Difference Between Cyst and Trophozoite

Main Difference – Cyst and Trophozoite

Cyst and trophozoite are two stages of the lifecycle of protozoans. Protozoa are a diverse group unicellular, eukaryotic microorganisms. Amoeba, Plasmodium, Paramecium, and ciliophora are examples of protozoans. The main difference between cyst and trophozoite is that cyst is the dormant stage that helps to survive in unfavorable environmental conditions whereas trophozoite is a growing stage that absorbs nutrients from the host. Cysts can be found in both bacteria and nematodes. They cause infections through the fecal-oral contamination. As the trophozoite stage is an active and feeding stage, it reproduces inside the host.

Key Areas Covered

1. What is a Cyst
– Definition, Facts, Role
2. What is a Trophozoite
– Definition, Facts, Role
3. What are the Similarities Between Cyst and Trophozoite
– Outline of Common Features
4. What is the Difference Between Cyst and Trophozoite
– Comparison of Key Differences

Key Terms: Cyst, Encystation, Excystation, Feeding, Reproduction, Resistance, Trophozoite

What is a Cyst

A cyst refers to a dormant stage of a protozoan that enables them to survive in unfavorable environmental conditions. Therefore, the cyst is the transmission state of protozoa. On that account, the cyst is infectious. The process of formation of the cyst is referred to as encystation. As encystation takes place in the rectum of the host, cysts can be identified in the feces. Cysts are spherical structures that are smaller than trophozoite. They are covered with a hard cyst wall with two layers. The contractile vacuole, macronucleus, and cilia are visible in young cysts. The organelle structures of the old cysts are granular. Metabolism and motility are arrested in cysts. The cyst of Entamoeba histolytica is shown in figure 1.

Figure 1: Entamoeba histolytica Cyst

Cysts also occur in bacteria and nematodes. In bacteria, cysts are less resistant than the spores. They allow the dispersal of bacteria. They germinate in favorable environmental conditions. The cysts occur in plant-parasitic nematodes as a part of their lifecycle.

What is a Trophozoite

A trophozoite refers to the growing stage of the parasitic protozoans that absorb nutrients from the host. The formation of the trophozoite from a cyst is referred to as excystation. The excystation takes place in the large intestine of the host at the subsequent ingestion of the cyst by the host. Trophozoites are not resistant structures. Generally, trophozoites are covered with cilia. Hence, they are motile. They exhibit rotary or boring motility. The trophozoite of Entamoeba histolytica is shown in figure 2.

Figure 2: Entamoeba histolytica Trophozoite

Trophozoites contain two, clearly visible nuclei. The macronucleus is kidney-shaped while the micronucleus is spherical. Trophozoites consist of an opening at the anterior end. It is known as the peristome. Other than feeding, the reproduction of protozoa occurs in the trophozoite stage either sexually by conjugation or asexually by binary fission. The largest parasitic protozoan in humans is Balantidium coli.

Similarities Between Cyst and Trophozoite

Cyst and trophozoite are two stages of the lifecycle of protozoans.
Both cyst and trophozoite are unicellular.
Both cyst and trophozoite consist of visible nuclei.
Both cyst and trophozoite contain contractile vacuoles.
Both cyst and trophozoite can be found in feces of infected individuals.

Definition

Cyst: A cyst refers to a dormant stage of a protozoan that helps to survive in unfavorable environmental conditions.

Trophozoite: A trophozoite refers to the growing stage of the parasitic protozoans that absorb nutrients from the host.

Formation

Cyst: The cyst is formed in a process called encystation.

Trophozoite: The trophozoite is formed in a process called excystation.

Type

Cyst: The cyst is a dormant stage of protozoans.

Trophozoite: The trophozoite is an active and reproductive stage of protozoans.

Stage

Cyst: Cysts are the transmission state of protozoans.

Trophozoite: Trophozoites are the disease-causing state of protozoans.

Shape

Cyst: A cyst is spherical in shape.

Trophozoite: A trophozoite is oval and pointed from the anterior end.

Size

Cyst: A cyst is 40-60 μm in diameter.

Trophozoite: A trophozoite is 50-130 μm long and 20-70 μm in wide.

Surface

Cyst: The cyst is covered with a thick, hard wall.

Trophozoite: The surface of the trophozoite is covered with cilia.

Motility

Cyst: Cysts are non-motile.

Trophozoite: Trophozoites exhibit rotary or boring motility.

Infectiousness

Cyst: The cyst is infective.

Trophozoite: Trophozoites are non-infective.

Reproduction

Cyst: Cysts are non-reproductive structures.

Trophozoite: Trophozoites reproduce through binary fission.

Nuclei

Cyst: Cysts contain a single, kidney-shaped, macronucleus.

Trophozoite: Trophozoites contain a kidney-shaped macronucleus and a spherical-shaped micronucleus.

Important Cell Structures

Cyst: The cyst contains a contractile vacuole and a cell wall with two layers.

Trophozoite: Trophozoites contain a funnel-shaped cytostome and two contractile vacuoles.

Resistance

Cyst: The cyst is resistance to water and desiccation.

Trophozoite: Trophozoites consist of a liable cell membrane, and are not resistance structures.

Survival Outside the Host

Cyst: The cysts can survive outside the host.

Trophozoite: The trophozoites cannot survive outside the host.

Conclusion

Cyst and trophozoite are two stages of the lifecycle of protozoans. The cyst is the dormant stage of the protozoans that resist unfavorable environmental conditions. The trophozoite is the active, reproductive stage of the protozoans that feeds on the host. This is the main difference between cyst and trophozoite.

Reference:

1. “Trophozoite.” Morphology, Available here.

Image Courtesy:

1. “Entamoeba histolytica 01” By Photo Credit:Content Providers(s): CDC/ Dr. George Healy – This media comes from the Centers for Disease Control and Prevention’s Public Health Image Library (Public Domain) via Commons Wikimedia
2. “Entamoeba histolytica” By Stefan Walkowski – Own work (CC BY-SA 3.0) via Commons Wikimedia