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| Nutrition in Bacteria |
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| Bacteria exhibit different modes of nutrition. On this basis, broadly two types of bacteria can be recognised autotrophic bacteria and heterotrophic bacteria. |
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| These are bacteria which are able to synthesize their own organic food from inorganic substances. They use carbon dioxide for obtaining carbon and utilise hydrogen sulphide (H2S) or ammonia (NH3) or hydrogen (H2) as the source of hydrogen to reduce carbon. These bacteria can be distinguished further into two types as follows: |
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| Photoautotrophic Bacteria |
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| The photoautotrophic bacteria possess photosynthetic pigments in membrane bound lamellae (or thylakoids) and utilise solar energy. The bacterial photosynthesis is different from that of green plants since here water is not used as a hydrogen donor. Hence oxygen is not released as a byproduct. For this reason, the process is described as anoxygenic photosynthesis. |
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| Chemosynthetic Bacteria |
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| These are bacteria which manufacture organic compounds from inorganic raw materials utilising energy liberated from the oxidation of inorganic substances. Following are the common types of chemo autotrophic bacteria. |
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| 1. Nitrifying bacteria which derive energy by oxidizing ammonia into nitrates. Eg: Nitrosomonas, Nitrobacter. |
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| 2. Sulphur bacteria which derive energy by oxidising hydrogen sulphide to sulphur. Eg: Thiobacillus, Beggiatoa. |
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| 3. Iron Bacteria which derive energy by oxidising ferrous ions into ferric form. Eg: Ferrobacillus, Gallionella. |
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| These are bacteria which are unable to manufacture their own organic food and hence are dependent on external source. These bacteria can be distinguished into three groups as follows: |
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 Saprophytic Bacteria: These bacteria obtain their nutritional requirements from dead organic matter. They breakdown the complex organic matter into simple soluble form by secreting exogenous enzymes. Subsequently they absorb the simple nutrients and assimilate them, during which they release energy. These bacteria have a significant role in the ecosystem, functioning as decomposers. |
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 The aerobic breakdown of organic matter is called as decay or decomposition. It is usually complete and not accompanied by the release of foul gases. Anaerobic breakdown of organic matter is called fermentation. It is usually incomplete and is always accompanied by the release of foul gases. Anaerobic breakdown of proteins is called putrifaction. |
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| The property of decomposition of organic compounds is employed in several industrial processes such as ripening of cheese, in the retting of fibres and in the curing of tobacco. |
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| These are bacteria which live in a mutually beneficial association with other organisms. Such bacteria derive the essential nutrients from their host organisms and in that process help the host through some of their biological activities. |
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| 1. The most familiar example of symbiotic bacteria are the nitrogen fixing bacteria found in the root nodules of leguminous plants. Bacteria such as Rhizobium and Pseudomonas reside in the root nodules and reduce atmospheric nitrogen directly to ammonia. This becomes the source of nitrogen for the host plants. The plants in return provide bacteria with nutrients and protection. |
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| 2. The bacteria found in the human alimentary canal Escherichia coli are nonpathogenic. These bacteria check the growth of harmful putrefying bacteria. In addition, these bacteria release vitamins K and B12 which are necessary for blood components. The human host provides shelter and food for these bacteria. |
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| 3. A similar example is that of cellulose digesting bacteria which occur in the alimentary canal of ruminant mammals such as cows and goats. |
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| These are bacteria which occur in the body of animals and plants, obtaining their organic food from there. Most of these bacteria are pathogenic, causing serious diseases in the host organisms either by exploiting them or by releasing poisonous secretions called toxins. |
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