Interactions Involving Microorganisms

Interactions Involving Microorganisms

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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 1 INTERACTIONS INVOLVING MICROORGANISMS Symbiosis is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, parasitic, etc. The organisms, each termed a symbiont, may be of the same or different species. Symbiosis can be obligatory, which means that one or both of the symbionts entirely depend on each other for survival, or facultative (optional) when they can generally live independently. Symbiosis is also classified by physical attachment; symbiosis in which the organisms have a bodily union is called conjunctive symbiosis, and symbiosis in which they are not in a union is called disjunctive symbiosis. When one organism lives on the surface of another, it is called ectosymbiosis; when one partner lives inside the tissues of another, it is termed endosymbiosis. The six possible types of symbiosis are (1) mutualism, (2) commensalism, (3) parasitism, (4) neutralism, (5) amensalism/antagonism, and (6) competition. These are distinguished by the degree of benefit or harm they cause to each partner. Another type of interaction – predation is also very important among microbes. The details of these interactions are as follows: 1. Mutualism: It is an interaction among species, where species derive a mutual benefit. 2. Commensalism: It is an interaction that benefits one organism and the other organism is neither benefited nor harmed. 3. Parasitism: It is a relationship between species, where one organism, the parasite, takes food and shelter from another organism, the host, and harms it in return. 4. Neutralism: In this case, two species interact but do not affect each other. 5. Amensalism: It is an interaction where an organism inflicts harm to another organism without any costs or benefits received by itself. In one type of amensalism – called antagonism, microorganisms secrete chemical substances to harm other microbes. 6. Competition: It is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition. 7. Predation: It is a biological interaction where one organism, the predator, kills and eats another organism, its prey. MICROBE – MICROBE INTERACTIONS
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 2 Interactions among microorganisms are of the following types: NEUTRALISM Neutralism occurs when microorganisms do not affect each other despite their growth in fairly close contact. Neutralism can occur in natural communities if the culture density is low, the nutrient level is high, and each culture has distinct requirements for growth. Neutralism may occur in early colonization of an environment without either harmful or beneficial interactions by the microorganisms introduced. MUTUALISM An example of mutualism among microorganisms is lichens: Lichens (Fungi & Algae / Cyanobacteria): Lichens are an example of a mutualism in which a fungus (usually a member of the Ascomycota or Basidiomycota phyla) lives in close contact with a photosynthetic organism (a eukaryotic alga or a prokaryotic cyanobacterium). Generally, neither the fungus nor the photosynthetic organism can survive alone outside of the symbiotic relationship. The body of a lichen, referred to as a thallus, is formed of hyphae wrapped around the photosynthetic partner. Figure: Thallus of lichen – This cross-section of a lichen thallus shows the (a) upper cortex of fungal hyphae, which provides protection; the (b) algal zone where photosynthesis occurs, the (c) medulla of fungal hyphae, and the (d) lower cortex, which also provides protection and may have (e) rhizines to anchor the thallus to the substrate.
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 3 The photosynthetic organism provides carbon and energy in the form of carbohydrates. Some cyanobacteria fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association. In return, the fungus supplies minerals and protection from dryness and excessive light by encasing the algae in its mycelium. The fungus also attaches the symbiotic organism to the substrate. Lichens display a range of colors and textures. They can survive in the most unusual and hostile habitats. They cover rocks, gravestones, tree bark, and the ground in the tundra where plant roots cannot penetrate. Lichens can survive extended periods of drought: they become completely desiccated and then rapidly become active once water is available again. Lichens fulfill many ecological roles, including acting as indicator species, which allow scientists to track the health of a habitat because of their sensitivity to air pollution. COMMENSALISM