Medical Microbiology

Host–PathogenInteractions 13

&Antiphagocytosis(seealsoFig. 1. 6 ,p. 23 ).

— Capsule.Rendersphagocytosismoredifficult.Capsulecomponentsmay

blockalternativeactivationofcomplementsothatC3bislacking(ligand

forC3breceptorofphagocytes)onthesurfaceofencapsulatedbacteria.

MicroorganismsthatusethisstrategyincludeStreptococcuspneumoniae

andHaemophilusinfluenzae.

— Phagocytetoxins.Examples:leukocidinfromstaphylococci,streptolysin

fromstreptococci.

— MacrophagesmaybedisabledbythetypeIIIsecretionsystem(seep. 1 7)

ofcertainGram-negativebacteria(forexamplesalmonellae,shigellae,

yersiniae,andcolibacteria).Thissystemisusedtoinjecttoxicproteins

intothemacrophages.

— Inhibitionofphagosome-lysosomefusion.Examples:tuberculosisbacteria,

gonococci,Chlamydiapsittaci.

— Inhibitionofthephagocytic“oxidativeburst.”NoformationofreactiveO 2

radicalsinphagocytes.Examples:Legionellapneumophilia,Salmonellaty-

phi.

&Serumresistance.ResistanceofGram-negativebacteriatocomplement.

Alipopolysaccharideintheoutermembraneismodifiedinsuchawaythatit

cannotinitiatealternativeactivationofthecomplementsystem.Asaresult,

themembraneattackcomplex(C5b6789),whichwouldotherwiselyseholes

intheoutermembrane,isnolongerproduced(seep.86ff.).

&Siderophores.Siderophores(e.g.,enterochelin,aerobactin)arelow-mo-

lecular-weightiron-bindingmoleculesthattransportFe3+activelyintothe

intracellularspace.Theycomplexwithiron,therebystealingthiselement

fromproteinscontainingiron(transferrin,lactoferrin).Theintricateiron

transportsystemislocalizedinthecytoplasmicmembrane,andinGram-

negativebacteriaintheoutermembraneaswell.Tothrive,bacteriarequire

10 –5mol/lfreeironions.Thefreeavailabilityofonlyabout 10 –20mol/lironin

humanbodyfluidsthuspresentsachallengetothem.

StrategiesagainstSpecificImmunity..................

&Immunotolerance.

— Prenatalinfection.Atthisstageofdevelopment,theimmunesystemisun-

abletorecognizebacterialimmunogensasforeign.

— Molecularmimicry.Molecularmimicryreferstothepresenceofmolecules

onthesurfaceofbacteriathatarenotrecognizedasforeignbytheim-

munesystem.Examplesofthisstrategyarethehyaluronicacidcapsule

ofStreptococcuspyogenesortheneuraminicacidcapsuleofEscherichia

coliK1andserotypeBNeisseriameningitidis.

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Kayser, Medical Microbiology © 2005 Thieme