Medical Microbiology

296 4 BacteriaasHumanPathogens

dermalandsubcutaneousinfections,andsepsis.Suchinfectionsonlyoccur

inpredisposedhosts,theyarefrequentlyseeninpatientswithsevereprimary

diseases.AnotherreasonwhyopportunisticEnterobacteriaceaehavebecome

soimportantinhospitalmedicineisthefrequentdevelopmentofresistance

toanti-infectiveagents,whichabilityenablesthemtopersistatlocations

whereuseofsuchagentsisparticularlyintensive,i.e.,inhospitals.Occur-

renceofmultipleresistanceinEnterobacteriaceaeisduetotheimpressive

geneticvariabilityoftheseorganisms(p. 17 0).Table4. 9 providesanoverview

ofthemostimportantopportunisticEnterobacteriaceae.

Vibrio,Aeromonas,andPlesiomonas..............................

&Vibriocholeraeisthemostimportantspeciesinthisgroupfromamedical

pointofview.CholeravibriosareGram-negative,comma-shaped,monotri-

chouslyflagellatedrods.Theyshowalkalitolerance(pH9),whichisusefulfor

selectiveculturingofV.choleraeinalkalinepeptonewater.Theprimarycho-

lerapathogenisserovarO:1.NonO:1strains(e.g.,O:139)causethetypical

clinicalpictureinrarecases.O:1vibriosarefurthersubdividedintothebio-

varscholeraeandeltor.Thediseasedevelopswhenthepathogensenterthe

intestinaltractwithfoodordrinkingwaterinlargenumbers(>– 108 ).Thevi-

briosmultiplyintheproximalsmallintestineandproduceanenterotoxin.

Thistoxinstimulatesaseriesofreactionsinenterocytes,theendresultof

whichisincreasedtransportofelectrolytesoutoftheenterocytes,whereby

waterisalsolostpassively.Massivewaterydiarrhea(upto 20 l/day)resultsin

exsiccosis.Theinitialtherapeuticfocusisthusonreplacementoflostelec-

trolytesandwater.Choleraoccursonlyinhumans.Preventivemeasurescon-

centrateonprotectionfromexposuretotheorganism.Akilledwholecell

vaccineandanattenuatedlivevaccineareavailable.Theyprovideonlya

moderatedegreeofprotectionoveraperiodofonlysixmonths.International

healthcaresourcesreportanincubationperiodoffivedays. &

ThebacteriainthesegroupsareGram-negativerodswithacommaorspiral

shape.Theirnaturalhabitatisinmostcasesdampbiotopesincludingthe

ocean.Someofthemcauseinfectionsinfish(e.g.,Aeromonassalmonicida).

ByfarthemostimportantspeciesintermsofhumanmedicineisVibriocho-

lerae.

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Kayser, Medical Microbiology © 2005 Thieme