Protozoa

Microscopic life forms are everywhere on Earth. These small forms of life are usually single-celled organisms: protozoa and bacteria. Though both protozoa and bacteria are single-celled organisms, bacteria do not have some of the internal structures (organelles) found in protozoa, such as nuclei and vacuoles, that mimic the organs of multicellular organisms.

With about 35,000 species, protozoa are remarkably diverse in form and activity. There are nearly as many kinds of protozoa as there are plants and animals in the visible world. Three main groupings of protozoa are based largely on how they move. Ciliates move by beating action of very small hair-like projections on the surface of their body. Flagellates have one or more long whip-like projections, called flagella. The beating motion of the flagella moves protozoa through water, much like the action of a fish's tail. Amoebae (Sarcodina) are flattened protozoa that move by bulging outward along their edges.¹ Many protozoa are permanently attached to surfaces. Sporozoa are parasitic protozoa which generally obtain nutrients by absorbing organic molecules from the host organism. They often have very complicated life cycles.

Unlike other single-celled organisms such as algae and fungi, protozoa do not have rigid cell walls. The shapes of protozoa are maintained, instead, by an interior protein skeleton called the cytoskeleton. The cytoskeleton is a network of protein fibers that provide structural rigidity but also flexibility. The flexible cell surface allows protozoa to engulf and digest bacteria and smaller eukaryotic microorganisms. Like algae, protozoa reproduce by transverse (ciliates) or longitudinal (flagellates) division, although in some protozoa sexual reproduction also occurs. During this division process, called binary fission, the organism divides into two equal-sized daughter cells.⁶

Protozoa graze on other life forms by ingesting and killing bacteria or smaller eukaryotes. Others, such as the protozoa that cause malaria, live by parasitizing larger animals.

Ciliated protozoan

Parasitic Protozoa

Most protozoa are free-living (they have no hosts). Parasitic protozoa live in or upon animal or human in cell (cytozoic) or in tissue (histozoic) and may cause diseases. Examples of infections caused by protozoa are malaria and sleeping thickness. Pathogenic protozoa have several common features. For example, many protozoa have both a dormant (immotile) cyst stage that permits survival when enviromental conditions are hostile, and a motile, actively feeding and reproducing, vegetative (trophozoite) stage.⁴ Following is the list of important parasitic protozoa:

• Intestinal flagellates: Trichomonas, Giardia, Histomonas.
• Hemoflagellates: Leishmania, Trypanosoma
• Sporozoa: Monocystis, Eimeria, Isospora, Plasmodium, Nosema, Babesia, Toxoplasma, Cryptosporidium.
• Rhizopoda: Entamoeba
• Ciliata: Balantidium, Nyctotherus, Opalina.
• Suctoria²

Three principal intestinal parasites cause diarrheal disease: the amoeba, Entamoeba histolytica; Giardia lamblia and several varieties of Cryptosporidium. They have been implicated in a number of outbreaks associated with drinking contaminated water. A good way to acquire Giardia is to drink the water from clear mountain streams during a hiking trip. Giardia can also be acquired from tap water where sanitation procedures are inadequate. Giardiasis is a diarrheal disease that can range in severity from a mild diarrhea to excessive dehydration and weight loss which can be fatal.³

Amebic dysentery is caused by ingesting cysts of Entamoeba histolytica from contaminated food or water. From the small intestine and colon where trophozoites may induce ulceration, they can spread to the liver and cause abscesses.

Leishmania protozoa live inside white blood cells that should swallow up and kill the protozoa. They do swallow them up but are unable to finish the job. The Leishmania thrive inside white blood cells. Over time, they destroy these cells and, much like HIV, wreak havoc on the immune system.⁵