Physiology of Bacteria

Most bacteria don’t contain chlorophyll and are unable to synthesize their own food. They are parasitic, saprophytic, or symbiotic. The parasitic and saprophytic bacteria obtain the unnecessary energy for their growth and other metabolic activities from the tissues of the host or the dead decaying substrate on which they are found by decomposing complex organic compounds like carbohydrates and proteins present in the tissues. Such bacteria are heterotrophic.

Some bacteria obtain energy for metabolic activities from the chemical reactions among inorganic substances. This phenomenon is known as ‘chemosynthesis‘ and the organisms are called ‘chemosynthetic‘ or ‘autotrophic’ bacteria.

1. Parasitic Bacteria: They derive their nutrition from the plants and animals on which they grow. As a result, certain enzymes are produced which decompose or kill the protoplasm of the host cells. Such effects of the parasites on the host become visible to the naked eye as disease symptoms. many well-known diseases of human beings like typhoid, tetanus, tuberculosis, pneumonia, and many other plants like ‘citrus canker’, ‘yellow rot of wheat’, and ‘ring rot of potato’ are due to parasite bacteria. They are also known as pathogenic bacteria. Some bacteria grow well only in the presence of oxygen, while others grow well in absence of oxygen. The former is known as aerobes and the latter anaerobes.

2. Saprophytic Bacteria: They grow on dead and decaying plants and animals, dung, rotten wood, stagnant, water, and many other decaying substances rich in organic matter. Certain enzymes secreted by the bacteria decompose the complex organic substances of the substrate, converting them into simpler ammonium compounds. They cause decay and are therefore also known as putrefying bacteria. The souring of milk, the manufacture of cheese, the preparation of butter from milk, and vinegar from sugarcane juice are various processes completed by the action of certain specific saprophytic bacteria.

Zygomonas ferments glucose-producing alcohol, lactic acid, and carbon dioxide and plays a significant role in the wine industry. Acetobacter oxidizes organic compounds to organic acids such as lactic acid thus having a significant role in the vinegar industry. Clostridium aceto-butylicum forms butyl alcohol from carbohydrates. Lactobacillus converts sugars into lactic acid. Canned food is spoiled by Bacillus Stearuothermophilus and Clostridium thermosaccharolyticium.

3. Symbiotic Bacteria: Some organisms live in close association with other organisms and both partners mutually benefit. This is termed symbiosis. Rhizobium spp. is a striking example of this type. They occur in root nodules of leguminous plants and help in fixing the free nitrogen of the atmosphere in the soil for the plants which in return provides carbohydrates and protection to the bacteria. They are also called nitrogen-fixing bacteria and add to the fertility of the soil.