Medical Microbiology

5 GeneralMycology

F.H.Kayser

GeneralCharacteristicsofFungi

&Fungiareeukaryoticmicroorganisms(domaineucarya)thatoccurubiqui-

touslyinnature.Onlyabout 200 ofthethousandsofspecieshavebeen

identifiedashumanpathogens,andamongtheseknownpathogenicspecies

fewerthanadozenareresponsibleformorethan 90 %ofallhumanfungal

infections.

Thebasicmorphologicalelementoffilamentousfungiisthehyphaanda

webofintertwinedhyphaeiscalledamycelium.Thebasicformofaunicel-

lularfungusistheyeastcell.Dimorphicfungiusuallyassumetheformof

yeastsintheparasiticstageandtheformofmyceliainthesaprophyticstage.

Thecellwallsoffungiconsistofnearly 90 %carbohydrate(chitin,glucans,

mannans)andfungalmembranesarerichinsteroltypesnotfoundinother

biologicalmembranes(e.g.,ergosterol).Filamentousfungireproduceeither

asexually(mitosis),byhyphalgrowthandtipextension,orwiththehelpof

asexualspores.Yeastsreproducebyaprocessofbudding.Sexualreproduc-

tion(meiosis)ontheotherhand,producessexualspores.Fungiimperfecti

ordeuteromycetesarethedesignationforatypeoffungiinwhichthefruc-

tificationformsareeitherunknownormissingentirely. &

DefinitionandTaxonomy

Fungiaremicroorganismsinthedomaineucarya(see.p.5).Theyshowless

differentiationthanplants,butahigherdegreeoforganizationthanthepro-

karyotesbacteria(Table5. 1 ).Thekingdomofthefungi(Mycota)comprises

over 50000 differentspecies,onlyabout 200 ofwhichhavebeenidentifiedas

humanpathogens.Onlyaboutadozenofthese“pathogenic”speciescause

90 %ofallhumanmycoses.Manymycoticinfectionsarerelativelyharmless,

forinstancethedermatomycoses.Inrecentyears,however,theincreasing

numbersofpatientswithvariouskindsofimmunedefectshaveresulted

inmorelife-threateningmycoses.

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Kayser, Medical Microbiology © 2005 Thieme