Medical Microbiology

PrinciplesofSterilizationandDisinfection 35

KillingofPrionsandThermophilicArchaea

Thestandardsterilizationmethodsusedinmedicalapplications(seebelow)are

capableofcausingirreversibledamagetomedicallyrelevantmicroorganisms

suchasbacteria,protozoans,fungi,andhelminthsincludingwormeggs.Much

moreextremeprocessesarerequiredtoinactivateprions,suchasautoclaving

at 1218 Cfor4.5hoursorat 1348 Cfor 30 minutes.Hyperthermophilicarchaea

formshavealsobeendiscoveredinrecentyears(seep.5)thatproliferateattem-

peraturesof 1008 Candhigherandcantolerateautoclavingat 1218 Cforonehour.

Theseextremelifeforms,alongwithprions,arenotcoveredbythestandarddefini-

tionsofsterilizationandsterility.

Disinfectionisaspecificallytargetedantimicrobialtreatmentwiththeob-

jectiveofpreventingtransmissionofcertainmicroorganisms.Thepurposeof

thedisinfectionprocedureistorenderanobjectincapableofspreadingin-

fection.

Preservationisageneraltermformeasurestakentopreventmicrobe-

causedspoilageofsusceptibleproducts(pharmaceuticals,foods).

Decontaminationistheremovalorcountreductionofmicroorganisms

contaminatinganobject.

Theobjectiveofasepticmeasuresandtechniquesistopreventmicrobial

contaminationofmaterialsorwounds.

Inantisepticmeasures,chemicalagentsareusedtofightpathogensinor

onlivingtissue,forexampleinawound.

TheKineticsofPathogenKilling

Killingmicroorganismswithchemicalagentsorbyphysicalmeansinvolvesa

first-orderreaction.Thisimpliesthatnopathogen-killingmethodkillsoffall

themicroorganismsinthetargetpopulationallatonceandinstantaneously.

Plottingthekillingrateagainstexposuretimeinasemilogcoordinatesystem

resultsinastraight-linecurve(Fig. 1. 7 ).

Sigmoidandasymptotickillingcurvesareexceptionstotheruleofexpo-

nentialkillingrates.Thesteepnessofthekillingcurvesdependsonthesen-

sitivityofthemicroorganismstotheagentaswellasonthelatter’seffective-

ness.Thesurvivor/exposurecurvedropsatasteeperanglewhenheatisap-

plied,andataflatteranglewithionizingradiationorchemicaldisinfectants.

Anothercontributingfactoristhenumberofmicroorganismscontaminating

aproduct(i.e.,itsbioburden):whenappliedtohigherorganismconcentra-

tions,anantimicrobialagentwillrequirealongerexposuretimetoachieve

thesamekillingeffect.

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Kayser, Medical Microbiology © 2005 Thieme