Medical Microbiology

252 4 BacteriaasHumanPathogens

Erysipelothrixrhusiopathiae,thepathogenthatcausesthezoonosis

swineerysipelas,isthecausativeorganisminthehumaninfectionerysipe-

loid,nowarareoccupationaldisease.

Gardnerellavaginalisisusuallyresponsible,incombinationwithother

bacteria,fornonspecificvaginitis(vaginosis). &

Listeriamonocytogenes...............................

TheonlylisteriaethatcausehumandiseaseareL.monocytogenesandthe

rarespeciesL.ivanovii.ThedesignationL.monocytogenesresultsfromthe

observationthatinfectionsofrodents,whicharemuchmoresusceptible

thanhumans,areaccompaniedbyamonocytosis.

Morphologyandculturing.ThesmallGram-positiverodsfeatureperi-

trichousflagellation.Theyshowgreatermotilityat 208 Cthanat 378 C.Cul-

turingismostsuccessfulunderaerobicconditionsonbloodagar.Following

incubationfor 18 hours,smallgraycoloniessurroundedbyinconspicuous

hemolyticzonesappear.ThezonesarecausedbylisteriolysinO.Listeriae

canalsoreproduceat 5 – 108 C,whichfactcanbeusedintheirselective

enrichment(“coldenrichment”).

Pathogenesis.Studiesofthemolecularprocessesinvolvedhaveusedmainly

systemicallyinfectedmice.

&Adherence.Tophagocyticcells(e.g.,macrophages)andnonphagocytic

cells(e.g.,enterocytes).

&Invasion.Endocytosis,inducedbytheproteininternalinonthesurfaceof

thelisteriae.Formationoftheendosome.

&Destructionoftheendosome.Thevirulencefactorlisteriolysinforms

poresintheendosomalmembrane,releasingthelisteriaeintothecytoplasm.

&Replicationofthelisteriaeinthecytoplasmofinfectedcells.

&Localintercellulardissemination.Polymerizationoftheactinofinfected

cellsatonepoleofthelisteriaetoformso-calledactintailsthatmovethe

listeriaetowardthemembrane.Formationoflongmembraneprotuberances

(knownaslisteriopods)containinglisteriae.Neighboringcellsengulfthelis-

teriopods,whereupontheprocessoflisteriareleasebymeansofendosome

destructionisrepeated.

&Disseminationisgenerallybymeansofhematogenousspread.

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Kayser, Medical Microbiology © 2005 Thieme