Autotrophic and Heterotrophic are the main mode of nutrition in bacteria. The autotrophic bacteria make their own food from outside sources of energy. They are further divided into chemo and photo autotrophs. While heterotrophs do not make their own food and they depend on readymade food from outside for their survival. saprotrophic, symbiotic and parasitic are their sub types.
Photoautotrophic bacteria: They have photosynthetic pigments which are bacteriochlorophyll and bacteriophaeophytin. These pigments occur in thylakoids. They are anaerobic type with no oxygen involvement in this type of process. As no oxygen is required the bacteria can survive in areas where there is low oxygen density. In this process instead of water hydrogen is used as a source of reducing power. The source of hydrogen is organic and inorganic compounds. This hydrogen is picked by NAD ions. In this type of nutrition photosynthetic pigments synthesize ATP.
Chemoautotrophic Bacteria: They make their food from inorganic compounds and make their energy from oxidation chemical reactions involved in external medium. This energy is used in carbon assimilation. Nitrifying, sulphur oxidizing and iron bacteria comes under this category. Energy released in this process is trapped inside ATP structure.
Nitrifying bacteria make energy by oxidation reaction involving ammonia and nitrate. Sulphur oxidizing bacteria oxidize sulphur compound hydrogen bi sulphide to sulphur. This oxidation process releases energy. An iron bacterium liberates energy by involving ferrous and ferric compounds. Some chemosynthetic bacteria depend upon liberation of carbon dioxide and water for energy.
Saprophytic bacteria: These are free organisms which depend upon organic remains for their food. This bacteria leads to fermentation (anaerobic break down of carbohydrates), putrefaction ( anaerobic break down of proteins) and decay (anaerobic break down of organic compounds). Their presence is beneficial as well as harmful for human interest. They serve to clean the environment. Fungi also play an important role in it. They dispose of the organic remains and are referred to as Natures Scavengers. They dispose sewage and cure tea, coffee and tobacco. They cause food poison, disintegration of food along with destruction of common household stuffs.
Symbiotic bacteria: They live in cordial environment which is suitable to both the organisms. They check the growth of putrefying bacteria and produce vitamin B and K. They live in human intestine like E. coli.
Parasitic bacteria: These bacteria live with other living beings. They depend on them for survival and derive their food from them. They may or may not be disease causing. Disease causing is referred as pathogenic which may cause disease by attacking host cells or releasing toxins. Toxins may be further sub divided into exotoxins and endotoxins.