Encystation in Protozoa and Microorganisms

Encystation is the formation of a layered hard crust or a cyst around to protect itself from unfavourable conditions. Organisms go through encystation for protection and reproduction. However, the morphology of cyst walls depends on the genus and species. The cysts are immotile and metabolically idle yet are innately fit for vegetation by shedding their cyst coating.

Encystation is just a defensive feature taken by the cell to survive the pressure forced by an adverse climate. We can observe encystations in many organisms like Amoeba, Entamoeba, etc. but it is studied deeply in Amoeba only.

Cyst Structure

Encystation in Amoeba

Amoeba is a protozoan which possesses pseudopodia for mobility and traps its food for nutrition. It reproduces by binary or multiple fission. It shows endocytosis and exocytosis for taking in food and secreting out waste, respectively.

Encystation: During the adverse environmental conditions, it withdraws its pseudopodia and forms a shell-like structure having hard layers around itself called a cyst. It shows no movement or nutrition but is vegetatively active inside the cyst.

Morphologically, cyst in amoeba has two layers of walls. The ectocyst is the external layer, and the endocyst, which is formed after the ectocyst, is the internal fibrillar layer. These layers are composed of acid-insoluble proteins and cellulose but the exact composition is not well known.

Some scientists suggested that cellulose was the only compound present in both layers of the cyst wall but later this was disapproved and recently it has been shown that the endocyst is thinner and fibrillar than the ectocyst which resembles the cellulose structure found in plant cell walls.

When this cyst matures and conditions are favourable, the cyst bursts open and releases many cells which divide multiple fission. These daughter cells of amoeba possess pseudopodia and are fit for survival and reproduction.

Cyst Formation in Amoeba

Encystation in Entamoeba Histolytica

Entamoeba histolytica is a parasitic protozoan which can cause amoebic dysentery affecting approximately 90 million people each year worldwide. It is transmitted through ingestion of food and water contaminated with the cyst form, which undergoes excystation in the small intestine to the trophozoite form that colonises the large intestine. They show encystation outside the host body and when they enter they start to reproduce after rupturing out from the cyst.

From this article, we can conclude that the sole purpose of encystation is reproduction and spreading the disease. Morphologically entamoeba has one layered cyst composed of chitin and encystation-specific chitin-binding lectins that cross-link chitin, degrade chitin, or self-aggregate that makes cyst walls impenetrable to small molecules, acids or other substances.

Encystment and Excystment

The formation of cyst is known as encystment. It occurs during unfavourable conditions. When the environment is favourable after encystment, the wall of the cyst ruptures this is called as excystation. This happens under favourable conditions and to prevent diseases caused by this protozoan (e.g., Amoeba), the process excystment should be blocked. It can be done by taking steps to interrupt their life cycles.

Important Questions

1. What does excystation mean?

Ans: When favourable conditions arrive, the protozoans tend to reproduce. They rupture their cyst wall and burst open with many cells divided by multiple fission. This is called an excystation.

2. Which organisms undergo encystation?

Ans: Protozoans like euglena, amoeba, and entamoeba histolytica undergo the process of encystation. They tend to become dormant during adverse conditions like high temperature, acidic environment, or high pressure and survive this by forming hard shell-like cyst around them.

Practice Questions

1. Histolytica does not show ________________.

1. Binary fission

2. Budding

3. Excystation

2. Sporulation in Amoeba mainly occurs during ___________.

1. Favourable conditions

2. Unfavourable conditions

3. The time of germination

Answers: 1(b) 2(a)

Key Features

• The formation of cyst is an integral part of the life cycle of many protozoans. This allows these organisms to become dormant and survive adverse environmental conditions.

• When the favourable conditions arrive, they may reproduce or spread the diseases in the host body.

• Encystation can be studied and observed deeply in amoeba only and to our knowledge, no research and investigations have yet been performed to determine the survival capacities in other protozoan cysts.