5.1 Unicellular Eukaryotic Parasites

Amoebozoa

The supergroup Amoebozoa includes protozoans that use amoeboid movement. Actin microfilaments produce pseudopodia, into which the remainder of the protoplasm flows, thereby moving the organism. The genus Entamoeba includes commensal or parasitic species, including the medically important E. histolytica, which is transmitted by cysts in feces and is the primary cause of amoebic dysentery. Another member of this group that is pathogenic to humans is Acanthamoeba, which can cause keratitis (corneal inflammation) and blindness. The notorious “brain eating amoeba,” Naegleria fowleri, is a considered a distant relative of the Amoebozoa and is classified in the phylum Percolozoa.

The Eumycetozoa are an unusual group of organisms called slime molds, which have previously been classified as animals, fungi, and plants (Figure 5.9). Slime molds can be divided into two types: cellular slime molds and plasmodial slime molds. The cellular slime molds exist as individual amoeboid cells that periodically aggregate into a mobile slug. The aggregate then forms a fruiting body that produces haploid spores. Plasmodial slime molds exist as large, multinucleate amoeboid cells that form reproductive stalks to produce spores that divide into gametes. One cellular slime mold, Dictyostelium discoideum, has been an important study organism for understanding cell differentiation, because it has both single-celled and multicelled life stages, with the cells showing some degree of differentiation in the multicelled form. Figure 5.10 and Figure 5.11 illustrate the life cycles of cellular and plasmodial slime molds, respectively.

Figure 5.9 (a) The cellular slime mold Dictyostelium discoideum can be grown on agar in a Petri dish. In this image, individual amoeboid cells (visible as small spheres) are streaming together to form an aggregation that is beginning to rise in the upper right corner of the image. The primitively multicellular aggregation consists of individual cells that each have their own nucleus. (b) Fuligo septica is a plasmodial slime mold. This brightly colored organism consists of a large cell with many nuclei.

Figure 5.10 The life cycle of the cellular slime mold Dictyostelium discoideum primarily involves individual amoebas but includes the formation of a multinucleate plasmodium formed from a uninucleate zygote (the result of the fusion of two individual amoeboid cells). The plasmodium is able to move and forms a fruiting body that generates haploid spores. (credit “photo”: modification of work by “thatredhead4”/Flickr)

Figure 5.11 Plasmodial slime molds exist as large multinucleate amoeboid cells that form reproductive stalks to produce spores that divide into gametes.

Chromalveolata

The supergroup Chromalveolata is united by similar origins of its members’ plastids and includes the apicomplexans, ciliates, diatoms, and dinoflagellates, among other groups (we will cover the diatoms and dinoflagellates in Algae). The apicomplexans are intra- or extracellular parasites that have an apical complex at one end of the cell. The apical complex is a concentration of organelles, vacuoles, and microtubules that allows the parasite to enter host cells (Figure 5.12). Apicomplexans have complex life cycles that include an infective sporozoite that undergoes schizogony to make many merozoites (see the example in Figure 5.4). Many are capable of infecting a variety of animal cells, from insects to livestock to humans, and their life cycles often depend on transmission between multiple hosts. The genus Plasmodium is an example of this group.

Figure 5.12 (a) Apicomplexans are parasitic protists. They have a characteristic apical complex that enables them to infect host cells. (b) A colorized electron microscope image of a Plasmodium sporozoite. (credit b: modification of work by Ute Frevert)

Other apicomplexans are also medically important. Cryptosporidium parvum causes intestinal symptoms and can cause epidemic diarrhea when the cysts contaminate drinking water. Theileria (Babesia) microti, transmitted by the tick Ixodes scapularis, causes recurring fever that can be fatal and is becoming a common transfusion-transmitted pathogen in the United States (Theileria and Babesia are closely related genera and there is some debate about the best classification). Finally, Toxoplasma gondii causes toxoplasmosis and can be transmitted from cat feces, unwashed fruit and vegetables, or from undercooked meat. Because toxoplasmosis can be associated with serious birth defects, pregnant people need to be aware of this risk and use caution if they are exposed to the feces of potentially infected cats. A national survey found the frequency of individuals with antibodies for toxoplasmosis (and thus who presumably have a current latent infection) in the United States to be 11%. Rates are much higher in other countries, including some developed countries.³ There is also evidence and a good deal of theorizing that the parasite may be responsible for altering infected humans’ behavior and personality traits.⁴

The ciliates (Ciliaphora), also within the Chromalveolata, are a large, very diverse group characterized by the presence of cilia on their cell surface. Although the cilia may be used for locomotion, they are often used for feeding, as well, and some forms are nonmotile. Balantidium coli (Figure 5.13) is the only parasitic ciliate that affects humans by causing intestinal illness, although it rarely causes serious medical issues except in the immunocompromised (those having a weakened immune system). Perhaps the most familiar ciliate is Paramecium, a motile organism with a clearly visible cytostome and cytoproct that is often studied in biology laboratories (Figure 5.14). Another ciliate, Stentor, is sessile and uses its cilia for feeding (Figure 5.15). Generally, these organisms have a micronucleus that is diploid, somatic, and used for sexual reproduction by conjugation. They also have a macronucleus that is derived from the micronucleus; the macronucleus becomes polyploid (multiple sets of duplicate chromosomes), and has a reduced set of metabolic genes.

