A. Heterotrophic bacteria (Heterotrophs):
They are most abundant in nature. They do not synthesize their own food but depend on other organisms or on dead organic matter for food. They may be parasites, saprophytes or symbiontes.
They live on other organisms called the host, from which they obtain food. e.g. Streptococcus, Clostridium, Mycobacterium tuberculosis etc. Disease causing parasites are called pathogens.
They are the major decomposers in nature. They obtain their food from detritus i.e. dead organic matters by Fermentation (anaerobic breakdown of carbohydrates), putrefaction (anaerobic breakdown of proteins) and decay (aerobic breakdown of organic compounds).
They live in symbiotic association with other plants for mutual benefit. A few species of Rhizobium, such as R. leguminosarum, live in the root nodules of leguminous plants forming intimate symbiotic relationship. Bacteria fix atmospheric nitrogen for the plant and in return get protection. Escherichia coli live in human intestine as commensal. It feeds on undigested matter, checks the growth of putrefying bacteria and produces vitamins B and K. The presence of E. coli in water supply indicates contamination by sewage.
B. Autotrophic Bacteria (Autotrophs):
They synthesize their own organic food from inorganic substances (CO2 and hydrogen donor) Autotrophic bacteria are of two types:
(i) photoautotrophic (photosynthetic) which make use of light energy and
(ii) chemoautotrophic (chemosynthetic) which utilize chemical energy.
(i) Photoautotrophic bacteria (Photoautotrophs):
They use solar energy for the synthesis of their own food. These bacteria are anaerobic, which could be purple or green. The purple bacteria possess pigment bacteriochlorophyll located in the membranes of thylakoids while green bacteria possess bacteriophaeophytin (chlorobium chlorophyll) located inside small sacs called chlorosomes.
Photoautotrophs carry out anoxygenic photosynthesis in which water is not used as reducing power. Instead sulphur compounds like hydrogen sulphide (H2S), hydrogen gas (H2), thiosulphates (Na2S203) or some organic compounds are used to obtain reducing power. Most photoautotrophs live near the bottoms of ponds and lakes where reduced sulphur or other compounds are in plenty and oxygen content is very low.
(ii) Chemoautotrophic bacteria (chemoautotrophs):
They do not have photosynthetic pigment and hence utilize chemical energy to reduce CO2 to organic food. The chemical energy is obtained from the oxidation of certain chemicals such as ammonia, nitrites, methane, carbon monoxide, molecular hydrogen, iron salts, sulphur and sulphur compounds (e.g., nitrifying bacteria Nitrosomonas, Nitrobacter, denitrifying bacteria Bacillus denitrificans, sulphur bacteria Thiobacillus thiooxidans, iron bacteria, hydrogen bacteria).