BSc 2nd Year Microbiology Eukaryotic Microbes Notes Study Material

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BSc 2nd Year Microbiology Eukaryotic Microbes Notes Study Material

Protozoa

They are a group of single-celled organisms, simple in structure, though functionally complex. More than 30,000 species have been reported which exist mostly in aquatic habitats. They are often included in zoology textbooks. They are considered in microbiology due to their unicellular structure, microscopic size and as agents of human diseases. The discipline concerned with the medically related protozoa and multicellular parasites is called parasitology.

Characteristics of Protozoa

(1) Largest organisms included in microbial world.

(2) Except a few, they lack chlorophyll or other photosynthetic pigments.

(3) Though unicellular, but able to perform all functions characteristic of multicellular organisms.

(4) Mostly live in water, damp soil or mud, in drainage ditches or puddles, in ponds or ocean.

(5) A great diversity in form as shown in Figure.

(6) Cells enclosed by a membrane which in some cases surrounded by a pellicle containing chitinlike material for rigidity. Cell wall, however, is absent. True nucleus, flagella, cilia are present. Freshwater forms take in water by osmosis and eliminate it via organelles called contractile vacuoles.

(7) Nutrition by ingestion i.e. engulf food particles by phagocytosis or through special organs of ingestion. A membrane encloses the particle, forming a food vacuole. The latter commonly joins with a submicroscopic organelle, lysosome which contains digestive enzymes.

(8) Generally heterotrophic, saprobic, some pathogenic. The feeding form is commonly known as the trophozite.

(9) Aerobic.

In Whittaker’s system, they along with certain algae are placed in the kingdom Protista. Four major classes are recognised.

(1) class Sarcodina (Amoebae)

(2) class Mastigophora (Flagellates)

(3) class Ciliophora (Ciliates)

(4) class Sporozoa (Nonmotile forms)

Sarcodina. This class includes amoebae, in which movement is by pseudopodia. These are projections of the cell membrane into which the cytoplasm flows slowly. The motion is called amoeboid motion. Reproduction is by binary fission preceded by a duplication of the nucleus. The feeding pattern is phagocytosis. Two large groups of marine amoebas are included in this class (i) Radiolaria, that are abundant in Indian and Pacific oceans; have spherical shells. (ii) The second group called Foraminifera have snail like shells and live in hard shell-like coverings composed of calcium carbonate. Amoebae also serve as research tools due to their large size and ease of cultivation. Their nucleus can be easily removed by microsurgery and transplanted to other cells. They cause amoebiasis and other infections.

Mastigophora. They possess one or more whip-like, undulating flagella to propel the cell forward. The structure of flagellum is similar to that of a sperm cell flagellum, with typical 9+2 arrangement. Undulations sweep down the flagella to the tip, and the lashing motion forces water outward to provide locomotion. This class includes the largest (about 50%) collection of protozoa. A common example is the green flagellate, Euglena gracilis, shown in Figure.

It has a single nucleus and flagellum, and an elongated cell structure. It occurs in fresh water ponds. It contains chloroplasts in the cytoplasm, thus photosynthetic. Another example of similar type is Astasia longa. This group also includes some important human parasites infecting nervous, urogenital and intestinal systems.

Ciliophora. They have complex cells that range in size from 10 µm to 3 mm. They are characterised by rows of hair-like appendages, the cilia, which structurally are similar to flagella showing 9+2 arrangement. However, they do not lash about. Rather, they beat synchronously similar to the motion of the teeth of a comb as you pass your finger down the row. This provides a smooth, even motion in contrast to the jerky motion of the flagellates.

Paramecium is slipper-shaped with a primitive gullet and mouth into which food particles are swept by cilia. There is single large macronucleus to one, or more micronuclei. During sexual conjugation, two cells make contact and a cytoplasmic bridge forms between them. A micronucleus in each cell undergoes two divisions to form four micronuclei per cell. Three micronuclei disintegrate and the remaining one undergoes division. An exchange of micronuclei takes place. The cells separate, the micronuclei fuse, and a new micronucleus forms in each cell. This recombination is observed during periods of environmental stress, a factor that suggests the formation of a genetically different and better adapted organism. At other times reproduction is by mitosis.

Another features of Paramecium is the kappa factors. These nucleic acid particles appear responsible for the synthesis of toxins. The toxins destroy those ciliates which lack these factors. Evidence suggests that these particles may be bacteria or viruses. Paramecium species also possess trichocysts – organelles than discharge filaments and trap the organism’s prey. Another feature is the contractile vacuoles, the bubble-like organelles which pump excess water from cytoplasm to external environment. Some are pathogenic.

Sporozoa. Their life cylces are very complex. Some stages in life cycle resemble bacterial or fungal spores due to which they owe this name. They are non-motile; occasionally, forms in life cycle have flagella or pseudopodia, which are short-lived. Adults are non-motile.

Most members of this group are parasites. Two members, those of malaria and toxoplasmosis are noteworthy.

Amoebiasis. The disease is characterised by painful ulcers of human intestine. The causal organism is Entamoeba histolytica (Sarcodina). It enters the body as cyst and passes through stomach acid, as shown in Figure.

