Medical Microbiology

ThePhysiologyofMetabolismandGrowthinBacteria 165

GrowthandCellDeath................................

Bacteriareproduceasexuallybymeansofsimpletransversebinaryfission.

Theirnumbers(n)increaselogarithmically(n= 2 G).Thetimerequiredfor

areproductioncycle(G)iscalledthegenerationtime(g)andcanvarygreatly

fromspeciestospecies.Fast-growingbacteriacultivatedinvitrohaveagen-

erationtimeof 15 – 30 minutes.Thesamebacteriamaytakehourstorepro-

duceinvivo.Obligateanaerobesgrowmuchmoreslowlythanaerobes;thisis

trueinvitroaswell.Tuberculosisbacteriahaveanin-vitrogenerationtimeof

12 – 24 hours.Ofcoursethegenerationtimealsodependsonthenutrientcon-

tentofthemedium.

Theso-callednormalgrowthcurveforbacteriaisobtainedbyinoculat-

inganutrientbrothwithbacteriathemetabolismofwhichisinitiallyquies-

cent,countingthematintervalsandenteringtheresultsinasemilogcoor-

dinatesystem(Fig.3. 16 ).Thelagphase(A)ischaracterizedbyanincreasein

bacterialmassperunitofvolume,butnoincreaseincellcount.Duringthis

phase,themetabolismofthebacteriaadaptstotheconditionsofthenutrient

medium.Inthefollowinglog(orexponential)phase(C),thecellcountin-

creaseslogarithmicallyuptoabout 109 /ml.Thisisfollowedbygrowthdecel-

erationandtransitiontothestationaryphase(E)duetoexhaustionofthe

nutrientsandtheincreasingconcentrationoftoxicmetabolites.Finally,death

phase(F)processesbegin.Thegenerationtimecanonlybedetermineddur-

ingphaseC,eithergraphicallyorbydeterminingthecellcount(n)attwo

differenttimesandapplyingtheformula:

g¼

t 2 "t 1

log 2 n 2 "log 2 n 1

:

NormalGrowthCurveofaBacterialCulture

Number of sur

viving cells (log)

(Hours) Time (Days)

A B

D

C

E

F

Fig.3. 16 A=lagphase,

B=accelerationphase,

C=log(exponential)phase,

D=decelerationphase,

E=stationaryphase,

F=deathphase.

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Kayser, Medical Microbiology © 2005 Thieme