BALANTIDIUM COLI: CHARACTERISTICS, LIFE CYCLE, MORPHOLOGY - BIOLOGY - 2025

Balantidium coli is a protozoan belonging to the phylum Ciliophora, considered one of the largest protozoa in existence. Described for the first time in 1857 by Malmsten, it has certain peculiar characteristics that make it a very useful organism for those specialists who carry out studies on the biology of protozoa.

This organism has infective capacity in humans, being the only ciliated protozoan that causes any pathology in them. Its innate host is the pig, but it has also been associated with other mammals such as horses and cows.

Source: By Euthman (Photo by Euthman), via Wikimedia Commons

Likewise, it has the peculiarity that it can reproduce by asexual and sexual mechanisms, which makes it a quite versatile and interesting living being.

Taxonomy

The taxonomic classification of Balantidium coli is as follows:

Domain: Eukarya

Kingdom: Protista

Phylum: Ciliophora

Class: Litostomatea

Order: Trichostomatida

Family: Balantidiidae

Genus: Balantidium

Species: Balantidium coli

General characteristics

Balantidium coli is a unicellular organism made up of a single eukaryotic cell. This means that its genetic material (DNA and RNA) is contained within a structure known as the cell nucleus.

It moves through the medium thanks to the currents that originate the movement of the cilia that cover its body. It has a spiral mobility, which allows it to be easily identified with the help of the microscope.

Likewise, Balantidium coli is considered a parasite. This is so because it requires a host to be able to develop properly. The host par excellence of B. coli is the pig.

This parasite is the only ciliated protozoan that is pathogenic for humans. In these it colonizes the large intestine and generates a disease known as Balantidiosis, which presents specific intestinal symptoms and is of care if it is not treated promptly.

Morphology

It is the largest known protozoan organism. It can measure 170 microns. Like many protozoa, throughout its life it can present two well differentiated phases: the trophozoite or vegetative form and the cyst.

The trophozoite is ovoid in shape and has small cilia on its entire surface. It also has a slightly more complex structural organization than other protozoa.

It has a primitive mouth, which is known by the name of cytostome, which is complemented by a kind of primitive digestive tube, known as cytopharynx. Similarly, it has another hole to excrete waste called cytoproct.

With the use of electron microscopy it has been possible to determine that it has two nuclei that are called macronucleus and micronucleus. These structures have a preponderant role in sexual reproduction known as conjugation.

On the other hand, the cyst is oval in shape and can measure up to 65 microns. When they are in their early stages, they present cilia, which can disappear during the maturation of the cyst.

The wall that covers them is very thick. This form of Balantidium coli is quite resistant to environmental conditions, so much so that it can survive for weeks.

Habitat

This is a parasite that is highly distributed throughout the globe. This is because its natural reservoir is the pig. However, the prevalence of infection in humans is frequent in those places where man is in frequent contact with these animals and they live with them.

Among the places with the highest incidence are South America, the Philippines and Mexico, among others.

Within the host, this organism has a predilection for the large intestine, especially the sigmoid colon and the cecum, since there are abundant nutrients for it, represented by bacteria, fungi and other microorganisms.

Nutrition

Balantidium coli is a heterotrophic organism. This implies that it is not capable of synthesizing its own nutrients, in such a way that it must feed on other organisms or substances produced by them.

This protozoan has a primitive digestive system outline that allows it to process nutrients optimally and efficiently.

The digestion process begins when food particles are brought into the cytosome by the movement of cilia found throughout the body. They are ingested and enter the body.

Inside they are included in a phagosome, which in turn fuses with a lysosome. This process is extremely important, since the latter contains the various digestive enzymes that will be responsible for degrading and transforming the ingested food into much smaller particles that are assimilated much more easily.

After they have undergone the enzymatic action of the lysosomes, the molecules obtained are used by the cell in various processes. As occurs in each and every digestive process in nature, residues remain that were not digested and therefore are of no use to the cell.