In soil, many microbial species such as many fungi degrade cellulose and lignin which cannot be utilized by many bacteria. But these bacteria can and do utilize the byproducts of the aforementioned breakdown of cellulose and lignin by fungi. This indirect interaction benefits many bacterial species and allows them to grow in environments in which their diet is deficient. ANTAGONISM (AMENSALISM) It is the relationship in which one species of an organism is inhibited or adversely affected by chemical substances produced by another species in the same environment. It may either be specific (production of antibiotics or bacteriocins) or non-specific (metabolic end-products): Production of Antibiotics: Several species of bacteria and fungi produce chemicals that inhibit the growth of other microorganisms. As a result of this production of antibiotics in nature, susceptible organisms are prevented from becoming dominant in the population and the producers of the antibiotics are given the competitive advantage for growth. An example of this is the production of penicillin by fungi; this antibiotic inhibits a type of cell wall (composed of peptidoglycan) found only in bacteria. Since the cell wall of fungi does not contain peptidoglycan (it is mainly composed of chitin), it is unaffected by the penicillin that it produced. Production of Bacteriocins: Many different taxonomic groups of bacteria produce bacteriocins – proteinaceous or peptidic toxins produced by bacteria to inhibit the growth of similar or
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 4 closely related bacterial strain(s). By targeting bacteria of similar physiological and nutritional activities, the bacteriocin producer will exclude susceptible bacteria from the habitat. Bacteriocins act more or less like a highly specialized antibiotic. Microbial End Products: It involves the end products of microbial metabolism which may inhibit the growth of other microorganisms. As a result of fermentation or modification of chemicals in the environment, bacteria produce chemicals that can serve to inhibit other organisms. For example, ethanol produced by Saccharomyces cerevisiae (yeast) during fermentation is inhibitory for many of its competitors. COMPETITION The competitive exclusion principle states that two species competing for the same limiting resource cannot coexist at constant population values. When one species has even the slightest advantage over another, the one with the advantage will dominate in the long term. Some examples of competition among microbial species are as follows: Competition among Paramecium Species: Laboratory competition experiments have been conducted using two species of Paramecium, P. aurelia and P. caudatum. The conditions were to add fresh water every day and input a constant flow of food. Although P. caudatum initially dominated, P. aurelia recovered and subsequently drove P. caudatum extinct via exploitative resource competition. Figure: Competitive exclusion of Paramecium caudatum by Paramecium aurelia. Competition among Soil Microbes: Soil is inhabited by different kinds of microorganisms, and therefore they exhibit competition among themselves for nutrients and space. In this kind of
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 5 situation, the best-adapted microorganism will predominate or eliminate the others which are dependent upon the same limited nutrient substance. Soil bacteria compete for space and suppress the growth of the fungal population. PARASITISM With microorganisms, the most widespread example of obligate parasitism is with viruses that attack specific cells by a process independent of an intermediate host, their detail is as follows: Viruses that attack Microbes: Viruses are highly specialized intracellular parasites that generally kill the host cell. All types of microorganisms (i.e., bacteria, fungi, protozoa, algae) have viruses and one role of viruses is to destroy susceptible populations. Viruses are responsible for killing about 20% of the ocean’s bacteria. It has been proposed that some viruses kill bacteria and may play an important role in shaping bacterial diversity. Bacteriophages prey on the bacterial species that are the most abundant, which keeps the dominant population under control. This phenomenon leads to greater genetic diversity as the less dominant organisms are not competitively excluded from a habitat by the dominant species. The killing of bacteria by phages with the release of cellular constituents also increases the availability of nutrients to other bacterial populations. Figure: Bacteriophages act as obligate parasites for bacteria.
