.in.the.
pathway.from.glucose.to.cellulose..These.are.the.phosphorylation.of.glucose.by.
glucokinase,.the.isomerization.of.glucose-6-phosphate.to.glucose-1-phosphate.
by. phosphoglucomutase,. the. synthesis. of. uridine. diphosphate. glucose.
(UDP-glucose). from. glucose-1-phosphate. by. UDP-glucose. pyrophosphorylase,.and.the.synthesis.of.cellulose.from.UDP-glucose.by.cellulose. .synthase.
(Ross.et.al..1991)..Although.a.large.fraction.(almost.50%).of.the.supplied.glucose.ends.up.in.cellulose,.a.significant.fraction.of.glucose.in.the.medium.is.
converted.to.gluconic.acid.in.the.early.stages.of.culture.of.G. xylinum .cells..
Even.though.carbon.from.gluconic.acid.does.end.up.in.cellulose. .in.the.later.
stages.of.culture,.one.of.the.goals.in.the.commercial.production.of.BNC.has.
been. to. isolate. strains. where. the. production. of. gluconic. acid. is. reduced..
However,.such.efforts.have.resulted.in.only.a.marginal.increase.in.the.yield.of.
cellulose..A.much.more.intriguing.aspect.of.carbon.metabolism.that.is.linked.
to. the. aerobic. lifestyle. of. G. xylinus. and. its. inability. . to. metabolize. glucose.
anaerobically. is. the. lack. of. phosphofructokinase. activity. . in. this. bacterium.
(Ross.et.al..1991)..Whether.this.alone.is.the.reason.why.G. xylinus.is.unable.to.
metabolize.glucose.anaerobically.can.now.be.determined.through.. analysis.
of. the. genome. sequences. available. for. two. different. strains. of. G. xylinus.
(Iyer et al..2010;.Ogino.et.al..2011).
Structure of Cellulose I is Determined by the
Cell-Directed Assembly of Glucan Chains
Microscopic. studies. of. bacterial. cells. producing. cellulose. show. the. presence. of. cellulose. ribbons. attached. to. the. longitudinal. axis. of. the. cell,. and.