BacterialCellCountandBacterialMass
Thecolonycountingmethod.Thenumberoflivingcellsinagivencultureor
materialcanbedeterminedbymeansofthecolonycountingmethod.Thesamples
aredilutedlogarithmicallybyadilutionfactorof 1 0.Usingthepourplatetechnique,
eachdilutionismixedwith 1 mlofliquidagarandpouredoutinaplate.Inthe
surfaceinoculationmethod,0. 1 mlofeachdilutionisplatedoutonanutrient
agarsurface.Theplatesareincubated,resultingincolonygrowth.Thenumber
ofcoloniescounted,multipliedbythedilutionfactor,resultsintheoriginalnumber
ofviablebacterialcells(CFU=colonyformingunits).
Bacterialmass.Thebacterialmasscanbeestablishedbyweighing(dryorwet
weight).Thesimplestwaytodeterminethemassisbymeansofphotometricad-
sorptionmeasurement.Theincreasesinmassandcellcountrunparallelduring
phaseConthegrowthcurve.TheMolecularBasisofBacterialGenetics.........................
&Bacteriapossesstwogeneticstructures:thechromosomeandtheplas-
mid.BothofthesestructuresconsistofasinglecircularDNAdoublehelix
twistedcounterclockwiseaboutitshelicalaxis.ReplicationofthisDNAmol-
eculealwaysstartsatacertainpoint(theoriginofreplication)andis“semi-
conservative,”thatis,onestrandineachofthetworesultingdoublestrandsis
conserved.Mostbacterialgenescodeforproteins(polypeptides).Noncoding
interposedsequences(introns),likethoseseenineukaryotes,aretheexcep-
tion.Certainbacterialgeneshaveamosaicstructure.Thephasesoftranscrip-
tionarepromoterrecognition,elongation,andtermination.Manybacterial
mRNAsarepolycistronic,meaningtheycontainthegeneticinformationfor
severalpolypeptides.Translationtakesplaceonthe 7 0Sribosomes.Special
mRNAcodonsmarkthestartandstopofpolypeptidesynthesis.Manygenes
thatcodeforfunctionallyrelatedpolypeptidesaregroupedtogetherinchro-
mosomeorplasmidsegmentsknownasoperons.Themostimportantreg-
ulatorymechanismisthepositiveornegativecontroloftranscriptioninitia-
tion.Thiscontrolfunctionmaybeexercisedbyindividuallocalizedgenes,the
genesofanoperonorgenesinaregulon. &166 3 GeneralBacteriology3
Kayser, Medical Microbiology © 2005 Thieme