Medical Microbiology

contributestoparasitepenetrationintohostcells.Thenucleusislocatedin

theposteriorhalfofthecell.Morerecentinvestigationshaverevealedthat

Toxoplasmaandseveralotherapicomplexanprotozoa(e.g.,Plasmodium,Sar-

cocystis)contain,inadditiontothechromosomalandmitochondrialge-

nomes,afurthergenomeconsistingofcircularDNA(35kb)localizedinaspe-

cialorganellecalledtheapicoplast.Thisorganelle,usuallylocatedanteriorto

thenucleusandclosetotheGolgiapparatus,issurroundedbyseveralmem-

branesandpossessesouterandinnermembranecomplexes.Thereisevi-

dencethattheapicoplasthasevolvedfromtheplastidsofendosymbiontic

greenorredalgae.Thefunctionoftheapicoplastremainsuncertainbutis

indispensableforthehostcellandmaybeanewandspecifictargetforche-

motherapy.Tachyzoitesalsomultiplyinexperimentalanimalsandcellcul-

tures.Thisstageishighlylabileoutsideofahostandusuallydoesnotsurvive

thestomachpassagefollowingingestion.

&Bradyzoites(cystozoites) arestagesproducedbyslowreproduction

withinthecysts(4– 8 !2–4lm).Thecystsdevelopintracellularlyinvarious

tissues(seebelow),havearelativelyresistantwall,growaslargeas 150 lm,

andcancontainuptoseveralthousandbradyzoites(Fig.9. 12 c).Theyhavea

longlifespaninthehost.Humansandanimalscanbeinfectedbyperoralin-

gestionofmeatcontainingcysts(Fig.9. 13 ).

&Oocystsareroundedandencystedstagesoftheorganism,surroundedby

aresistantcystwall,andareapprox. 9! 14 lminsize.Theyarethefinal

productofasexualreproductivecycleintheintestinalepitheliaofFelidae.

Theycontainazygoteandareshedinfeces(Fig.9. 13 ).Sporulationtakesplace

Toxoplasmagondii 509

Toxoplasmagondii

a b c

50

μm

Pole ring Conoid

Rhoptries

10

μm

5 μ

m

Apicoplast

Fig.9. 12 aTachyzoites,bultrastructureofatachyzoite,ccystwithbradyzoites.

9

Kayser, Medical Microbiology © 2005 Thieme