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).