Bacteria exhibit different modes of nutrition. On this bases, two types of bacteria can be seen- Autotrophic and Heterotrophic.
1. Autotrophic Bacteria: These are bacteria which are able to synthesize their own organic food from inorganic substances. These bacteria can be distinguished further into two types
- Photosynthetic: These bacteria posses photosynthetic pigments in membrane bound lamellae(thylakoids) and utilize solar energy as a source of 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 by product. For this reason, the process is described as anoxygenic photosynthesis.
- The green sulphur bacteria such as Chlorobium contains a pigment called bacterioviridin, similar to chlorophyll.
- The purple sulphur bacteria like Thirospirulum contain the pigments bacteriochlorophyll and carotenoids.
- The purple nonsulphur bacteria such as Rhodospirilum also contain the pigment bacteria chlorophyll but they accomplish reduction of carbon dioxide in the presence of organic compounds.
- Chemosynthetic bacteria: These are bacteria which manufacture organic compounds from inorganic raw materials utilizing energy liberated from the oxidation of inorganic substances. Fallowing are the some types of chemoautotrophic bacteria
- Nitrifying bacteria which drive energy by oxidizing ammonia into nitrates. example: Nitrosomonas
- Sulphur bacteria which drive energy by oxidising hydrogen sulphide or molecular sulphur. example: Thiobacillus
- Iron bacteria which drive energy by converting ferrous ions into ferric form. Example: Ferrobacillus
- Hydrogen bacteria which utilize free molecular hydrogen and oxidize it to water. Example: Hydrogenomonas
- Carbon bacteria which utilize methane as the source of carbon. Example: Methanomonas.
- Methanogenic bacteria which produce methane from hydrogen and carbon dioxide which is utilized for the synthesis of biogas. Example: Methanobacterium.
2. Heterotrophic Bacteria: 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 fallows.
- Saprophytic bacteria: These bacteria obtain their nutritional requirements from dead and decaying organic matter. They breakdown the complex organic matter into simple soluble form by secreting exogenous enzymes. Subsequently, they absorb simple nutrients and assimilate them, during which they release energy. These bacteria have a significant role in the ecosystem, work as decomposer.
- Symbiotic bacteria: 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. Example: Nitrogen fixing bacteria like Rhizobium resides inside inside root nodules and reduce atmospheric nitrogen directly to ammonia. This become the source of nitrogen for host plant. The plant in return provide bacteria with nutrients and protection.
- Parasitic bacteria: These are bacteria which occur in the body of animal and plants, obtaining their organic food. Most of these bacteria are pathogenic, causing serious diseases in the host organisms either by exploiting them or by releasing poisonous toxins.