Medical Microbiology

12 1 GeneralAspectsofMedicalMicrobiology

pili,attachmentfimbriae,adhesionproteinsintheoutermembraneofGram-

negativebacteria,cellwall-associatedproteinsinGram-positivebacteria).

Thisisaspecificprocess,meaningthattheadhesionstructure(orligand)

andthereceptormustfittogetherlikeakeyinakeyhole.

InvasionandSpread....................................

&Invasion.Bacteriamayinvadeahostpassivelythroughmicrotraumataor

macrotraumataintheskinormucosa.Ontheotherhand,bacteriathatinvade

throughintactmucosafirstadheretothisanatomicalbarrier,thenactively

breachit.Differentbacterialspeciesdeployavarietyofmechanismstoreach

thisend:

— Productionoftissue-damagingexoenzymesthatdestroyanatomicalbar-

riers.

— Parasite-directedendocytosis,initiatedbyinvasinsonthesurfaceofthe

bacterialcells,causesthecytoskeletonoftheepithelialcelltoformpseu-

dopodsthatbringaboutendocytosis.

— PhagocytosisofenteropathogenicbacteriabyMcellsintheintestinalmu-

cosa(cellsthatcaningestsubstancesfromtheintestinallumenbywayof

phagocytosis).

&Spread.

— Localtissuespreadbeginningattheportalofentry,helpedalongbytis-

sue-damagingexoenzymes(hyaluronidase,collagenase,elastase,and

otherproteases).

— Cell-to-cellspread.Bacteriatranslocatedintotheintracellularspaceby

endocytosiscauseactintocondenseintofilaments,whichthenarray

atoneendofthebacteriumandpushupagainsttheinnersideofthe

cellmembrane.Thisisfollowedbyfusionwiththemembraneofthe

neighboringtissuecell,whereuponthebacteriumentersthenewcell

(typicalofListeriaandShigella).

— Translocationofmacrophage-resistantbacteriawithmacrophagesinto

intestinallymphoidtissuefollowingtheiringestionbyMcells.

— Lymphogenousorhematogenousgeneralization.Thebacteriatheninvade

organsforwhichtheypossessaspecifictropism.

StrategiesagainstNonspecificImmunity..............

Establishmentofabacterialinfectioninahostpresupposesthecapacityof

theinvaderstoovercomethehost’snonspecificimmunedefenses.Themost

importantmechanismsusedbypathogenicbacteriaare:

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Kayser, Medical Microbiology © 2005 Thieme