Ciliates are able to reproduce through conjugation, in which two cells attach to each other. In each cell, the diploid micronuclei undergo meiosis, producing eight haploid nuclei each. Then, all but one of the haploid micronuclei and the macronucleus disintegrate; the remaining (haploid) micronucleus undergoes mitosis. The two cells then exchange one micronucleus each, which fuses with the remaining micronucleus present to form a new, genetically different, diploid micronucleus. The diploid micronucleus undergoes two mitotic divisions, so each cell has four micronuclei, and two of the four combine to form a new macronucleus. The chromosomes in the macronucleus then replicate repeatedly, the macronucleus reaches its polyploid state, and the two cells separate. The two cells are now genetically different from each other and from their previous versions.

Figure 5.13 This specimen of the ciliate Balantidium coli is a trophozoite form isolated from the gut of a primate. B. coli is the only ciliate capable of parasitizing humans. (credit: modification of work by Kouassi RYW, McGraw SW, Yao PK, Abou-Bacar A, Brunet J, Pesson B, Bonfoh B, N’goran EK & Candolfi E)

Figure 5.14 Paramecium has a primitive mouth (called an oral groove) to ingest food, and an anal pore to excrete it. Contractile vacuoles allow the organism to excrete excess water. Cilia enable the organism to move.

Figure 5.15 This differential interference contrast micrograph (magnification: ×65) of Stentor roeselie shows cilia present on the margins of the structure surrounding the cytostome; the cilia move food particles. (credit: modification of work by “picturepest”/Flickr)

Öomycetes have similarities to fungi and were once classified with them. They are also called water molds. However, they differ from fungi in several important ways. Öomycetes have cell walls of cellulose (unlike the chitinous cell walls of fungi) and they are generally diploid, whereas the dominant life forms of fungi are typically haploid. Phytophthora, the plant pathogen found in the soil that caused the Irish potato famine, is classified within this group (Figure 5.16).

Figure 5.16 A saprobic oomycete, or water mold, engulfs a dead insect. (credit: modification of work by Thomas Bresson)

Excavata

The third and final supergroup to be considered in this section is the Excavata, which includes primitive eukaryotes and many parasites with limited metabolic abilities. These organisms have complex cell shapes and structures, often including a depression on the surface of the cell called an excavate. The group Excavata includes the subgroups Fornicata, Parabasalia, and Euglenozoa. The Fornicata lack mitochondria but have flagella. This group includes Giardia lamblia (also known as G. intestinalis or G. duodenalis), a widespread pathogen that causes diarrheal illness and can be spread through cysts from feces that contaminate water supplies (Figure 5.3). Parabasalia are frequent animal endosymbionts; they live in the guts of animals like termites and cockroaches. They have basal bodies and modified mitochondria (kinetoplastids). They also have a large, complex cell structure with an undulating membrane and often have many flagella. The trichomonads (a subgroup of the Parabasalia) include pathogens such as Trichomonas vaginalis, which causes the human sexually transmitted disease trichomoniasis. Trichomoniasis often does not cause symptoms in males, but they are able to transmit the infection. In females, it causes vaginal discomfort and discharge and may cause complications in pregnancy if left untreated.

The Euglenozoa are common in the environment and include photosynthetic and nonphotosynthetic species. Members of the genus Euglena are typically not pathogenic. Their cells have two flagella, a pellicle, a stigma (eyespot) to sense light, and chloroplasts for photosynthesis (Figure 5.17). The pellicle of Euglena is made of a series of protein bands surrounding the cell; it supports the cell membrane and gives the cell shape.

The Euglenozoa also include the trypanosomes, which are parasitic pathogens. The genus Trypanosoma includes T. brucei, which causes African trypanosomiasis (African sleeping sickness and T. cruzi, which causes American trypanosomiasis (Chagas disease). These tropical diseases are spread by insect bites. In African sleeping sickness, T. brucei colonizes the blood and the brain after being transmitted via the bite of a tsetse fly (Glossina spp.) (Figure 5.18). The early symptoms include confusion, difficulty sleeping, and lack of coordination. Left untreated, it is fatal.

Figure 5.17 (a) This illustration of a Euglena shows the characteristic structures, such as the stigma and flagellum. (b) The pellicle, under the cell membrane, gives the cell its distinctive shape and is visible in this image as delicate parallel striations over the surface of the entire cell (especially visible over the grey contractile vacuole). (credit a: modification of work by Claudio Miklos; credit b: modification of work by David Shykind)

Figure 5.18 Trypanosoma brucei, the causative agent of African trypanosomiasis, spends part of its life cycle in the tsetse fly and part in humans. (credit “illustration”: modification of work by Centers for Disease Control and Prevention; credit “photo”: DPDx/Centers for Disease Control and Prevention)

Chagas’ disease originated and is most common in Latin America. The disease is transmitted by Triatoma spp., insects often called “kissing bugs,” and affects either the heart tissue or tissues of the digestive system. Untreated cases can eventually lead to heart failure or significant digestive or neurological disorders.

The genus Leishmania includes trypanosomes that cause disfiguring skin disease and sometimes systemic illness as well.