A microbial cyst is a resting or dormant

Dormant

Dormant means lacking activity. It can refer to:*Dormancy in an organism's life cycle*Dormant volcano, a volcano that is inactive but may become active in the future...

 stage of a microorganism, usually a bacterium or a protist

Protist

Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of suspended animation

Suspended animation

Suspended animation is the slowing of life processes by external means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means. Extreme cold can be used to precipitate the slowing of an individual's functions; use...

 in which the metabolic processes of the cell are slowed down and the cell ceases all activities like feeding and locomotion. Encystment also helps the microbe to disperse easily, from one host

Host (biology)

In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

 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.

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.

Cyst formation in bacteria

In bacteria (for instance, Azotobacter

Azotobacter

Azotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes which play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is...

 sp.), encystment occurs by changes in the cell wall

Cell wall

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...

; the cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 contracts and the cell wall thickens. Bacterial cysts differ from endospore

Endospore

An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...

s in the way they are formed and also the degree of resistance to unfavorable conditions. Endospores are much more resistant than cysts.

Cyst formation in protists

Protists, especially protozoa

Protozoa

Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

n parasites, are often exposed to very harsh conditions at various stages in their life cycle. For example, Entamoeba histolytica

Entamoeba histolytica

Entamoeba histolytica is an anaerobic parasitic protozoan, part of the genus Entamoeba. Predominantly infecting humans and other primates, E. histolytica is estimated to infect about 50 million people worldwide...

, a common intestinal parasite that causes dysentery

Dysentery

Dysentery is an inflammatory disorder of the intestine, especially of the colon, that results in severe diarrhea containing mucus and/or blood in the faeces with fever and abdominal pain. If left untreated, dysentery can be fatal.There are differences between dysentery and normal bloody diarrhoea...

, has to endure the highly acidic environment of the stomach before it reaches the intestine and various unpredictable conditions like desiccation

Desiccation

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...

 and lack of nutrients while it is outside the host. An encysted form is well suited to survive such extreme conditions, although protozoan cysts are less resistant to adverse conditions compared to bacterial cysts. In addition to survival, the chemical composition of certain protozoan cyst walls may play a role in their dispersal. The sialyl

Sialic acid

Sialic acid is a generic term for the N- or O-substituted derivatives of neuraminic acid, a monosaccharide with a nine-carbon backbone. It is also the name for the most common member of this group, N-acetylneuraminic acid...

 groups present in the cyst wall of Entamoeba histolytica confers a net negative charge to the cyst which prevents its attachment to the intestinal wall and thus causing its elimination in the feces. Other protozoan intestinal parasites like Giardia lamblia

Giardia lamblia

Giardia lamblia is a flagellated protozoan parasite that colonizes and reproduces in the small intestine, causing giardiasis. The giardia parasite attaches to the epithelium by a ventral adhesive disc, and reproduces via binary fission...

and Cryptosporidium

Cryptosporidium

Cryptosporidium is a protozoan that can cause gastro-intestinal illness with diarrhea in humans.Cryptosporidium is the organism most commonly isolated in HIV positive patients presenting with diarrhea...

also produce cysts as part of their life cycle. In some protozoans, the unicellular organism multiplies during or after encystment and releases multiple trophozoites upon excystation.

Cyst formation in nematodes

Some soil-dwelling plant parasitic nematode

Nematode

The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

s, such as the soybean cyst nematode

Soybean cyst nematode

The soybean cyst nematode , Heterodera glycines, is a plant-parasitic nematode and a devastating pest of the soybean worldwide. The nematode infects the roots of soybean, and the female nematode eventually becomes a cyst...

, or the potato cyst nematode

Potato cyst nematode

Potato root nematodes or potato cyst nematodes are 1-mm long roundworms belonging to the genus Globodera, which comprises around 12 species. They live on the roots of plants of the Solanaceae family, such as potatoes and tomatoes. PCN cause growth retardation and, at very high population...

 form cysts as a normal part of their lifecycle.

Composition of the cyst wall

The composition of the cyst wall is variable in different organisms. The cyst walls of bacteria are formed by the thickening of the normal cell wall with added peptidoglycan

Peptidoglycan

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...

 layers whereas the walls of protozoan cysts are made of chitin

Chitin

Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

, a type of glycoprotein

Glycoprotein

Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...

. Nematode cyst walls are composed of chitin reinforced by collagen.