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:
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.
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.
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.