In small and large intestines the amoebae emerge and begin to feed on tissue. This feeding form is known as the trophozite. Lesions coalesce to form deep ulcer, causing appendicitis-like pain. If not controlled, the amoebae eventually reach the blood vessels of intestinal wall. Now the stool becomes bloody, causing the problem of parasitemia. If amoebae localise in the lung or liver, these may be fatal. Amoebiasis may be treated with paromomycin and metronidazole and tetracycline.(BSc Microbiology Eukaryotic Microbes Notes Study Material)

Leishmaniasis (kala-azar). The causal organism is Leishmania donovani (Mastigophora). It occurs in two stages: the nonflagellated amastigote or Leishman-Donovan body, and the flagellated promastigote or leptomonad stage. The first occurs in humans, the second in the insect vector.

Kala-azar (black fever) is transmitted by the sandfly of the genus Phlebotomus which initially acquires the protozoa when it takes a blood meal from an infected patient. The parasites fill the insect’s gut and then passes to the salivary Eland from which they are injected to the next victim.

The disease is characterised by infection of the phagocytes of the body’s reticuloendothelial system (RES). These include the white blood cells and various tissue phagocytes called macrophages, which are found in the spleen, liver and lymph nodes and other organs. When the phagocytes engulf the parasites, the latter multiply within phagocyte’s cytoplasm causing it to burst. The new parasites are then engulfed by other phagocytes and the cycle is repeated.

Symptoms are fever, swelling of lymph nodes, especially in the neck, and progressive anemia, weakness and emaciation. As the number of phagocytes declines the bone marrow attempts to replace them at the expense of other white blood cells. The lowered white blood cell count is referred to as leukopenia.

The treatment includes antimony compounds and either of two normally antifungal agents, nystatin or amphotericin B.

Malaria. The disease was described as early as 5th century B.C. There are four recognised species of Plasmodium that are related to malaria: P. vivax, ovale. P. malariae and P. falciparum, all transmitted by female Anopheles mosquito. This insect consumes human blood to acquire a component for production of her eggs.

The parasite has a very complex life cycle. It takes place partly in mosquito and partly in the human blood. The parasite enters the blood stream in the sporozite form and immediately invades the liver, where it is very difficult to treat. After several hours, many of the merozites emerge and penetrate the red blood cells. Following a complex series of transformations thousands of RBCs rupture simultaneously and release tens of thousands of new parasites. At this point, the victim suffers a malaria attack. There is severe cold, temperature rises rapidly to 104°F – 106°F and there is severe headache with mild delirium. After next two or three hours, massive perspiration ends the hot stage. The patient has a sound sleep, till next attack.

During this period, the parasite enters a new set of RBC, and repeats the ansformation cycle. Plasmodium vivax and P. ovale spend about 48 hr in RBCs and there is a 48 hr interval between attacks. This malaria is called tertian malaria (three days). For P. malariae, the cycle is of about 72 hr – therefore, quartan malaria (four days). The cycle of P. falciparum is not defined and attack may occur at irregular intervals. This type of malaria is called estivo-autumnal malaria.

Death from malaria may be due to several factors, related to loss of red blood cells. The anemia is substantial. The RBC fragments accumulate in the small vessels of brain, kidneys, heart muscle, liver and other vital organs, causing their blockage. Heart attacks, cerebral hemorrhages and venal shutdowns are not uncommon.(BSc Microbiology Eukaryotic Microbes Notes Study Material)

Quinine is used to treat malaria since its discovery in 1640. Synthetic drug chloroquine is also used for the clinical phase of malaria, and primaquine for dormant phase. Recently mefloquine is also used. Most recently Walter Reed Army Institute for Research in Washington, U.S.A. has developed halofantrine, a new anti-malaria drug that is effective to all forms of multidrug-resistant malaria Halfan, as the compound known, is shown to cure more than 95% for both, P. vivax and P. falciparum.

Multicellular Parasites of Humans

These parasites are the roundworms and flatworms, commonly found throughout the world. They range in size from the tiny flukes to the tapeworms that sometimes reach 20 feet in length. Due to their infectious nature, they are considered in microbiology. Together with the unicellular protozoa they constitute the discipline known as parasitology.

Multicellular parasites are grouped in two phyla of animal kingdom: platyhelminthes (flatworms) and nemathelminthes (roundworms). The former have a broad or flat body, whereas the latter have a long threadlike appearance. The flatworms include the cestodes, known as tapeworms and the trematodes, the flukes.(BSc 2nd Year Microbiology Eukaryotic Microbes Notes Study Material)

The life cycle of the parasitic worms involves several hosts. The final host – definitive host is the one in which the adult sexually mature is developed. We shall consider here mainly those worms whose definitive host is human. The adults usually produce eggs that are released from the body after fertilisation. The eggs develop into larvae in one or more intermediate hosts. The cycle is completed when the larvae reach again to the human body by water or soil contact or by consumption of contaminated food or water.

