Medical Microbiology

36 1 GeneralAspectsofMedicalMicrobiology

Standardsterilizationmethodsextendbeyondkillingallmicroorganisms

onthetargetobjectstoprojectatheoreticalreductionofrisk,i.e.,thenumber

oforganismspersterilizedunitshouldbeequaltoorlessthan 10 –6.

TheDvalue(decimalreductiontime),whichexpressesthetimerequired

toreducetheorganismcountby 90 %,isahandyindexforkillingeffective-

ness.

Theconcentration(c)ofchemicalagentsplaysasignificantroleinpatho-

gen-killingkinetics.Therelationbetweenexposuretime(t)andciscalledthe

dilutioncoefficient(n):t!cn=constant.Eachagenthasacharacteristiccoef-

ficientn,forinstancefiveforphenol,whichmeanswhencishalvedtheex-

posuretimemustbeincreasedbyafactorof 32 toachievethesameeffect.

Thetemperaturecoefficientdescribestheinfluenceoftemperatureonthe

effectivenessofchemicalagents.Thehigherthetemperature,thestronger

theeffect,i.e.,theexposuretimerequiredtoachievethesameeffectisre-

duced.Thecoefficientoftemperaturemustbedeterminedexperimentally

foreachcombinationofantimicrobialagentandpathogenspecies.

MechanismsofAction

Whenmicroorganismsarekilledbyheat,theirproteins(enzymes)areirre-

versiblydenatured.Ionizingradiationresultsintheformationofreactive

groupsthatcontributetochemicalreactionsaffectingDNAandproteins.Ex-

posuretoUVlightresultsinstructuralchangesinDNA(thyminedimers)that

preventitfromreplicating.Thisdamagecanberepairedtoacertainextent

BacterialDeathKinetics

Sur

viving cell count in log units

Time exposed to antimicrobial agent

Bacteria species A

Bacteria species B

Fig. 1. 7 Thedeathrate varies

among bacterial species. The

highertheinitialconcentration

ofabacterialculture,thelonger

an appliedantimicrobialagent

willrequiretoachievethesame

effect.

1

Kayser, Medical Microbiology © 2005 Thieme