Toxoplasma gondii - Life cycle and Invasion
- Toxoplasma gondii is the pathogen of toxoplasmosis in humans and animals.
- The film demonstrates the morphology, invasion behaviour and life cycle of the parasite in tissue culture, partly in computer animation.
The following development stages are shown in real-time and time-lapse sequences:
1 -Formation of the oocysts (sporogony).
2- Release of sporozoites (excystation).
3 -Invasion of tachyzoites in both fibroblasts and macrophages.
4- Multiplication of the tachyzoites in fibroblasts (endodyogeny).
5- Phagocytosis of tachyzoites by macrophages.
6- Release of bradyzoites from tissue cysts.
Toxoplasma gondii: zoites in cell culture
Initially, the microcinematographic shots show isolated Toxoplasma gondii zoites, subsequently zoites in cell culture. The Toxoplasma gondii stages that develop in the intermediate host within the tissue cysts by means of repeated endodyogenias are termed zoites (also cystomerozoites, cystozoites or bradyzoites). These zoites are transfer stages, which are infectious for both the primary host (Felidae, cats) and the intermediate hosts of Toxoplasma gondii.
Toxoplasma gondii: sporulation of the oocyst
Toxoplasma gondii passes through the typical coccidian cycle in the intestinal epithelium of its primary host (Felidae - cats). Non-sporulated oocysts are subsequently excreted with the faeces. An animation illustrates these processes. Under favourable environmental conditions sporulation occurs in the oocyst in the open air. Microcinematographic shots document the sporulation of an oocyst. By ingesting oocysts both the primary host and the secondary one can infect themselves.
Excystation of the oocyst in the small intestine
After ingestion of sporulated Toxoplasma gondii oocysts, the oocysts and the sporocyst walls are digested in the small intestine and the sporozoites contained in them are released. They infect the host cells. The photomicrographs document two excystations.
Locomotion and morphology of the sporozoites
Based on microphotographs the active locomotion of the sporozoites (released sporozoites and sporozoites in fibroblast culture) is shown as an introduction, and using an animation their cell organelles are described.
Intermediate host: infection and multiplication
Using an animation the modes of infection of Toxoplasma gondii are described. Both the sporozoites of the sporulated oocysts and the bradyzoites from infected meat of other intermediate hosts (as e.g. pigs) are infectious stages. In the intermediate host asexual reproduction occurs intracellularly: the acute stage of toxoplasmosis. In a first infection of pregnant women, Toxoplasma gondii can be transmitted to the foetus (infantile toxoplasmosis) in utero (transplacentally).
Toxoplasma gondii: invasion of host cells
In a cell culture the active entry of the Toxoplasma gondii zoites into the host cell is shown, and subsequently the process is explained in detail in an animation.
Formation of tachyzoites in an intermediate host
Microcinematographic shots documents the penetration and multiple division of the Toxoplasma gondii sporozoites in host cells (cell culture of human fibroblasts). After penetration the parasitophorous vacuole, in which the sporozoites asexually reproduce themselves by cellular fission (endodyogeny), is formed. Finally, the host cell bursts due to the mass propagation within the parasitophorous vacuole; the tachyzoites are released and can infect additional cells. In conclusion, the clip shows the penetration of a tachyzoite into a macrophage in cell culture.
Toxoplasma: phagocytosis by macrophages
In the course of an immune response an the organism infected by Toxoplasma gondii, macrophages phagocytize Toxoplasma gondii zoites. This process is documented in the clip in cell culture. In contrast to the penetration of the parasite into a host cell within a few seconds, the killing of the causative agent by phagocytizing macrophages takes considerably longer.
Toxoplasma gondii: cycle of intermediate host
Toxoplasma gondii belongs to those opportunistic parasitic causative agents that are able to remain in the host organism for long periods in transfer stages (bradyzoites in tissue cysts). The host is not or barely harmed by this. Under favourable conditions, such as a weakening of the immune systems (immunosuppression) of the host, a sudden mass propagation can occur. This can be fatal to the host organism. Toxoplasma gondii''s cycle in its intermediate host is illustrated by means of an animation. Mikrophotographic images show isolated cysts with Toxoplasma gondii bradyzoites.
Toxoplasma gondii: cycle in primary host
Cats (Felidae) are Toxoplasma gondii''s primary hosts. Subsequent to ingestion of sporulated oocysts or of tissue cysts, which are present in infected meat, the zoites are released in the small intestine and pass through the typical coccidian phases of schizogony and gamogony. The zygote that was produced by gamogony is excreted as an oocyst. Subsequent to sporulation of the oocyst outside the host, an infectious stage has been produced and the cycle can begin again. The cycle in the primary host is described with the aid of an animation, and the sporulation of an oocyst is documented with microcinematographic shots.