Medical Microbiology

196 3 GeneralBacteriology

SpectrumofAction........................................

Eachanti-infectiveagenthasacertainspectrumofaction,whichisarange

ofbacterialspeciesshowingnaturalsensitivitytothesubstance.Someanti-

infectiveagentshaveanarrowspectrumofaction(e.g.,vancomycin).Most,

however,havebroadspectraliketetracyclines,whichaffectalleubacteria.

Efficacy....................................................

Theefficacyofananti-infectiveagent(syn.kineticsofaction)definesthe

wayitaffectsabacterialpopulation.Twobasiceffectsaredifferentiated:

bacteriostasis,i.e.,reversibleinhibitionofgrowth,andirreversiblebacteri-

cidalactivity(Fig.3. 30 ).Manysubstancescandevelopbothformsofefficacy

dependingontheirconcentration,thetypeoforganism,andthegrowth

phase.Manyofthesedrugsalsohaveapostantibioticeffect(PAE)reflecting

thedamageinflictedonabacterialpopulation.Aftertheanti-infectiveagent

isnolongerpresent,thebacterialcellsnotkilledrequirearecoveryphase

beforetheycanreproduceagain.ThePAEmaylastseveralhours.

Abacteriostaticagentalonecannevercompletelyeliminatepathogenic

bacteriafromthebody’stissues.“Healing”resultsfromthecombinedeffects

oftheanti-infectiveagentandthespecificandnonspecificimmunedefenses

ofthehostorganism.Intissuesinwhichthisdefensesystemisinefficient

(endocardium),inthemiddleofapurulentlesionwherenofunctionalpha-

gocytesarepresent,orinimmunocompromisedpatients,bactericidalsub-

stancesmustberequired.Theclinicalvalueofknowingwhetheranantibac-

terialdrugisbacteriostaticorbactericidalisreadilyapparent.

Allofthebacteriafromaninfectionfocuscannotbeeliminatedwithout

supportfromthebody’simmunedefensesystem.Abacterialpopulation

alwaysincludesseveralcellswithphenotypicresistancethatisnotgeno-

typicallyfounded.Thesearetheso-calledpersisters,whichoccurinin-vitro

culturesatfrequenciesrangingfrom1:1 06 to1:1 08 (Fig.3. 30 ).Thecauseof

suchpersistenceisusuallyaspecificmetabolicpropertyofthesebacteria

thatpreventsbactericidalsubstancesfromkillingthem.Followingdisconti-

nuationoftherapy,suchpersisterscanleadtorelapses.Infectionswith

L-formsshowaspecialtypeofpersistencewhentreatedwithantibiotics

thatblockmureinsynthesis(p. 1 56).

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Kayser, Medical Microbiology © 2005 Thieme