Medical Microbiology

204 3 GeneralBacteriology

ResistanceTests........................................

Twostandardtestsystemsareusedtodeterminethein-vitroresistancelevels

ofbacteria.

Indilutionseriestests,theminimuminhibitoryconcentration(MIC)of

ananti-infectiveagentrequiredtoinhibitproliferationofabacterialpopu-

lationisdetermined.Afactor 2 geometricaldilutionseriesoftheagentispre-

paredinanutrientmedium,inoculatedwiththetestorganismandincu-

bated,whereuponthelowestgrowth-inhibitingconcentrationlevel(mg/l)

isdetermined.Threestandardizeddilutionmethodsareavailable.Inthe

agardilutiontest,nutrientagarplatescontainingantibioticareinoculated

(“spotted”)withthetestorganisms.Inthemicrobrothdilutiontest,thefinal

volumeisusually 100 llpermicroplatewell.Thistesttypecanalsobeauto-

mated.Thefinalvolumeinamacrobrothdilutiontestis 2 mlpertube.

Duetothecomplexityandtime-consumingnatureoftheabovetesttypes,

routinelaboratoriesoftenusetheagardiffusiontest.Thisinvolvesdiffuse

inoculationofthenutrientagarplatewiththeteststrain.Thendisksoffilter

papercontainingtheanti-infectiveagentsareplacedontheagar.Afterthe

platesthuspreparedareincubated,theinhibitionzonesaroundthedisks(i.e.,

whetherornottheydevelopandtheirsize)provideinformationonthere-

sistanceofthemicroorganismstested(Fig.3. 31 ).Thisispossiblebecauseof

thelinearrelationbetweenthelog 2 MICandthediameteroftheinhibition

zones(Fig.3. 32 ).

Tointerprettheresults,theMICsorinhibitionzonesarebroughtinto

relationwiththesubstanceconcentrationspresentatasiteofinfectionat

standarddosagelevels.Thiscalculationisbasedonknownaveragesforvar-

iouspharmacokineticparameters(serumconcentration,half-life)andphar-

macodynamicparameters(bactericidalactivityornot,postantibioticeffect,

AgarDiffusionTest

Fig.3. 31 Thismethod,also

knownasthe“disktest,”is

usedtotesttheresistance

ofabacterialculturetovar-

ious anti-infective agents.

Themethodprovidesabasis

forclassificationofabacterial

strainas“susceptible,”“resis-

tant,”or“intermediate”ac-

cordingtothedimensionof

theinhibitionzone.

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Kayser, Medical Microbiology © 2005 Thieme