Introduction to Microbiology
The Protozoa
All protozoal species are assigned to the kingdom
Protista in the Whittaker classification. The protozoa are then placed into various groups primarily on the basis of how they move. The groups are called phyla (singular, phylum) by some microbiologists, and classes by others. Members of the four major groups are illustrated in Figure
1 .
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Mastigophora. The Mastigophora are protozoa having one or more flagella. Reproduction in these protozoa generally occurs by fission, although sexual reproduction is observed in some species. Members of the group include the organisms that cause intestinal distress ( Giardia lamblia) and sleeping sickness ( Trypanosoma brucei) in humans. Some of these protozoa have only a single flagellum (for example, Trypanosoma), but others may have up to eight (for example, Giardia). Flagellar motion tends to be jerky, so flagellates can be identified in live specimens by this characteristic movement.
Also in this group is Euglena gracilis, a flagellated organism that also has chlorophyll pigments for photosynthesis. For this reason, the organism is considered both a unicellular alga and a protozoan. Since it has the characteristics of both animals (motion) and plants (photosynthesis), many evolutionists consider Euglena to be a source of all complex organisms.
Flagellates are very common in nature, and numerous species may be observed in pond and swamp water, aquarium water, and birdbath water. The flagellum may either push the protozoan ahead or lead the cell along. A complex basal structure at one end of the cell anchors the flagellum to the cell.
Sarcodina. Members of the group Sarcodina are amoebas. These organisms move by pseudopodia, although flagella may be present in the reproductive stages. Cytoplasmic streaming assists movement. Asexual reproduction occurs by fission of the cell. Sarcodina includes two marine groups known as foraminiferans and radiolarians. Both groups were present on earth when the oil fields were in formative stages, and marine geologists use them as potential markers for oil fields.
Another member, Entamoeba histolytica, is the cause of amoebic dysentery. This organism can cause painful lesions of the intestine if it is contracted from contaminated water.
Ciliophora. The Ciliophora group includes protozoa that move by means of cilia. Two types of nuclei, the macronucleus and the micronucleus, are often found in the cells, and a contractile vacuole is usually present. Conjugation may be used for sexual reproduction, and binary fission also occurs. The distinctive rows of cilia vibrate in synchrony and propel the organism in one direction.
Paramecium is a member of this group, as are many other free-living ciliates such as Tetrahymena. Paramecium cells are known for their ability to conjugate and exchange genetic material, thereby acquiring new genetic characteristics. The cells also have kappa factors, which are chemical substances known to destroy other protozoal cells, and they produce trichocysts, which are submicroscopic stinging particles. Indeed, the physiology of a single Paramecium cell is similar to that of a multicellular organism.
Apicomplexa. Members of the group Apicomplexa form spores at one stage in their life cycle. For this reason, the group is also known as Sporozoa. Reproduction is a complex phenomenon in this group, and all species are parasites. Members of the group display no means of locomotion in the adult form. The agents of malaria and toxoplasmosis are classified here. Another important pathogen is Pneumocystis carinii, a cause of sometimes-lethal pneumonia in AIDS patients; over half the deaths known to be associated with AIDS have been caused by P. carinii.
