Medical Microbiology

Vibrio,Aeromonas,andPlesiomonas 297

Vibriocholerae(Cholera)..............................

Morphologyandculture.CholeravibriosareGram-negativerodbacteria,

usuallyslightlybent(comma-shaped), 1 .5– 2 lminlength,and0.3–0.5lm

wide,withmonotrichousflagellation(Fig.4. 19 ).

CulturingofV.choleraeispossibleonsimplenutrientmediumsat 378 Cin

anormalatmosphere.Owingtoitspronouncedalkalistability,V.choleraecan

beselectivelyculturedoutofbacterialmixturesatpH9.

Antigensandclassification.V.choleraebacteriaaresubdividedintoserovars

basedontheirOantigens(lipopolysaccharideantigens).Theserovarpatho-

genisusuallyserovarO:1.StrainsthatdonotreacttoanO:1antiserumare

groupedtogetherasnonO:1vibrios.NonO:1strainswererecentlydescribed

inIndia(O:139)asalsocausingtheclassicclinicalpictureofcholera.O:1vi-

briosarefurthersubclassifiedinthebiovarscholeraeandeltorbasedonphy-

siologicalcharacteristics.Thevareltorhasaverylowlevelofvirulence.

Choleratoxin.Choleratoxinisthesolecauseoftheclinicaldisease.Thissub-

stanceinducestheenterocytestoincreasesecretionofelectrolytes,aboveall

Cl–ions,wherebypassivewaterlossalsooccurs.Thetoxinbelongstothe

groupofABtoxins(seep. 1 6).SubunitBofthetoxinbindstoenterocytere-

ceptors,theactivetoxinsubunitAcausestheadenylatecyclaseintheenter-

ocytestoproducecAMPcontinuouslyandinlargeamounts(Fig.4. 20 ).cAMP

inturnactsasasecondmessengertoactivateproteinkinaseA,whichthen

activatesthespecificcellproteinsthatcontrolsecretionofelectrolytes.The

toxingenesctxAandctxBarecomponentsoftheso-calledCTXelement,

whichisintegratedinthenucleoidoftoxiccholeravibrios(seelysogeniccon-

version,p. 1 86)aspartofthegenomeofthefilamentousprophageCTXf.The

CTXelementalsoincludesseveralregulatorgenesthatregulatebothproduc-

Vibriocholerae

Fig.4. 19 Comma-shaped

rodbacteriawithmono-

trichousflagellation(SEM

image).

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Kayser, Medical Microbiology © 2005 Thieme