The bacteria mostly lack chlorophyll and are unable to synthesize carbohydrates except in some forms. On the basis of the modes of nutrition the bacteria are grouped into two types:
(A) Autotrophic bacteria.
(B) Heterotrophic bacteria.
(A) Autotrophic bacteria:
These bacteria synthesize all their food from inorganic substances (H2O, C02, H2S salts). The autotrophic bacteria are of two types:
(i) Phytosynthetic bacteria.
(ii) Chemosynthetic bacteria.
(i) Phytosynthetic bacteria:
Few purple sulphur (e.g., Chromatium) bacteria possess pigments, such as, purple pigment, the bacteriopurpurin, and green pigment, the bacterial chloroyhyll etc. Bacterioviridin occurs hi green sulphur bacteria, e.g., Chlorobium. Such bacteria synthesize their carbohydrate food in presence of sunlight by photosynthesis and are known as chlorophyll bacteria.
2H2S + CO2 ® (CH2O)2 + 2S + H2O
(ii) Chemosynthetic bacteria:
These bacteria get their energy for food synthesis from the oxidation of certain inorganic chemicals. Light energy is not used. The energy obtained from the chemical reactions is exothermic. The Chemosynthetic bacteria are of the following types:
(a) Sulphomonas (Sulphur bacteria): These bacteria get their energy by oxidation of hydrogen sulphide into H2SO4, e.g., Thiobadllus, Beggiatoa.
CO2 + 2H2S ® 2S + H2O + CH2O + Energy
3CO2 + 2S + 8H2O ® 2 H2S04 + 2(CH10) + 3H2O + Energy
(b) Hydromonas (Hydrogen bacteria): These convert hydrogen into water, e.g., Bacillus pantotrophus.
H2 + ½O2 ® H2O + Energy
(c) Ferromonas (Iron bacteria): These bacteria get their energy by oxidation of ferrous compounds into ferric forms,. e.g., Leptothrix.
2Fe(HCO3)2 + H2O + O ® 2Fe (OH)3 + 4CO2 + Energy
4FeCO3 + O2 + 6 H2O ® 4Fe(OH)3 + 4CO2 + Energy
(d) Mcthanomonas (Methane bacteria): These bacteria get their energy by oxidation of methane into water and carbon dioxide.
(e) Nitrosomonas (Nitrifying bacteria): These bacteria get their energy by oxidation of ammonia and nitrogen compounds into nitrates. Nitrosomonas oxidises NH3 to nitrites.
NH3 + ½O2 ® H2O + HNO2 + Energy
Nitrobacter converts nitrites to nitrates.
NO2 + ½O2 ® NO2 + Energy
(B) Heterotrophic bacteria:
The heterotrophic bacteria obtain their-ready made food from organic substances, living or dead. These are of three types:
(i) Saprophytic bacteria.
(ii) Parasitic bacteria.
(iii) Symbiotic bacteria.
(i) Saprophytic bacteria:
These bacteria obtain their food from the dead organic decaying substances such as leaves, fruits, vegetables, meat, animal faeces, leather, humus etc. They secrete enzymes to digest the food and absorb it. The breakdown of carbohydrates is fermentation and of proteins the putrefaction. The former produces alcohols, acetic and other organic acids by fermentation of carbohydrates. Putrefaction decomposes proteins into ammonia, methane, H2S, carbonic acids. The enzymes secreted break down the complex compounds into simpler soluble compounds, which are easily absorbed. Examples are Bacillus acidi lacti, Acetobacter etc.
(ii) Parasitic bacteria:
These bacteria obtain their food from the tissues of living organisms, the hosts. They may be harmless or may cause serious diseases. The disease-producing bacteria are pathogenic which cause various diseases in plants and animals. Examples are Bacillus typhosus, B. anthracis, B.tetani. B. diplheriae, B. tuberculosis, B. pneumoniae, Vibrio cholerae, Pseudomonas citri etc.
(iii) Symbiotic bacteria:
These bacteria live in close association with other organisms as symbionts. They are beneficial to the organisms. The common examples are the nitrogen-fixing bacteria, e.g., Bacillus radicicola, B. azotobacter, Rhizobium, Ctostridium etc. Rhizobium spp.,B. radicicola and B. azotobacter live inside the roots of leguminous plants and form bacteria nodules for fixation of nitrogen from the air.