Medical Microbiology

Host–PathogenInteractions 21

PIsarefoundinbothGram-positiveandGram-negativebacteria.Theseare

DNAregionsupto 200 kbthatoftenbearseveraldifferentvirgenesandhave

specificsequenceslocatedattheirends(e.g.,ISelements)thatfacilitatelat-

eraltranslocationoftheislandsbetweenbacterialcells.Theroleplayedby

lateraltransferoftheseislandsintheevolutionaryprocessisfurtherunder-

linedbythefactthattheGCcontentsinPIsoftendifferfromthoseinchro-

mosomalDNA.

DefensesagainstInfection.................................

Amacroorganismmanifestsdefensivereactionsagainstinvasionbymicroor-

ganismsintwoforms:asspecific,acquiredimmunityandasnonspecific,

innateresistance(seealsoChapter2,BasicPrinciplesofImmunology,p.43).

NonspecificDefenseMechanisms......................

Table 1. 6 liststhemostimportantmechanisms.

&Primarydefenses.Themainfactorsinthefirstlineofdefenseagainstin-

fectionaremechanical,accompaniedbysomehumoralandcellularfactors.

Thesedefensesrepresentanattemptonthepartofthehostorganismtopre-

ventmicroorganismsfromcolonizingitsskinandmucosaandthusstaveoffa

generalizedinvasion.

&Secondarydefenses.Thesecondlineofdefenseconsistsofhumoraland

cellularfactorsinthebloodandtissues,themostimportantofwhicharethe

professionalphagocytes.

&Phagocytosis.“Professional”phagocytosisisrealizedbypolymorphonu-

clear,neutrophilic,eosinophilicgranulocytes—alsoknownasmicrophages—

andbymononuclearphagocytes(macrophages).Thelatteralsoplayanim-

portantroleinantigenpresentation(seep.62).Thetotalmicrophagecell

countinanadultisapproximately2.5! 1012 .Only 5 %ofthesecellsarelo-

catedintheblood.Theyarecharacterizedbyahalf-lifeofonlyafewhours.

Microphagescontainbothprimarygranules,whicharelysosomescontaining

lysosomalenzymesandcationicpeptides,andsecondarygranules.Bothmi-

crophagesandmacrophagesarecapableofameboidmotilityandchemotac-

ticmigration,i.e.,directedmovementalongaconcentrationgradienttoward

asourceofchemotacticsubstances,inmostcasesthecomplementcompo-

nentsC3aandC5a.Otherpotentiallychemotacticsubstancesincludesecre-

toryproductsoflymphocytes,productsofinfectedanddamagedcellsorthe

N-formylpeptides(fMet-PheandfMet-Leu-Phe).

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Kayser, Medical Microbiology © 2005 Thieme