Eukaryotic Micro-Organisms - the Protozoa As eukaryotes, these organisms have a basic cellular structure with distinct organelles and at least one nucleus. PROTOZOA ARE MOTILE, SINGLE-CELLED ORGANISMS Important point - all protozoa, because of their size (unicellular) are effectively aquatic, either marine, freshwater, or parasitic. In some way, they live in a wet environment. 1) Very heterogeneous group with many phyla that includes important human parasites that cause disease (malaria, sleeping sickness, beaver fever, dysentery) and some of the most structurally complex cells on the planet 2) Actual phylogenetic relations & classification hotly debated; several 'groupings of convenience' based mainly on locomotion that contain parasitic species:
<ul><li>Flagellates (~6,900 spp.)- use 1 or more flagella, anterior/posterior ends and includes: </li></ul><ul><li>Phytoflagellates (have chloroplasts - so plant-like); </li></ul><ul><ul><ul><li>EXAMPLES: dinoflagellates (red tides), Euglena , Volvox (colonial) </li></ul></ul></ul><ul><ul><ul><li>Zooflagellates (lack chloroplasts, most are parasitic) </li></ul></ul></ul><ul><ul><ul><li>EXAMPLES: Trypanosoma , Giardia </li></ul></ul></ul>
<ul><li>Flagella and Cilia </li></ul><ul><li>Move because microtubules slide past each other </li></ul><ul><li>Outer membrane </li></ul><ul><li>Inner Axoneme </li></ul><ul><li>9+2 microtubule structure </li></ul><ul><li>Flagella </li></ul><ul><li>Long structure </li></ul><ul><li>Whip-like motion </li></ul><ul><li>Propulsion base to tip or tip to base </li></ul><ul><li>Cilia </li></ul><ul><li>Short, stiff </li></ul><ul><li>Oar-like motion </li></ul><ul><li>Metachronal waves (sequential activity) </li></ul>
REPRODUCTION-THE SEX LIVES OF LITTLE BUGS! Remember how important sex is for hosts, but what about parasites? How much protozoan sex is there, actually? Fission, fusion, or a bit of both? Nice fruiting bodies baby! Your pseudopods ain’t so bad either, bub! Try yourself, cowboy!
Production of new individuals in many protozoa proceeds without sexual union i.e., by fission or cellular splitting. Fission is not analogous to parent/offspring, but rather sibling/sibling-2 n individuals from 1 over ‘ n ’ fission cycles. Chromosomal replication and division is by mitosis, so that individuals retain the same genetic makeup in the absence of mutation. Cycles Individuals 1 3 2 4 8 2 If haploid, then chromosome number is 1N-this system common in trypanosomes Binary Fission - production of 2 equal sister cells Replication of organelles after fission Symmetrogenic division
Multiple fission - schizogony-multiple nuclear divisions, followed by division of the cytoplasm and organelles into the same number of units, followed by membrane formation around each - but still clonal, e.g. malaria during the exoerythrocytic stage
<ul><li>The amoebas - Phylum Sarcodina; Order - Amoebida </li></ul><ul><li>Uninucleate </li></ul><ul><li>Usually with mitochondria although lacking in Entamoeba </li></ul><ul><li>No flagellated stage </li></ul><ul><li>Asexual reproduction </li></ul><ul><li>Parasites of the intestinal tracts of vertebrates and invertebrates </li></ul><ul><li>Nucleus is vesicular (many inclusions) </li></ul><ul><li>Amoebic dysentery is one of the most common diarrheas </li></ul><ul><li>3rd most common cause of parasitic death in the world </li></ul><ul><li>500 million people per year are infected at any one time with ~100,000 deaths/annum </li></ul><ul><li>Trophozoite (feeding stage) amoebas crawl around in intestine and invade mucosa where they consume host tissue and cause lesions </li></ul>
Amoebiasis - Causal Agent : Several species of the protozoan parasite genus Entamoeba infect humans, but Entamoeba histolytica is the only species known to cause disease. The other (non-pathogenic) species are important because they may be confused with E. histolytica in diagnostic investigations. 12 - 15 um - hartmanni 20 - 30 um - histolytica 5 - 9 um - hartmanni 10 - 20 um - histolytica Bulls-eye nucleus Heavy cyst wall
Infection by Entamoeba histolytica occurs by ingestion of mature cysts 1 in fecally contaminated food, water, or hands. Excystation 2 occurs in the small intestine and trophozoites 3 are released, which migrate to the large intestine. The trophozoites multiply by binary fission and produce cysts 4 , which are passed in the feces. Because of the protection conferred by their walls, the cysts can survive days to weeks in the external environment and are responsible for transmission. (Trophozoites can also be passed in diarrheal stools, but are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment.) In many cases, the trophozoites remain confined to the intestinal lumen ( A non-invasive infection) of individuals who are asymptomatic carriers, passing cysts in their stool. In some patients the trophozoites invade the intestinal mucosa B intestinal disease), or, through the bloodstream, extraintestinal sites such as the liver, brain, and lungs ( C extra-intestinal disease), with resultant pathologic manifestations. It has been established that the invasive and noninvasive forms represent two separate species, respectively E. histolytica and E. dispar, however not all persons infected with E. histolytica will have invasive disease. These two species are morphologically indistinguishable. Transmission can also occur through fecal exposure during sexual contact (in which case not only cysts, but also trophozoites could prove infective). Geographic Distribution: Worldwide, with higher incidence of amebiasis in developing countries. In industrialized countries, risk groups include male homosexuals, travelers and recent immigrants, and institutionalized populations.