Medical Microbiology

176 3 GeneralBacteriology

Intheconjugationprocess,theconjugativeelementsthemselvesarewhat

areprimarilytransferred.However,theseelementscanalsomobilizechro-

mosomalgenesorotherwisenontransferableplasmids.Conjugationisseen

frequently in Gram-negative rods (Enterobacteriaceae), in which the

phenomenonhasbeenmostthoroughlyresearched,andenterococci.

TheF-factorinEscherichiacoli.Thisistheprototypeofaconjugativeplasmid.

Thisfactorcontainstheso-calledtra(transfer)genesresponsiblebothfor

ModelofaHypotheticalConjugativeMultiple-ResistancePlasmid

Fig.3. 23 Multipleresistanceplasmidscanresultfromsuccessiveintegrationof

transposableresistanceDNAorintegrationofresistanceintegrons(seeFigs.3. 19

and3. 20 ,p. 173 and 17 4)

Tn 21 TransposonoftheTn 21 family,codesforresistancetosulfonamides

(sulI)andcontainsanRintegron(In 4 ).

In 4 Codesforchloramphenicolacetyltransferase(=cmlA),an

aminoglycoside acetyltransferase (= aacC1) and an aminoglycoside

adenylyltransferase(=aadA2);alsocontainsanopenreadingframe(orfE)

ofunknownfunction.

Tn 3 Transposon3;codesforabetalactamase(=bla).

Tn 10 Transposon 1 0;codesforresistancetotetracyclines(=tet).

repA Codesforthereplicationenzymeoftheplasmid.

tra PlasmidDNAregioncontaining 25 tragenes;tragenesareresponsible

forthetransferandreplicationprocess(seeFig.3. 22 ).

3

Kayser, Medical Microbiology © 2005 Thieme