On the basis of their mode of nutrition bacteria are classified into two types namely autotrophs and heterotrophs.
I. Autotrophic bacteria: Bacteria which can synthesis their own food are called autotrophic bacteria. They may be further subdivided as
A. Photoautotrophic bacteria – Bacteria use sunlight as their source of energy to synthesize food. They may be:
1. Photolithotrophs: In Photolithotrophs the hydrogen donor is an inorganic substance.
Green sulphur bacteria: In this type of bacteria the hydrogen donor is H S and possess pigment called Bacterioviridin.
Example: Chlorobium.
Purple sulphur bacteria: For bacteria belong to this group the hydrogen donor is thiosulphate, Bacteriochlorophyll is present. Chlorophyll containing chlorosomes are present.
Example: Chromatium.
2. Photoorganotrophs: They utilize organic acid or alcohol ‘as hydrogen donor.
Example: Purple non-sulphur bacteria – Rhodospirillum.
B. Chemoautotrophic bacteria – They do not have photosynthetic pigment hence they cannot use sunlight energy. These type of bacteria obtain energy from organic or inorganic substance.
1. Chemolithotrophs: This type of bacteria oxidize inorganic compound to release energy.
Examples:
- Sulphur bacteria – Thiobacillus thiooxidans
- Iron bacteria – Ferrobacillus ferrooxidans
- Hydrogen bacteria – Hydrogenomonas and
- Nitrifying bacteria – Nitrosomonas and Nitrobacter.
2. Chemoorganotrophs: This type of bacteria oxidize organic compounds to release energy.
Examples:
- Methane bacteria – Methanococcus
- Acetic acid bacteria – Acetobacter and
- Lactic acid bacteria – Lactobacillus
II. Heterotrophic bacteria: They are parasites (Clostridium and Mycobacterium), Saprophytes (Bacillus mycoides) or symbiotic (Rhizobium in root nodules of leguminous crops).
