Medical Microbiology

24 1 GeneralAspectsofMedicalMicrobiology

DefectsinImmuneDefenses...........................

Hostswithdefectsintheirspecificand/ornonspecificimmunedefensesare

pronetoinfection.

&Primarydefects.Congenitaldefectsinthecomplement-dependentpha-

gocytosissystemarerare,asareBandTlymphocytedefects.

&Secondarydefects.Sucheffectsareacquired,andtheyaremuchmore

frequent.Examplesincludemalnutrition,veryoldandveryyounghosts,me-

tabolicdisturbances(diabetes,alcoholism),autoimmunediseases,malignan-

cies(abovealllymphomasandleukemias),immunesysteminfections(HIV),

severeprimarydiseasesofparenchymatousorgans,injuryofskinormucosa,

immunosuppressivetherapywithcorticosteroids,cytostaticsandimmuno-

suppressants,andradiotherapy.

Oneresultofprogressinmodernmedicineisthatincreasingnumbersofpa-

tientswithsecondaryimmunedefectsarenowreceivinghospitaltreatment.

Such“problempatients”arefrequentlyinfectedbyopportunisticbacteria

thatwouldnotpresentaseriousthreattonormalimmunedefenses.Often,

thepathogensinvolved(“problembacteria”)havedevelopedaresistanceto

numerousantibiotics,resultingindifficultcoursesofantibiotictreatmentin

thispatientcategory.

NormalFlora................................................

Commensals(seeTable 1. 3 ,p.9)areregularlyfoundincertainhumanmicro-

biotopes.Thenormalhumanmicrofloraisthusthetotalityofthesecommen-

sals.Table 1. 7 liststhemostimportantmicroorganismsofthenormalflora

withtheirlocalizations.

Bacteriaarethepredominantcomponentofthenormalflora.They

proliferateinvariedprofusiononthemucosaandmostparticularlyinthe

gastrointestinaltract,whereover 400 differentspecieshavebeencounted

todate.Thecountofbacteriapergramofintestinalcontentis 101 –1 05 in

theduodenum, 103 –1 07 inthesmallintestine,and 1010 –1 012 inthecolon.

Over 99 %ofthenormalmucosalfloraareobligateanaerobes,dominated

bytheGram-neg.anaerobes.Althoughlifeispossiblewithoutnormalflora

(e.g.,pathogen-freeexperimentalanimals),commensalscertainlybenefit

theirhosts.Onewaytheydosoiswhenorganismsofthenormalflora

managetopenetrateintothehostthroughmicrotraumas,resultingina

continuousstimulationoftheimmunesystem.Commensalsalsocompete

forlivingspacewithovertlypathogenicspecies,afunctionknownascolo-

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Kayser, Medical Microbiology © 2005 Thieme