Microbial cyst
[2] Environmental conditions that may trigger encystment include, but are not limited to: lack of nutrients or oxygen, extreme temperatures, desiccation, adverse pH, and presence of toxic chemicals which are not conducive for the growth of the microbe.
"[5] Leeuwenhoek later continued his work with rotifers to discover that when he returned the dried bodies to their preferred aquatic conditions, they resumed their original shape and began swimming again.
[5] In 1743, John Turberville Needham observed the revival of the encysted larval stage of the wheat parasite, Anguillulina tritici and later published these findings in New Microscopal Discoveries (1745).
[5] Several others repeated and expanded upon this work, informally referring to their studies on the “phenomenon of reviviscence.”[5] In the late 1850s, reviviscence became embroiled in the debate surrounding the theory of spontaneous generation of life, leading two highly involved scientists on either side of the issue to call upon the Biological Society of France for an independent review of their opposing conclusions on the matter.
[5] In 1872, Wilhelm Preyer introduced the term ‘anabiosis’ (return to life) to describe the revitalization of viable, lifeless organisms to an active state.
[5] As part of his 1959 review of Leeuwenhoek’s original findings and the evolution of the science surrounding microbial cysts and other forms of metabolic suspension, D. Keilin proposed the term ‘cryptobiosis’ (latent life) to describe: “...the state of an organism when it shows no visible signs of life and when its metabolic activity becomes hardly measurable, or comes reversibly to a standstill.”[5] As microbial research began to gain popularity exponentially, details about ciliated protist physiology and cyst formation led to increased curiosity about the role of encystment in the life cycle of ciliates and other microbes.
The study of the encystment process was mostly confined to the 1970s and '80s, resulting in the lack of understanding of genetic mechanisms and additional defining characteristics, though they are generally thought to follow a different formation sequence than endospores.
De novo synthesis of cyst wall precursors in the endoplasmic reticulum also frequently indicate a ciliate is undergoing encystment.
The extremophile Rhodospirillum centenum, an anoxygenic, photosynthetic, nitrogen-fixing bacterium that grows in hot springs was found to form cysts in response to desiccation as well.
[3] Cytoplasmic dehydration, high autophagic activity, nuclear condensation, and decrease of cell volume are all indicators of encystment initiation in ciliated protists.
Due to the hard outer shell of the cyst, Cryptosporidium and Giardia are resistant to common disinfectants used by water treatment facilities such as chlorine.
