Medical Microbiology

20 1 GeneralAspectsofMedicalMicrobiology

Gram-positiveandGram-negativebacteria.Itdenotesamodeofcommuni-

cationbetweenbacterialcellsthatenablesabacterialpopulationtoreact

analogouslytoamulticellularorganism.

Accumulationofagivendensityofalow-molecular-weightpheromone

(autoinducer)enablesabacterialpopulationtosensewhenthecritical

celldensity(quorum)hasbeenreachedthatwillenableittoinvadethe

hostsuccessfully,atwhichpointtranscriptionofvirulencedeterminantsis

initiated.

TheGeneticsofBacterialPathogenicity...............

Thevirulencegenesofpathogenicbacteriaarefrequentlycomponentsof

mobilegeneticelementssuchasplasmids,bacteriophagegenomes,orcon-

jugativetransposons(seep. 17 0ff.).Thismakeslateraltransferofthesegenes

betweenbacterialcellspossible.Regionsshowingahighfrequencyofviru-

lencegenesinabacterialchromosomearecalledpathogenicityislands(PI).

Quorum-SensingCommunicationinBacteria(Cell-to-CellSignals)

Autoinducer

synthase Regulator Activation of transcription

Autoinducer Autoinducer/regulator complex (activator)

Cell wall/cell membrane

I gene R gene Virulence gene (virulence regulon)

Fig. 1. 5 Cell-to-cellsignalingismadepossiblebyactivationoftwogenes.The

Igenecodesforthesynthaseresponsibleforsynthesisoftheautoinducer.The

autoinducer(oftenanN-acylhomoserinelactone)candiffusefreelythrough

thecellmembrane.TheRgenecodesforatranscriptionalregulatorprotein

thatcombineswiththeautoinducertobecomeanactivatorfortranscription

ofvariousvirulencegenes.

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Kayser, Medical Microbiology © 2005 Thieme