38 1 GeneralAspectsofMedicalMicrobiology
Thesignificantheatenergycontentofsteam,whichistransferredtothe
coolersterilizationitemswhenthesteamcondensesonthem,explainswhyit
issuchaneffectivepathogenkiller.Inaddition,theproteinsofmicroorgan-
ismsaremuchmorereadilydenaturedinamoistenvironmentthanunder
dryconditions.
Radiation
&Nonionizingradiation.Ultra-violet(UV)rays(280– 200 nm)areatypeof
nonionizingradiationthatisrapidlyabsorbedbyavarietyofmaterials.UV
raysarethereforeusedonlytoreduceairbornepathogencounts(surgical
theaters,fillingequipment)andfordisinfectionofsmoothsurfaces.
&Ionizingradiation.Twotypesareused:
— Gammaradiationconsistsofelectromagneticwavesproducedbynuclear
disintegration(e.g.,ofradioisotope^60 Co).
— Corpuscularradiationconsistsofelectronsproducedingeneratorsand
acceleratedtoraisetheirenergylevel.
Radiosterilizationequipmentisexpensive.Onalargescale,suchsystemsare
usedonlytosterilizebandages,suturematerial,plasticmedicalitems,and
heat-sensitivepharmaceuticals.Therequireddosedependsonthelevelof
productcontamination(bioburden)andonhowsensitivethecontaminating
microbesaretotheradiation.Asarule,adoseof2.5! 104 Gy(Gray)iscon-
sideredsufficient.
OneGyisdefinedasabsorptionoftheenergyquantumonejoule(J)
perkg.
Filtration
Liquidsandgasescanalsobesterilizedbyfiltration.Mostoftheavailable
filterscatchonlybacteriaandfungi,butwithultrafinefiltersvirusesand
evenlargemoleculescanbefilteredoutaswell.Withmembranefilters,re-
tentiontakesplacethroughsmallpores.Thebest-knowntypeisthemem-
branefiltermadeoforganiccolloids(e.g.,celluloseester).Thesematerialscan
beprocessedtoproducethinfilterlayerswithgaugedandcalibratedpore
sizes.Inconventionaldepthfilters,liquidsareputthroughalayeroffibrous
material(e.g.,asbestos).Theeffectivenessofthistypeoffilterisduelargely
totheprincipleofadsorption.Becauseofpossibletoxicsideeffects,theyare
nowpracticallyobsolete.
1
Kayser, Medical Microbiology © 2005 Thieme