Medical Microbiology

Morphologyandlifecyle.Microsporareproduceintracellularlybymeansof

repeated,asexualbinaryormultiplefission(merogony),thenformsporesin

asubsequentphase(sporogony)(andsexualstagesaswellinThelohania).The

developmentalstagesarelocatedfreelyinthehostcellcytoplasm(Enterocy-

tozoon,Nosema)ortheyinhabitaparasitophorousvacuole(Encephalitozoon);

inothergenera(Pleistophora,Trachipleistophora)theintracellularstagesare

separatedfromthecytoplasmbyanamorphouslayer(pansporoblasticmem-

brane).Sporogonybeginswithformationofsporonts,whicharederivedfrom

merogoniccellsandpossessathickercellwall.Thesporontsdividetoform

sporoblasts,followedbymorphologicaldifferentiationintospores.

ThefinestructureofthesesporesistypicalofMicrospora(Fig.9.20a,b).

Thetwo-layeredsporewall(exosporeandendospore)enclosestheuninucle-

ate(rarely:binucleate)infectiveparasitestagecalledasporoplasmor“ame-

boidorganism”andacomplexexpulsionapparatusconsistingofacoiledpo-

lartubuleandthepolaroplast,amembranousanchoringcomponent.Thesize

ofthesporesofMicrosporaspeciesinfectinghumansvariesbetweenabout 1

and 4 lm.Thenumberandpositionofthepolartubulewindingsasseenon

theelectronmicroscopicallevel(Fig.9.20a,b)areofdiagnosticimportance.

Thesporesareeliminatedinfeces,urine,orsputumandcanremainviable

forseveralweeksoutsideofthehost.Followingperoralingestionbyasuit-

ablehost,thepolaroplastswellsup,theinternalpressureinthesporein-

Microspora 539

Microspora

Sporoblast

5 μ

m

Polar tubule

Exospore

Endospore

a b^1 μm c^10 μm

Fig.9. 20 aSpore(modifiedfromBinfordCH,ConnorD,eds.PathologyofTropical

andExtraordinaryDiseases.Vol. 1 .Washington:ArmedForcesInstituteofPathol-

ogy; 1987 :336);bimmaturesporeofEncephalitozoonhellem(TEM);csporesof

Enterocytozoonbieneusiinstoolsmear,chromotropicstainaccordingtoWeber.

9

Kayser, Medical Microbiology © 2005 Thieme