Medical Microbiology

150 3 GeneralBacteriology

DNATopologyinBacterialCells

TheDNAdoublehelix(onewinding/1 0 basepairs)isalsowoundcounterclockwise

aboutitshelicalaxis(onewinding/1 5 helicalwindings).Thisso-calledsupercoilingis

necessarytosavespaceandenergy.OnlysupercoiledDNAcanbereplicatedand

transcribed.Topoisomerasessteerthesupercoilingprocess.DNAgyraseandtopo-

isomeraseIVaretopoisomerasesthatoccuronlyinbacteria.The4-quinolones,an

importantgroupofanti-infectionsubstances,inactivatetheseenzymesirreversibly.

BacteriaDuringCellDivision

Fig.3. 5 Thenucleoid(nucleusequivalent)

ofbacteriaconsistsofatangledcircular

DNAmoleculewithoutanuclearmembrane.

Transmissionelectronmicroscopy(TEM)

imageofstaphylococci.

Plasmids

Fig.3. 6 aOpencircularform(OC).Theresult

ofaruptureinoneofthetwonucleicacid

strands.

bTwisted(CCC=covalentlyclosedcircular),

nativeform(TEMimage).

a

b

3

Kayser, Medical Microbiology © 2005 Thieme