These are released to the outside through an opening located later that is known as cytoproct.

Reproduction

Two types of reproduction have been described in Balantidium coli, an asexual (binary fission) and a sexual type (conjugation). The one that has been observed most frequently is binary fission, this being of the transverse type.

Binary fission is a process by which a cell divides, creating two cells exactly the same as the parent cell. In order for it to occur, the first step is the duplication of the genetic material found within the cell nucleus.

Once this has occurred, the cell begins to undergo a division of its cytoplasm, a process known as cytosinesis. In this particular case, the division occurs transversely, that is, perpendicular to the axis of the spindle. Finally the cell membrane also divides and two eukaryotic cells 100% equal to the cell that gave them life originate.

In the case of conjugation, an exchange of genetic material occurs between two Balantidium coli cells. The first thing that happens is that within each cell the micronuclei undergo successive divisions. At the end there are two pronuclei in each one, one that will migrate to the other cell and the other that will not.

Subsequently, both cells put their cytosomes in contact and exchange micronuclei. This done, both cells separate. Inside each one, the foreign micronuclei that entered merge with the remaining micronucleus, forming a zygotic nucleus that will undergo successive divisions until it returns to being a cell with a macro-nucleus and a micro-nucleus.

Lifecycle

Two forms can be seen in the life cycle of Balantidium coli: the trophozoite and the cyst. Of the two, the latter is the infectious form.

The cysts are ingested by the host through water or food that has not been processed correctly, following minimum hygiene measures. For this reason they are infested with cysts of this parasite.

Once inside the host, at the level of the stomach, by the action of gastric juices, the protective wall begins to disintegrate, a process that ends at the level of the small intestine. Already here, the trophozoites are released and reach the large intestine to initiate colonization of it.

In the large intestine, trophozoites develop and begin to reproduce through the process of binary fission (asexual reproduction). They can also reproduce by a sexual mechanism known as conjugation.

Gradually they are dragged through the intestine, while they metamorphose back into cysts. These are expelled together with the feces.

It is important to clarify that not all individuals follow this path. Some of the trophozoites formed remain on the colon wall and multiply there, generating a clinical picture in which liquid stools predominate.

epidemiology

Balantidium coli is a pathogenic organism that is capable of generating infections in humans, specifically in the large intestine. The pathology they cause in humans is known as Balantidiasis.

Transmission

The transmission mechanism is through ingestion of the cysts, in contaminated water or food. After transit through the digestive tract, it reaches the large intestine, where, thanks to the production of a chemical called hyaluronidase, it can penetrate the mucosa and settle there and cause various lesions.

Clinical picture

Sometimes people are infected with the parasite but do not show any symptoms. Therefore they are asymptomatic carriers.

In symptomatic cases, the following symptoms occur:

• Diarrhea episodes. This can be mild, present mucus and in some cases even blood.
• Abdominal pain
• Vomiting
• Headache
• Anemia
• Lack of appetite and consequently weight loss.

Diagnosis

To diagnose this pathology, it is enough to analyze the feces. If the person is infected, there will be cysts and trophozoites in the stool.

Treatment

Treatment encompasses various medications, the most commonly used being metronidazole, tetracycline, iodoquinol, and nitasoxanide, among others.

References

1. Arean V and Koppisch E. (1956). Balantidiasis. A review and report of cases. J. Pathol. 32: 1089-1116.
2. Beaver P, Cupp E and Jung P. (1990). Medical Parasitology. 2nd ed. Salvat editions. pp. 516.
3. Devere, R. (2018). Balantidiosis: Some historical and epidemiological notes in Latin America with special reference to Venezuela. Know 30. 5-13
4. Gállego Berenguer, J. (2007). Manual of Parasitology: Morphology and Biology of parasites of sanitary interest. Edicions Universitat de Barcelona. 2nd ed. Pp. 119-120
5. Kreier, J. and Baker, J. (1993). Parasitic Protozoa. Academic Press. Second edition.