Medical Microbiology

ThePrinciplesofAntibioticTherapy 205

etc.).Theinterpretationalsotakesintoaccountclinicalexperiencegained

fromtherapyofinfectionswithpathogensofgivensuceptibility.Suchdata

areusedtoestablishgeneralguidelinevaluesdefiningtheboundarybetween

susceptibleandresistantbacteria.

Theminimumbactericidalconcentration(MBC)isthesmallestconcen-

trationofasubstancerequiredtokill99.9%ofthecellsinaninoculum.

TheMBCisdeterminedusingquantitativesubculturesfromthemacro-

scopicallyuncloudedtubesor(microplate)wellsofanMICdilutionseries.

CombinationTherapy.....................................

Combinationtherapyisthetermforconcurrentadministrationoftwoor

moreanti-infectiveagents.Somegalenicpreparationscombinetwocompo-

nentsinafixedratio(example:cotrimoxazole).Normally,however,thein-

InhibitionZoneandMinimumInhibitoryConcentration(MIC)

Minimum inhibit

or

y concentration (log

) 2

suscep

tible

int

er

• mediat

e

re

sis

ta

nt

resistant intermediate susceptible

Inhibition zone diameter (mm)

Regression line

Fig.3. 32 Eachpointrepresentsabacterialstrain.Thesizeoftheinhibitionzone

isdeterminedintheagardiffusiontest(disktest)andtheminimuminhibitory

concentration(MIC)inthedilutiontest.TheMICcorrespondslogarithmically

(log 2 )withthediameteroftheinhibitionzone.

3

Kayser, Medical Microbiology © 2005 Thieme