Medical Microbiology

TheGeneticVariabilityofBacteria 177

theformationofconjugalpilionthesurfaceofFcellsandforthetransfer

process.Thetransferoftheconjugativeplasmidtakesplaceasshown

hereinschematicsteps(Fig.3. 22 ).

OccasionalintegrationoftheFfactorintothechromosomegivesitthe

conjugativepropertiesoftheFfactor.Suchanintegrationproducesasort

ofgiantconjugativeelement,sothatchromosomalgenescanalsobetrans-

ferredbythesamemechanism.CellswithanintegratedFfactoraretherefore

calledHfr(“highfrequencyofrecombination”)cells.

Conjugativeresistanceandvirulenceplasmids.Conjugativeplasmidsthat

carrydeterminantscodingforantibioticresistanceand/orvirulenceinaddi-

tiontothetragenesandrepAareofconsiderablemedicalimportance.Three

characteristicsofconjugativeplasmidspromoteahighlyefficienthorizontal

spreadofthesedeterminantfactorsamongdifferentbacteria:

&Highfrequencyoftransfer.Duetothe“transferreplication”mechanism,

eachreceptorcellthathasreceivedaconjugativeplasmidautomaticallybe-

comesadonorcell.Eachplasmid-positivecellisalsocapableofmultipleplas-

midtransferstoreceptorcells.

&Widerangeofhosts.Manyconjugativeplasmidscanbetransferredbe-

tweendifferenttaxonomicspecies,genera,orevenfamilies.

&Multipledeterminants.Manyconjugativeplasmidscarryseveralgenes

determiningthephenotypeofthecarriercell.Theevolutionofahypothetical

conjugativeplasmidcarryingseveralresistancedeterminantsisshownsche-

maticallyinFig.3. 23.

Conjugativetransposons.TheseareDNAelements(p. 17 3)thatareusually

integratedintothebacterialchromosome.TheyoccurmainlyinGram-posi-

tivecocci,buthavealsobeenfoundinGram-negativebacteria(Bacteroides).

Conjugativetransposonsmaycarrydeterminantsforantibioticresistanceand

thuscontributetohorizontalresistancetransfer.Inthetransferprocess,the

transposonisfirstexcisedfromthechromosomeandcircularized.Thenasin-

glestrandofthedoublehelixiscutandthelinearizedsinglestrand—analo-

goustotheFfactor—istransferredintothereceptorcell.Conjugativetrans-

posonsarealsocapableofmobilizingnonconjugativeplasmids.

Restriction,Modification,andGeneCloning.........

Theabovedescriptionsofthemechanismsofgeneticvariabilitymightmake

theimpressionthatgenespassfreelybackandforthamongthedifferent

bacterialspecies,renderingthespeciesdefinitionsirrelevant.Thisisnot

thecase.Anumberofcontrolmechanismslimitthesegeneticexchange

processes.Amongthemostimportantarerestrictionandmodification.Re-

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Kayser, Medical Microbiology © 2005 Thieme