V. PROTOZOA Unicellular, chemoheterotrophic, eukaryotic organisms

of kingdom Protista (3-2000 m). Protozoan means “first animal”. 20,000 species, only a few are pathogens. Most are free-living organisms that inhabit water and

soil. Some live in association with other organisms as parasites or symbionts.

Reproduce asexually by fission, budding, or schizogony. Some exhibit sexual reproduction (e.g.: Paramecium). Trophozoite: Vegetative stage which feeds upon

bacteria and particulate nutrients. Cyst: Some protozoa produce a protective capsule

under adverse conditions (toxins, scarce water, food, or oxygen).

V. PROTOZOA (Continued) Nutrition Most are heterotrophic aerobes. Intestinal

protozoa can grow anaerobically. Some ingest whole algae, yeast, bacteria, or

smaller protozoans. Others live on dead and decaying matter. Parasitic protozoa break down and absorb nutrients from their hosts.

Some transport food across the membrane. Others have a protective covering (pellicle)

and required specialized structures to take in food.◦ Ciliates take in food through a cytostome.

Digestion takes place in vacuoles. Waste may be eliminated through plasma

membrane or an anal pore.

Medically Important Protozoa 1. Amoeboflagellates (Phylum Sarcomastigophora) Move using pseudopods (false feet) or flagella.

A. Amoebas (Subphylum Sarcodina) Move by extending blunt, lobelike projections

(pseudopods). Amoebas engulf food with pseudopods and

phagocytize it. Several species cause amoebic dysenteries of

varying degrees of severity.◦ Entamoeba hystolytica: Feeds on red blood cells.

Produces dysentery and extraintestinal cysts.◦ Dientamoeba fragilis: Found in 4% of humans.

Usually commensal. Can cause chronic, mild diarrhea.

Other diseases include:◦ Meningoencephalitis: Caused by Naegleria

fowleri. Penetrate nasal mucosa of swimmers in warm waters. Mortality rate almost 100%.

◦ Keratitis: Caused by Acanthamoeba. Can cause blindness. Associated with use of contact lenses.

B. Flagellates (Subphylum Mastigophora) Move by one or more whiplike flagella. Some

parasitic flagellates have up to eight flagella. Most are spindle shaped with flagella projecting from

anterior end. Outer membrane is a tough pellicle. Food is

ingested through an oral groove or cytosotome. Important pathogens:◦ Trichomonas vaginalis: Causes genital and urinary

infections. Has undulating membrane. Lacks a cyst stage. Transmitted sexually or by fomites.

◦ Giardia lamblia: Causes a persistent intestinal infection (giardiasis) with diarrhea, nausea, flatulence, and cramps. In U.S. most common cause of waterborne diarrhea. About 7% of U.S. population are healthy carriers.

◦ Trypanosoma brucei gambiense: Hemoflagellate (blood parasite). Causes African sleeping sickness.

◦ Trypanosoma cruzi: Hemoflagellate that causes Chaga’s disease, a cardiovascular disease common in Texas and Latin America.

Medically Important Protozoa (Continued)

2. Apicomplexans (Phylum Apicomplexa) Not motile in their mature form. Obligate intracellular parasites. Have specialized organelles at tip (apex)

of cells that penetrate host tissues.

Complex life cycles. May have more than

one host. Definitive host: Harbors

sexually reproducing form. Intermediate

host: In which asexual reproduction

occurs.

Medically Important Protozoa (Continued) 2. Apicomplexans (Phylum Apicomplexa) Important pathogens:◦ Plasmodium vivax and falciparum: Cause malaria in

humans (intermediate host). Initially treated with quinine, drug resistance is a major problem today. Major cause of worldwide mortality: Kill 3 million people/year and infect 500 million.Transmitted by Anopheles mosquito (definitive host).DDT was used extensively in 1960s in an attempt to eradicate the mosquito vector.Successful vaccine not available yet.

Medically Important Protozoa (Continued) 2. Apicomplexans (Phylum Apicomplexa) Important pathogens:

◦ Toxoplasma gondii: Causes toxoplasmosis in humans. Causes blindness and lymphatic infections in adults. Dangerous to pregnant women, causes severe neurological defects in unborn children. Cats are part of life cycle, oocysts excreted in feces. Contact with infected feces or meat are means of transmission.

◦ Cryptosporidium: Causes respiratory and gallbladder infections in immunosuppressed individuals. Found in intestines of mammals and water. Major cause of death in AIDS patients.

◦ Cyclospora cayetensis: New parasite (1996) caused diarrhea associated with raspberries.

Medically Important Protozoa (Continued)

3. Ciliates (Phylum Ciliophora)

Move and obtain food using cilia.

Only known human pathogen is Balantidium

coli, which causes a severe intestinal

infection in pigs and humans.

4. Microsporans (Phylum Mycrospora)

Obligate intracellular parasites, lack

mitochondria and microtubules.

Discovered in 1984 to cause chronic diarrhea

and conjunctivitis, mainly in AIDS patients.

IV. SLIME MOLDS Have both fungal and animal characteristics.

◦ Amoeboid stage◦ Produce spores like fungi

Eukaryotic, classified as protists. Cellular and plasmodial (acellular) slime molds. Life Cycle of Cellular Slime Molds:◦ Amoeba stage: Germinate from a spore.◦ Slug stage: Many amoebas aggregate and sheath

forms. Migration.◦ Fruiting body: Releases spores which germinate into

amoebas.

Plasmodial (Acellular) Slime Molds◦ Mass of protoplasm with many nuclei (plasmodium).◦ Capable of sexual reproduction.