Medical Microbiology

ThePrinciplesofAntibioticTherapy 201

TheProblemofResistance................................

Definitions.............................................

Clinicalresistance.Resistanceofbacteriatotheconcentrationofanti-infec-

tiveagentsmaintainedattheinfectionsiteinthemacroorganism.

Naturalresistance.Resistancecharacteristicofabacterialspecies,genus,or

family.

Acquiredresistance.Strainsofsensitivetaxacanacquireresistancebywayof

changesintheirgeneticmaterial.

Biochemicalresistance.Abiochemicallydetectableresistanceobservedin

strainsofsensitivetaxa.Thebiochemicalresistanceoftencorrespondsto

theclinicallyrelevantresistance.Biochemicallyresistantstrainssometimes

showlowlevelsofresistancebelowtheclinicallydefinedboundaryseparat-

ingresistantandsensitivestrains.Suchstrainsmaybemedicallysusceptible.

Incidence,Significance................................

Problematicbacteria.Strainswithacquiredresistanceareencounteredfre-

quentlyamongEnterobacteriaceae,pseudomonads,staphylococci,andente-

rococci.Specificinfectiontherapydirectedatthesepathogensisoftenfraught

withdifficulties,whichexplainsthelabelproblematicbacteria.Theyare

responsibleformostnosocomialinfections(p.342f.).Usuallyharmlessin

otherwisehealthypersons,theymaycauselife-threateninginfectionsin

highlysusceptible,so-calledproblematicpatients.Problematicbacteria

areoftencharacterizedbymultipleresistance.Resistancetoanti-infective

agentsisobservedlessfrequentlyinnonhospitalbacteria.

Geneticvariability.Thebasiccauseofthehighincidenceofantibioticresis-

tanceexperiencedwithproblematicbacteriaisthepronouncedgenetic

variabilityoftheseorganisms,themechanismsofwhicharedescribedin

thesection“Geneticvariability”(p. 171 andp. 17 4).Mostimportantare

themechanismsofhorizontaltransferofresistancedeterminantsresponsible

fortheefficientdistributionofresistancemarkersamongthesebacteria.

Selection.Theoriginanddistributionofresistantstrainsisbasedtoasignif-

icantextentonselectionofresistancevariants.Themoreoftenanti-infective

substancesareadministeredtherapeutically,thegreaterthenumberof

strainsthatwilldevelopacquiredresistance.Eachhospitalhasacharacter-

isticflorareflectingitsprescriptionpractice.Aphysicianmustbefamiliar

withtheresistancecharacteristicsofthishospitalflorasothattheright

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Kayser, Medical Microbiology © 2005 Thieme