Symbiosis: Exercise 25 PDF

Symbioses Microbes are ubiquitous, in the water, air, and soil, as well as on and in hosts. We often think of these microbes as living and growing distinct from other organisms, and indeed many techniques described in this manual are used to isolate a single microbial species and analyze its charact...

Symbioses Microbes are ubiquitous, in the water, air, and soil, as well as on and in hosts. We often think of these microbes as living and growing distinct from other organisms, and indeed many techniques described in this manual are used to isolate a single microbial species and analyze its characteristics while it is in pure culture. While this can provide extremely valuable information, it is also important to explore the microbe's various functions in its natural environment, where it is often surrounded by a network of other microbes and/or a host and may be a member of a symbiotic relationship.  Symbiosis is defined as an interaction between two different organisms living in dose association. Often, symbiotic interactions have evolved over time, although this is not always the case. There are four types of symbiotic relationships: mutualism, commensalism, pathogenesis/parasitism, and amensalism (Table 25.1). Mutualism occurs when both organisms receive a net benefit from the interaction. Some mutualistic relationships have evolved such that one or both members can no longer live independently (obligate mutualism), whereas others can live independently but benefit from living together (facultative mutualism). Commensalism occurs when one organism receives a net benefit and the other organism is not significantly affected. Although some relationships are defined as commensalistic, the host may actually receive benefits that have not yet been discovered. Pathogenesis/parasitism occurs when one organism receives a net benefit and the other organism receives a net loss from the interaction. Pathogenesis can cause the death of the host, but these relationships generally involve a period of intense dispersal of the pathogen/parasite either by introducing aerosols through coughing, infecting water sources, or contamination of fomites through intense diarrhea or vomiting. Amensalism occurs when one organism is inhibited or killed while the other is unaffected. Symbiotic relationships exist on a spectrum. Depending on the conditions and location, an organism can move along that spectrum; for example, commensal nasopharyngeal bacteria may become pathogenic when the immune system of the host is weakened. See Table 25.1 for macroscopic and microscopic examples of each type of symbiosis. Table 25.1. Comparison of Symbiotic Relationships. TYPE OF MACROSCOPIC EXAMPLE MICROSCOPIC EXAMPLE SYMBIOSIS Chemosynthetic bacteria (+) provide Sea anemones(+) and H2Sand CO2 as nutrients for deep sea Mutualism clownfish (+) tubeworms (+) near hydrothermal vents. Termites require their symbiotic Obligate Certain flowering plants require microbiota in order to acquire Mutualism insect pollinators to reproduce nutrients from wood. Nitrogen-fixing bacteria can live Animals eating fruit and within the roots of legumes, receiving Facultative dispersing the seeds is useful, but nutrients and fixing nitrogen for the Mutualism not necessary for both organisms plant, but both can live without the interaction. Cattle egrets(+) feeding on Bacteria living in the human Commensalism insects disturbed by cattle (0) nasopharynx (0) without providing grazing benefit or causing disease. Mycobacterium tuberculosis(+) Pathogenesis/ Aphids (+) feeding on sap causing the disease tuberculosis in Parasitism from plants(-) humans(-). Black walnut trees (0) secreting The bread mold (Penicillium) (O) a substance that kills other secreting the antibiotic penicillin that Amensalism herbaceous plants(-) in its inhibits other bacteria from growing root zone (-). Typically, the costs and/or benefits of symbioses are nutritional, reproductive, defensive, dispersal/motility, or some combination thereof. The mutualism that occurs between humans and the normal human gut microbiome is a good example that encompasses several of these categories (Table 25.2). Table 25.2. The Different Costs and Benefits of the Human Microbiome Mutualism. Some food is consumed by microbes(-) Gains food from host(+) Nutritional Microbes digest inaccessible Some cells are digested by nutrients and make them host(-) accessible to humans(+) Normal gut microbiome fills environmental niches and keeps Relatively safe environment, Defensive potentially harmful bacteria temperature regulated (+) from invading, or actively attack invading cells Carried from place to place and Dispersal/Motility No benefit or cost (O) dispersed regularly in fecal matter(+)  Nitrogen and Living Organisms All living organisms require nitrogen (N). This element is necessary to biosynthesize nucleic acids, amino acids, peptidoglycan cell walls in prokaryotes, and enzymes used in photosynthesis. Although our atmosphere contains 78% nitrogen, plants cannot use the form of nitrogen found in the atmosphere. The atmosphere contains dinitrogen (N2) and all plants require microbes to transform it to a usable form before their roots can take it up. Some plants associate with specialized symbiotic bacteria that "fix" the dinitrogen by reducing it to ammonia (NH3). Plants can also use forms of nitrogen that non-symbiotic microorganisms have transformed. Another plant-microbe interaction in the nitrogen cycle will be discussed in Exercise 43. In that pathway, decomposer microorganisms convert organic forms of nitrogen found in decaying material, such as dead leaves and animal carcasses, to inorganic forms such as ammonium ( N H + 4 ) and nitrate (NO3-). Plants rely on microbes to make these molecular forms of nitrogen available. Rhizobia bacteria are root symbionts that associate with plants in the legume family (Fabaceae). You may have more legumes in your diet than you are aware of. Peas, green beans, soy, alfalfa sprouts, bean sprouts, chickpeas, garbanzo beans, edamame, peanuts, and kidney beans are all legumes. From soy we can also produce tofu, tempeh, soy sauce, and some vegetable oils. Farmers use crop rotations with alfalfa, soy, or clover to replenish soil with nitrogen before planting another nitrogen-demanding crop such as corn. Mimosa trees and locust trees and the wildflower lupine are all in the legume family, as well. The functional group name rhizobia includes the prefix "rhiz," which comes from the ancient Greek word for "root." Thus, many species of bacteria can be functionally identified as rhizobia. They live in soils and infect plant roots, causing the plants to produce nodules (Figure 25.1), organs where the bacteria reside. Inside the nodule, the bacteria reduce atmospheric nitrogen found in air pockets within soil and use the enzyme nitrogenase to convert dinitrogen to ammonia (NH3). They exchange ammonia with the plant for sugars in this mutualism. All rhizobia are gram-negative rods found in the order Rhizobiales within the phylum Alphaproteobacteria. " figure 25.1. This section of a legume root has multiple visible nodules formed by rhizobia bacteria. Three are identified with the"