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 6 PREDATION Myxobacteria: Many species of the soil-dwelling myxobacteria are predators of other microbes. Myxobacteria use gliding motility to search the soil matrix for prey and produce a wide range of antibiotics and lytic compounds that kill and decompose prey cells and break down complex polymers, thereby releasing substrates for growth. MICROBE – PLANT INTERACTIONS Following are the types of interactions between microorganisms and plants (with examples): MUTUALISM Examples of mutualistic interactions between plants and microbes include mycorrhiza (between fungi and vascular plant roots) and leguminous plant root nodules (between leguminous plant roots and Rhizobium spp.), their details are as follows: Mycorrhiza: Mycorrhiza, which comes from the Greek words “myco” meaning fungus and “rhizo” meaning root, refers to the association between vascular plant roots and their symbiotic fungi. Figure: Left – Endomycorrhiza and Right – Ectomycorrhiza. About 90 percent of all plant species have mycorrhizal partners. In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 7 the soil to channel water and minerals from the soil into the plant, thereby increasing a plant’s nutrient uptake. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus. There are mainly two types of Mycorrhizae: (1) Ectomycorrhizae (“outside” mycorrhiza) depend on fungi enveloping the roots in a sheath (called a mantle) and a net of hyphae that extends into the roots between cells. (2) Endomycorrhizae (“inside” mycorrhiza) involve fungi that penetrate root cells with their hyphae and are the site of the metabolic exchanges between the fungus and the plant. Legume Root Nodules: Legumes have a symbiotic relationship with bacteria called rhizobia (Rhizobium spp.), which create ammonia from atmospheric nitrogen (nitrogen fixation). Many legumes have root nodules that provide a home for symbiotic nitrogen-fixing bacteria called rhizobia. This relationship is particularly common in nitrogen-limited conditions. The Rhizobia convert nitrogen gas from the atmosphere into ammonia, which is then used in the formation of amino acids and nucleotides. The steps involved in root nodule formation are as follows:  Rhizobia normally live in the soil and can exist without a host plant.  When legume plants encounter low nitrogen conditions and want to form a symbiotic relationship with rhizobia they release flavinoids into the soil.  Rhizobia respond by releasing nodulation factor (sometimes just called nod factor), which stimulates nodule formation in plant roots.  Exposure to the nod factor triggers the formation of deformed root hairs, which permit rhizobia to enter the plant.  Rhizobia then form an infection thread, which is an intercellular tube that penetrates the cells of the host plant, and the bacteria then enter the host plant cells through the deformed root hair.  Rhizobia can also enter the root by inserting themselves between cracks between root cells; this method of infection is called crack entry.  Bacteria enter the root cells from the intercellular spaces, also using an infection thread to penetrate cell walls. Infection triggers rapid cell division in the root cells, forming a nodule of tissue. Both partners benefit from this mutualistic association. Once the rhizobia have established themselves in the root nodule, the plant provides carbohydrates in the form of malate and
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 8 succinate, and the rhizobia provide ammonia for the formation of amino acids. Many legumes are popular crops specifically because they require very little fertilizer: their rhizobia fix nitrogen for them. Used properly some legumes can even serve as fertilizer for later crops, binding nitrogen in the plant remains in the soil. Figure: Left – Root nodules of a leguminous plant. Right – Formation of root nodules in leguminous plants by Rhizobium spp. (mutualistic). PARASITISM Many microbes act as parasites for host plants, some examples are as follows: Bacterial Parasite – Agrobacterium and Crown Gall Disease: Crown Gall Disease is caused by a bacteria called Agrobacterium tumefaciens. The disease manifests as a tumor-like growth usually at the junction of the root and shoot. A. tumefaciens can transfer part of its DNA to the host plant, through a plasmid – a bacterial DNA molecule that is independent of a chromosome. The new DNA segment integrates semi-randomly into the genome of the plant and causes the plant to produce unusual amino acids and plant hormones that provide the bacteria with carbon and nitrogen. Fungal Parasite – Ustillago and Smut Disease: Spores (teliospores) of Ustilago tritici (loose smut of wheat) are carried by the wind from infected wheat ears to healthy flowers, where they germinate. The resulting hyphae penetrate lower ovaries. Inside the ovary, the mycelium spreads and becomes dormant and remains so in the seed (grain). When such infected seeds are sown next season, the hyphae also grow within the growing plant and form smut spores inside the kernel, thus destroying them completely. The covering of the grain breaks exposing the black spores mass, which may be dispersed by wind.
