Medical Microbiology

TheGeneticVariabilityofBacteria 175

Bacteriophagesarevirusesthatinfectbacteria(p. 1 82ff.).Duringtheirrep-

licationprocess,DNAsequencesfromthehostbacterialcellmayreplaceallor

partofthegenomeinthephagehead.Suchphageparticlesarethendefective.

TheycanstilldockonreceptorcellsandinjecttheirDNA,buttheinfected

bacterialcellwillthenneitherproducenewphagesnorbedestroyed.

Conjugation............................................

ConjugationisthetransferofDNAfromadonortoareceptorinaconjugal

processinvolvingcell-to-cellcontact.Conjugationismadepossiblebytwo

geneticelements:theconjugativeplasmidsandtheconjugativetransposons.

Conjugation

a c

b d

Fig.3. 22 Transfer/replicationprocessofaconjugativeplasmid.

aConjugation:connectionbetweentwobacterialcellsbymeansofsexpili.This

initialstepalonedoesnotnecessarilyalwaysleadtoeffectiveconjugation.

bEffectiveconjugation:formationofaspecificconjugalbridgebetweendonorcell

andreceptorcell.

cPlasmidmobilizationandtransfer:anendonucleasecleavesonestrandofthe

circularDNAdoublehelixataspecificpoint(b).Thesinglestrandwiththe“leader

region”entersthereceptorcell.

dSynthesis:thedouble-strandedstructureofboththetransferredsinglestrand

andtheremainingDNAstrandisrestoredbymeansofcomplementaryDNAsyn-

thesis.Thereceptorcell,nowplasmid-positive,iscalledatransconjugant.

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Kayser, Medical Microbiology © 2005 Thieme