Parasites.ThecausativeagentofamebosisisthepathogenicE.histolytica.
ThisspeciesismorphologicallyidenticalwiththeapathogenicE.dispar.
TheycanbedifferentiatedbymeansofzymodemeandDNAanalysisand
withmonoclonalantibodies.Thetwospeciesoccurintheformoftropho-
zoites(vegetativestages)andcysts(Figs.9. 10 and9. 11 c).
&ThetrophozoitesofE.histolyticaarecellsofvariableshapeandsize(1 0 –
60 lm)thatusuallyformasingle,broadpseudopod(protrusionofcellmem-
braneandcytoplasm)thatisoftenquicklyextendedinthedirectionofmove-
ment.StainedpreparationsofthegenusEntamoebashowacharacteristic
ring-shapednucleuswithacentralnucleolusandchromatingranulaon
thenuclearmembrane.Trophozoitesthathavepenetratedintotissuesoften
containphagocytosederythrocytes.
&Thespherical,nonmotilecysts( 1 0–1 6 lm)havearesistantcystwall.At
firsteachcystcontainsauninucleateameba,withglycogeninvacuolesand
theso-calledchromidialbodies,whicharecigar-shaped.Thenucleusdivides
oncetoproducethebinuclearformandlateronceagaintoproducethein-
fectivetetranuclearcyst(Fig.9. 11 c).Thecystsareeliminatedinthestoolof
infectedpersons,eitheraloneortogetherwithtrophozoites.
Lifecycleandpathogenesis.ThecycleofE.histolyticaisshowninFig.9. 10.
&Symptomaticintestinalamebosis.Followingperoralingestionofama-
tureE.histolyticacyst,thetetranuclearamebaisreleased,dividestoproduce
fouroreightuninucleatetrophozoites,whichthencontinuetomultiplyand
encyst(Fig.9. 10 ).Thetrophozoitescolonizethelargeintestinemucosaorlu-
men.Theirpotentialforinvadinganddestroyingtissueishighandisbasedon
thefollowingcharacteristicsandprocesses:adhesionoftrophozoitestoin-
testinalcellsbymeansofsurfacelectins,killingofcellswithpore-forming
peptides(amebapore,typesA–C)anddissolutionoftheextracellularmatrix
bycysteineproteases.Thisenablestheamebastopenetrateintotheintestinal
wall,wheretheymultiplyandcausepathologicalchanges(necroticfoci,ul-
cers,inflammatoryreactions)(seebelow).
&Asymptomaticintestinalamebosis.ThisconditionisusuallycausedbyE.
dispar,lessfrequentlybyE.histolytica.CharacterizingE.disparas“apatho-
genic”isnotentirelyaccurate,sincetheseorganismscancauseslightintest-500 9 ProtozoaFig.9. 10 1 CystofE.histolytica,followingperoralingestion,instomach; 2 ameba
emergingfromacyst; 3 dividingstageofameba; 4 uninucleatetrophozoitesresult
fromdivision;4ainvasivestagewithphagocytosederythrocytes,extraintestinal;
4blesionsintheintestinalwall; 5 theamebasencyst; 6 cystsexcretedwithfeces
anddifferenttransmissionroutes; 7 peroralingestionofcysts. "9
Kayser, Medical Microbiology © 2005 Thieme