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 9 Figure: Life cycle of loose smut (Ustillago tritici). Viral Parasite – Tobacco Mosaic Virus: Tobacco mosaic virus (TMV) is an RNA virus that infects a wide range of plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns, such as “mosaic” – like mottling and discoloration on the leaves (hence the name). Tobacco mosaic virus is usually spread from plant to plant via "mechanical" wounds caused by contaminated hands, clothing, or tools such as pruning shears and hoes. This is because TMV occurs in very high concentrations in most plant cells. When plants are handled, the tiny leaf hairs and some outer cells are inevitably damaged and leak sap onto hands, tools, and clothing. Seeds from infected plants can also carry the virus on their seed coats. When the seed germinates, the virus may enter the seedling through small cuts caused by transplanting and handling, or during the germination/emergence process. Once inside the plant, the virus releases its genetic code (RNA). The plant mistakes this for its RNA and starts to produce viral proteins. The virus then spreads to neighboring cells through microscopic channels in the cell walls (plasmodesmata), and eventually enters the translocation system of the plant (xylem and phloem). From here, it spreads to the entire plant. Symptoms first appear about 10 days after infection. The plants do not usually die, but growth can be seriously stunted. In the case of tomatoes, certain TMV strains can cause deformed fruit, non-uniform fruit color, and delay ripening. Specific symptoms depend on the host plant, the age of the infected plant, environmental conditions, the virus strain, and the genetic
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 10 background of the host plant. However, common signs include mosaic-like patches (mottling) on the leaves, curling of leaves, and the yellowing of plant tissues. MICROBE – ANIMAL INTERACTIONS Following are the types of interactions between microorganisms and animals (with examples): MUTUALISM Some examples of mutualistic associations between microbes and animals include microbes in the rumen of ruminants, Trichonympha spp. in the gut of lower termites/wood roaches, etc. These examples are discussed as follows: Ruminant Animals: Ruminant animals (such as deer and cows) digest food in a four-chambered stomach with the help of special bacteria, protozoa, and fungi. Ruminants are mammals that digest plant-based food by processing it in a series of chambers in their stomachs. Ruminating mammals include cattle, goats, sheep, giraffes, bison, moose, elk, yaks, water buffalo, deer, camels, alpacas, llamas, and antelope. Ruminants differ from non-ruminants (called monogastric animals) because they have a four-chambered stomach. The four compartments are called: (1) rumen, (2) reticulum, (3) omasum, and (4) abomasum. The rumen and the reticulum are connected and work in concert and are therefore sometimes called the “reticulorumen”. Ruminants have unique species of bacteria, yeasts, protozoa, and fungi in their rumens. The plant matter consumed by ruminants is high in cellulose, but vertebrates cannot produce cellulase which is the enzyme required to break down cellulose. Thus ruminants depend on the symbiotic microbes in their guts to break down cellulose for digestion. There is no oxygen in the rumen, so bacteria in the rumen are typically anaerobes or facultative anaerobes. Trichonympha & Lower Termites / Wood Roaches: Trichonympha spp. is found as an endosymbiont in the gut of lower termites and the wood roach. Trichonympha spp. is a vital part of the hindgut microbiota of these organisms. Lower termites and wood roaches have a diet composed almost exclusively of wood and wood-related items, such as leaf litter, and therefore, need to digest large quantities of cellulose, lignocellulose, and hemicellulose. However, they do not have the enzymes necessary to do this. Trichonympha spp. and other endosymbionts in the hindgut of these organisms help with the digestion of wood related
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 11 particles. Trichonympha spp. (along with other species), converts cellulose into sugar using glycoside hydrolases. The sugar is then converted into acetate, hydrogen, and carbon dioxide via oxidation. Acetate is the main energy source for lower termites and wood roaches, so without the activity of Trichonympha spp., its host would not be able to survive. Figure: Termites have endosymbiotic flagellates (i.e. Trichonympha spp.) in their hindgut. Squid & Aliivibrio (Bioluminescence): Squids host light-generating Allivibiro bacteria in a special organ so that they can illuminate themselves and blend in with the environment. A special category of symbiotic relationships involve bioluminescence, where light-producing bacteria are hosted by another organism. One of the best-studied examples of bioluminescence is the Hawaiian bobtail squid (Euprymna scolopes) and its mutualistic bacteria, Aliivibrio fischeri. Aliivibrio fischeri inhabits a special light organ in the squid’s mantle. The bacteria are fed a sugar and amino acid solution by the squid. In return, they produce light to hide the squid’s shadow when viewed from below, allowing the squid to match ambient light conditions. Bobtail squid hatchlings do not have Aliivibrio fischeri naturally in their bodies. They are born with a special light organ structure, with ciliated cells at the opening designed to trap passing A. fischeri, but must obtain the bacteria from seawater. To do this, the squid secretes a special mucus whenever its cells detect peptidoglycan (which is found in the cell walls of bacteria). The mucus collects near the opening of the light organ which traps passing bacteria. The squid weeds out
