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Concept
Cyste (biologie)
Résumé
A microbial cyst is a resting or dormant stage of a microorganism, usually a bacterium or a protist or rarely an invertebrate animal, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed and the cell ceases all activities like feeding and locomotion. Encystment, the formation of the cyst, also helps the microbe to disperse easily, from one host to another or to a more favorable environment. When the encysted microbe reaches an environment favorable to its growth and survival, the cyst wall breaks down by a process known as excystation. In excystment, the exact stimulus is unknown for most protists.
Unfavorable environmental conditions such as lack of nutrients or oxygen, extreme temperatures, lack of moisture and presence of toxic chemicals, which are not conducive for the growth of the microbe trigger the formation of a cyst.
The main functions of cysts are to protect against adverse changes in the environment such as nutrient deficiency, desiccation, adverse pH, and low levels of oxygen, they are sites for nuclear reorganization and cell division, and in parasitic species they are the infectious stage between hosts.
In bacteria (for instance, Azotobacter sp.), encystment occurs by changes in the cell wall; the cytoplasm contracts and the cell wall thickens. Bacterial cysts differ from endospores in the way they are formed and also the degree of resistance to unfavorable conditions. Endospores are much more resistant than cysts.
Bacteria do not always form a single cyst. Varieties of cysts formation events are known. As an example Rhodospirillium centenum can change the number of cell per cyst usually ranging from 4 to 10 cells per cyst depending on environment.
Protists, especially protozoan parasites, are often exposed to very harsh conditions at various stages in their life cycle.
Source officielle
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