Roundworms

The roundworms or nematodes, vary from the tiny Trichinella worm, cause of trichinosis, to the foot-long Ascaris, like an earthworm. Some are threadlike as filarial worm, others such as hookworm are more rounded. Mose of these live in human intestines and reproduce sexually by forming eggs, which are often excreted to the environment. After the development of larvae, the parasite is passed to next victim. Filarial worm depends on mosquito for transfer.

Pinworm disease. This is caused by a common nematode, Enterobius vermicularis, and the most common parasite of humans. The female, is about 10 mm long and the male of about half that size. Both live in the latter part of small intestine and in the large intestine. Typical symptoms are diarrhea.

The parasite releases thousands of fertilised eggs by female to the region of anus. Scratching the area brings the eggs to the nails and fingertips causing reinfection through mouth. Others may be infected from eggs deposited in food, or on clothing or bed linens. Usually the anal area itches intensely, which is the common sign of disease.

Roundworm disease. This is commonly caused by Ascaris lumbricoides, the largest of intestinal nematodes. Tropical and subtropical areas are the common foci of this disease. The nematode is like an earthworm. The female is over a foot long and the male somewhat smaller with a curled tail. Heavy infection may cause intestinal obstruction due to presence of tightly compacted masses of the parasite. The female is a prolific egg-producer, sometimes producing 200,000 per day. After fertilisation the eggs are excreted to soil where they hatch to larvae. These attach to plants from which are again ingested. In many parts of the world human feces – night soil is used as a fertiliser to vegetables. This helps in the spread of the parasite. Water pollution from the soil and contact with contaminated fingers are other ways of transmission. Roundworms larvae may also pass from intestine to other parts of the body. They may be passed to lungs causing a pneumonialike disorder.

Hookworm disease. This is caused by two nematodes: the old world hookworm, Ancyclostoma duodenale, and the New World hookworm, Necator americanus. Of these the former is common in Europe, Asia and the U.S.A. Both are about 10 mm long with a set of hooks or sucker devices, which allow them to attach firmly to intestine. They may induce ulcers and consume blood, causing amemia. Hookworm eggs pass into soil where the larvae emerge as long rhabditi-form larvae. These convert to hair-like flariform larvae. The flariform larvae attach to skin during contact with moist vegetation in the soil and penetrate the skin layers to the bloodstream. They localise in the lungs and are carried up the bronchi, from where they are swallowed. Infection then occurs in the intestine.

Filiariasis. The worm of this disease is a threadlike nematode – Wuchereria bancrofti, present in hot, humid climates. The worm breeds in tissues of circulatory system causing extensive inflammation and damage of lymphatic vessels. If the parasite persists longer in body, the legs or scrotum may become swollen and distorted with lymph fluid. A condition called elephantiasis may develop.(BSc 2nd Year Microbiology Eukaryotic Microbes Notes Study Material)

The females are about 100 mm long and males about half that length. Within the female, fertilised eggs give tiny eel-like embryos called microfilariae. These enter the bloodstream and are ingested by mosquitos during blood meal. They undergo further development in the muscle of the mosquito and are injected back into human during blood meal. The adults are formed in lymph vessels to establish infection.(BSc 2nd Year Microbiology Eukaryotic Microbes Notes Study Material)

Flatworms

They belong to the phylum platyhelminthes. The intestinal tract of these worms consists of a blind sac. Single worm contains both male and female sex organs (hermaphroditic). The life cycle is rather complex involving several different forms and a number of intermediate hosts. Human intermediate hosts.

The tapeworms (cestodes) consist of a head region called a scolex, a neck, and a series of segments called proglottids. The scolex holds tightly to the host tissue, and new proglottids are produced in the neck area. Sex organs are located in mature proglottids in the centre of worm. The most distal proglottids, called gravid proglottids, are filled with fertilised eggs.

The flukes (trematodes) are broad leaf-like with oral suckers for attachment to host tissue. From fertilised eggs many ciliated forms called a miracidium develop. The miracidium penetrates a snail where it is transformed into a cercaria. It is then released as an encysted metacercaria and makes its way back to the human in this form.(BSc Microbiology Eukaryotic Microbes Notes Study Material)

Beef and pork tapeworm diseases. Human is the definitive host for both the beef tapeworm, Taenia saginata and the pork tapeworm T. solium. The former is about 25 feet long, whereas the latter about 8 feet long with up to 2,000 proglottids. The main damage is caused to intestine, causing obstruction. The life cycle involves the release of gravid proglottids to soil, from where they are consumed by cattle or pigs. Embryos are released from the eggs and these are passed to the muscle of animal where they encyst. These may then be consumed in poorly cooked beef or pork.

Liver fluke disease. This is caused by Fasciola hepatica, leaf-like flat-worm, commonly found in sheep and cattle. Eggs from the animal may be deposited in water. If snails are present, the conversion from miracidium to cercaria to metacercaria may occur.

The cysts collected on vegetation, may be then ingested by humans. The intestinal wall is penetrated, followed by the migration of parasite to the liver where it localises. There may be significant damage to liver. Liver flukes are common in Far East Countries.

BSc 2nd Year Microbiology Eukaryotic Microbes Notes Study Material

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