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 12 unwanted bacteria in several ways, i.e. the squid also creates a hostile environment at the entrance to the light organ by secreting an enzyme that splits hydrogen peroxide, creating a toxic environment for most bacteria. Aliivibrio fischeri can capture hydrogen peroxide before the squid can use it as a toxin, and thus can survive in the hostile chemical environment. Once A. fischeri has passed these hurdles at the opening of the light organ, it can colonize chambers of the light organ and begin enjoying the benefits of symbiosis. Mutualistic Associations between Fungi and Insects: Fungi have evolved mutualisms with numerous insects. Arthropods (jointed, legged invertebrates, such as insects) depend on the fungus for protection from predators and pathogens, while the fungus obtains nutrients and a way to disperse spores into new environments. The association between species of Basidiomycota (fungi) and scale insects is one example. The fungal mycelium covers and protects the insect colonies. The scale insects foster a flow of nutrients from the parasitized plant to the fungus. In a second example, leaf-cutting ants of Central and South America farm fungi. They cut disks of leaves from plants and pile them up in gardens. Fungi are cultivated in these disk gardens, digesting the cellulose in the leaves that the ants cannot break down. Once smaller sugar molecules are produced and consumed by the fungi, the fungi in turn become a meal for the ants. The insects also patrol their garden, preying on competing fungi. Both ants and fungi benefit from the association. The fungus receives a steady supply of leaves and freedom from competition, while the ants feed on the fungi they cultivate. COMMENSALISM Examples of commensalism between microbes and animals are as follows: Colonization of Aspergillus in Upper Gastrointestinal Tract of Humans: Numerous genera of bacteria and fungi live on and in the human body as part of its natural flora. The fungal genus Aspergillus is capable of living under considerable environmental stress and thus is capable of colonizing the upper gastrointestinal tract where relatively few examples of the body's gut flora can survive due to highly acidic or alkaline conditions produced by gastric acid and digestive juices. While Aspergillus normally produces no symptoms, in individuals who are immunocompromised or suffering from existing conditions such as tuberculosis, a condition called aspergillosis can occur, in which populations of Aspergillus grow out of control.
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 13 Commensal strains of Staphylococcus aureus: Many strains of Staphylococcus aureus are commensals (although most are pathogens), and are present on roughly 20 to 30% of the human population as part of the skin flora. S. aureus also benefits from the variable ambient conditions created by the body's mucous membranes, and as such can be found in the oral and nasal cavities, as well as inside the ear canal. PARASITISM Many microbial species are parasites of animals, some examples are as follows: Bacterial Parasites: Some examples of bacterial parasites of animals (including humans) include: (1) Bacillus anthracis, the cause of anthrax, is spread by contact with infected domestic animals; its spores, which can survive for years outside the body, can enter a host through an abrasion or may be inhaled. (2) Borrelia spp., the cause of Lyme disease and relapsing fever, is transmitted by ticks from the diseases’ reservoirs in animals such as deer. (3) Campylobacter jejuni, a cause of gastroenteritis, is spread by the fecal-oral route from animals, or by eating insufficiently cooked poultry, or by contaminated water. (4) Haemophilus influenzae, an agent of bacterial meningitis and respiratory tract infections such as influenza and bronchitis, is transmitted by droplet contact. (5) Treponema pallidum, the cause of syphilis, is spread by sexual activity. Protozoan Parasites: Protozoa such as Plasmodium, Trypanosoma, and Entamoeba, are endoparasitic. (1) Plasmodium is a genus of obligate parasitic protozoans of vertebrates and insects. The life cycles of Plasmodium species involve development in a blood-feeding insect host (usually mosquitoes such as Anopheles) which then injects parasites into a vertebrate host. Parasites grow within a vertebrate body tissue (often the liver) before entering the bloodstream to infect red blood cells. The ensuing destruction of host red blood cells can result in a disease, called malaria. During this infection, some parasites are picked up by a blood- feeding insect, continuing the life cycle. (2) Trypanosoma is a genus of unicellular parasitic flagellate protozoa. The name is derived from the Greek trypano- (borer) and soma (body) because of their corkscrew-like motion. Most trypanosomes require more than one obligatory host to complete the life cycle and most are transmitted via a vector. The majority of species are transmitted by blood-feeding invertebrates (such as the Tsetse fly), but there are different
