8 1 GeneralAspectsofMedicalMicrobiology&Damagetohosttissuesduetodirectbacterialcytotoxicity,exotoxins,and
exoenzymes(aggressins).
&Damageduetoinflammatoryreactionsinthemacroorganism:activation
ofcomplementandphagocytosis;inductionofcytokineproduction(modu-
lins).
Theabovebacterialpathogenicityfactorsareconfrontedbythefollowing
hostdefensemechanisms:
&Nonspecificdefensesincludingmechanical,humoral,andcellularsys-
tems.Phagocytosisisthemostimportantprocessinthiscontext.
&SpecificimmuneresponsesbasedonantibodiesandspecificreactionsofT
lymphocytes(seechapteronimmunology).
Theresponseofthesedefensestoinfectionthusinvolvesthecorrelationofa
numberofdifferentmechanisms.Defectivedefensesmakeiteasierforan
infectiontotakehold.Primary,innatedefectsarerare,whereasacquired,sec-
ondaryimmunedefectsoccurfrequently,pavingthewayforinfectionsby
microorganismsknownas“facultativepathogens”(opportunists). &BasicTerminologyofInfectiology..........................
Tables 1. 3 and 1. 4 listthemostimportantinfectiologicaltermstogetherwith
briefexplanations.
Thetermspathogenicityandvirulencearenotclearlydefinedintheir
relevancetomicroorganisms.Theyaresometimesevenusedsynonymously.
Ithasbeenproposedthatpathogenicitybeusedtocharacterizeaparticular
speciesandthatvirulencebeusedtodescribethesumofthedisease-causing
propertiesofapopulation(strain)ofapathogenicspecies(Fig. 1. 1 )
Pathogenicityandvirulenceinthemicroorganismcorrespondtosuscept-
ibilityinahostspeciesanddispositioninaspecifichostorganism,whereby
anindividualmaybeanywherefromhighlydisposedtoresistant.DeterminantsofBacterialPathogenicityandVirulence...
Relativelylittleisknownaboutthefactorsdeterminingthepathogenicityand
virulenceofmicroorganisms,andmostofwhatwedoknowconcernsthe
disease-causingmechanismsofbacteria.1
Kayser, Medical Microbiology © 2005 Thieme