III. PROTOZOA
B. MEDICALLY IMPORTANT PROTOZOA
Fundamental Statements for this Learning Object:
1. Protozoan diseases include amoebic dysentery, giardiasis, balantidiasis, cryptosporidiosis African sleeping sickness, acanthamoebiasis, toxoplasmosis, and genitourinary trichomoniasis.
2. Many of the same factors that enable bacteria to colonize a host also enable protozoans to colonize a host.
3. Many of the same factors that enable bacteria to harm the host enable protozoans to harm the host.
LEARNING OBJECTIVES FOR THIS SECTION
Protozoa are unicellular eukaryotic microorganisms lacking a cell wall and belonging to the Kingdom Protista. The vegetative, reproducing, feeding form of a protozoan is called a trophozoite (def). Under certain conditions, some protozoa produce a protective form called a cyst (def) that enables them to survive harsh environments. Cysts allow some pathogens to survive outside their host.
Medically Important Protozoa
1. The Sarcomastigophora (Amoeboflagellates)
The amoebas (subphylum Sarcodina) move by extending lobelike projections of their cytoplasm called pseudopodia (def).
a. Entamoeba histolytica (see Fig. 1), which causes a gastrointestinal infection called amoebic dysentery . The organism produces protective cysts which pass out of the intestines of the infected host and are ingested by the next host. It is transmitted by the fecal-oral route.
b. Acanthamoeba can cause rare, but severe infections of the eye, skin, and central nervous system. Acanthamoeba kerititis (see Fig. 2) is an infection of the eye that typically occurs in healthy persons and can result in blindness or permanent visual impairment. Granulomatous amebic encephalitis (GAE) is an infection of the brain and spinal cord typically occurring in persons with a compromised immune system. Acanthamoeba is found in soil, dust, and a variety of water sources including lakes, tap water, swimming pools, and heating and air conditioning units. It typically enters the eyes and most cases are associated with contact lens use, but it can also enter cuts or wounds and be inhaled.
c. Naegleria fowleri (sometimes called the"brain-eating amoeba"), is another amoeba that can cause a rare but devastating infection of the brain called primary amebic meningoencephalitis (PAM). The amoeba is commonly found in warm freshwater rivers, lakes, rivers, and hot springs, as well as in the soil. It typically causes infections when contaminated water enters the body through the nose where it can subsequently travel to the brain.
The flagellates (subphylum Mastigophora) move by means of flagella. Some also have an undulating membrane (def).
a. Giardia lamblia (see Fig. 3) can cause a gastrointestinal infection called giardiasis. Cysts pass out of the intestines of the infected host and are ingested by the next host. It is transmitted by the fecal-oral route.
- Scanning electron micrograph of Giardia (see Fig. 4);courtesy of CDC.
b. Trichomonas vaginalis (see Fig. 5) infects the vagina and the male urinary tract causing an infection called genitourinary trichomoniasis. It does not produce a cysts stage and is usually transmitted by sexual contact.
c. Trypanosoma brucei gambiens (see Fig. 6) causes African sleeping sickness and is transmitted by the bite of an infected Tsetse fly. The disease primarily involves the lymphatic and nervous systems of humans.
2. The Ciliophora
The ciliates move by means of cilia.
a. The only pathogenic ciliate is Balantidium coli (see Fig. 7) which causes a diarrhea-type infection called balantidiasis. Cysts pass out of the intestines of the infected host and are ingested by the next host. It is transmitted by the fecal-oral route.
3. The Apicomplexans
The apicomplexans are not motile in their mature forms, reproduce both asexually and sexually, and often have complex life cycles for transmission from host to host. They possess a complex of organelles called apical complexes (def) at their apex that contain enzymes used in penetrating host tissues.
a. Species of Plasmodium (see Fig. 8A and Fig. 8B) cause malaria and are transmitted by the bite of an infected female Anopheles mosquito. They reproduces asexually by schizogony (def) in human liver cells and red blood cells but also reproduce sexually by gametes (def) in the mosquito (see Fig. 1), the life cycle of Plasmodium. In the case of malaria caused by P. vivax and P. ovale, a dormant form or hypnozoite remains in the liver and may cause later relapses.
b. Toxoplasma gondii is another intracellular apicomplexan and causes toxoplasmosis. It can infect most mammals and is contracted by inhaling or ingesting cysts from the feces of infected domestic cats, where the protozoa reproduce both asexually and sexually, or by ingesting raw meat of an infected animal. Toxoplasmosis is usually mild in people with normal immune responses but can infect the brain, heart, or lungs of people who are immunosuppressed. It can also be transmitted congenitally (def) and infect the nervous system of the infected child.
c. Cryptosporidium is an intracellular (def) parasite that causes a gastrointestinal infection called cryptosporidiosis, although in people who are immunosuppressed it can also cause respiratory and gallbladder infections. It is transmitted by the fecal-oral route.
4. Virulence Factors that Promote Colonization of Protozoans
Virulence factors that promote protozoal colonization of the host include the ability to:
1. contact host cells;
2. adhere to host cells and resist physical removal;
3. invade host cells;
4. compete for nutrients;
5. resist innate immune defenses such as phagocytosis and complement; and
6. evade adaptive immune defenses.
Examples of virulence factors that promote protozoal colonization include:a. Some protozoa, such as Entamoeba histolytica,Trichomonas vaginalis, Giardia lamblia, and Balantidium coli use pseudopodia, flagella or cilia to swim through mucus and contact host cells.
b. Protozoa use adhesins (def) associated with their cytoplasmic membrane to adhere to host cells, colonize, and resist flushing.
c. Some protozoa, such as the apicomplexans (Plasmodium (inf), Toxoplasma gondii (inf), and Cryptosporidium (inf)) possess a complex of organelles called apical complexes at their apex that contain enzymes used in penetrating host tissues and cells.
d. Protozoans such as Trypanosoma brucei gambiens (inf) and Plasmodium species (inf) are able to change their surface antigens (def) during their life cycle in the human. As the protozoa change the amino acid sequence and shape of their surface antigens, antibodies (def) and cytotoxic T-lymphocytes (def) made against a previous shape will no longer fit and the body has to start a new round of adaptive immunity against the new antigen shape.
e. Some protozoa, such as Entamoeba histolytica (inf) shed their surface antigens so that antibodies made by the body against these surface antigens are tied up by the shed antigens.
Medscape article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.
Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.
Last updated: Feb., 2020
Please send comments and inquiries to Dr.
Gary Kaiser