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 14 mechanisms among the varying species. Trypanosomes infect a variety of hosts and cause various diseases, including sleeping sickness, caused by Trypanosoma brucei, Chagas disease, caused by Trypanosoma cruzi. (3) Entamoeba is a genus of amoeboid protozoa found as internal parasites or commensals of animals. Entamoeba histolytica predominantly infects humans and other primates causing amoebiasis. Mammals such as dogs and cats can become infected transiently (temporarily) but are not thought to contribute significantly to transmission. The word histolysis means disintegration and dissolution of organic tissues. Figure: Life cycle of Plasmodium. Fungal Parasites: Some fungal parasites of animals are as follows: (1) Candida spp. causes infections in individuals with deficient immune systems. Candida albicans is a kind of diploid yeast that commonly occurs among the human gut microflora. C. albicans is an opportunistic pathogen in humans. Abnormal over-growth of this fungus can occur, particularly in immunocompromised individuals. (2) Histoplasma capsulatum can cause histoplasmosis in humans, dogs, and cats (the disease affects primarily the lungs). The fungus is most prevalent in the Americas, India, and southeastern Asia. Infection is usually due to inhaling contaminated air. (3) Pneumocystis jirovecii (or Pneumocystis carinii) can cause a form of pneumonia in
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 15 people with weakened immune systems, such as premature children, the elderly, and AIDS patients. Viral Parasites: Some viral parasites of animals are as follows: (1) Rabies virus is a virus that causes rabies in humans and animals. Rabies transmission can occur through the saliva of animals (i.e. via a dog bite) and less commonly through contact with human saliva. It has an extremely wide host range. In the wild, it has been found infecting many mammalian species, while in the laboratory it has been found that birds can be infected, as well as cell cultures from mammals, birds, reptiles, and insects. The first symptoms of rabies may be very similar to those of the flu including general weakness or discomfort, fever, or headache. These symptoms may last for days. There may be also discomfort or a prickling or itching sensation at the site of bite, progressing within days to symptoms of cerebral dysfunction, anxiety, confusion, agitation. As the disease progresses, the person may experience delirium, abnormal behavior, hallucinations, and insomnia. It may also be inactive in its host's body and become active after a long period. (2) Poxviruses cause smallpox in humans. In this disease, raised fluid- filled vesicles are formed on the body which become pustules later on and form pitted scars, the pocks. (3) Human immunodeficiency viruses (HIV) cause acquired immunodeficiency syndrome (AIDS). AIDS is a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive. Without treatment, the average survival time after infection with HIV is estimated to be 9 to 11 years, depending on the HIV subtype. In most cases, HIV is a sexually transmitted infection and occurs by contact with or transfer of blood, pre-ejaculate, semen, and vaginal fluids. Non-sexual transmission can occur from an infected mother to her infant during pregnancy, during childbirth by exposure to her blood or vaginal fluid, and through breast milk. Within these bodily fluids, HIV is present as both free virus particles and virus within infected immune cells. HIV infects vital cells in the human immune system, such as helper T cells, macrophages, and dendritic cells. HIV infection leads to low levels of T cells, which causes a loss of cell-mediated immunity, and the body becomes progressively more susceptible to opportunistic infections. PREDATION Certain fungi prey upon nematodes, they are called nematophagous fungi. Some animals feed on microorganisms, such as the cave mollies. They are discussed as follows:
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  SYED MUHAMMAD KHAN(BS HONS. ZOOLOGY) pg. 16 Nematophagous Fungi: Nematophagous fungi are carnivorous fungi specialized in trapping and digesting nematodes. Around 160 species are known. There exist both species that live inside the nematodes from the beginning and others that catch them, mostly with glue traps or in rings, some of which constrict on contact. Some species possess both types of traps. Another technique is to stun the nematodes using toxins. The habit of feeding on nematodes has arisen many times among fungi, as is demonstrated by the fact that nematophagous species are found in all major fungal groups. Nematophagous fungi can be useful in controlling those nematodes that eat crops, i.e. as a bio-nematicide. Figures: Left – a nematode, trapped by a nematophagous fungus. Right – the process of capture of nematodes. Microbial Grazing by Cave Mollies: Microorganisms constitute choice food sources for a variety of herbivorous animals who graze on them. One unusual grazing example occurs in Mexico, where cave mollies (Poecilia mexicana – a small fish) feed on sulfur bacteria in the stream that runs through the cave. Microorganisms form a key part of the food web in this cave system, and grazing may help structure the microbial communities of this system.