1. Mode of living in Bacteria
Considering the oxygen demands of bacteria, it can be classified into:
Aerobes.
They require free supply of oxygen.
2. Anaerobes
They grow in complete absence of oxygen.
3. Facultative aerobes
They can live either in presence or absence of oxygen.

2. Mode of living in Bacteria
If we consider the mode of nutrition, bacteria can be divided into two categories:
1. Autotrophs. They can build up complex organic substances such as carbohydrates from simple inorganic sources (CO2 and water).
2. Heterotrophs. They cannot build up carbohydrates from simple inorganic sources. They depend on ready made organic materials derived from plants , animals of humans. They can live on such compounds , break it down , enzymatically , into simpler compounds which can be absorbed. The process of break down of such complex organic substances releases energy which can be used by such bacteria in building up its complex protoplasmic material from the simple absorbed substances.

3. I: Autotrophic Bacteria
The autotrophs can be divided into two types:
I: Photoautotrophs II: Chemoautotrophs.

4. Autotrophic Bacteria Light Energy Green sulphur bacteria
Photoautotrophs.
They contain in their cells a chlorophyll known as bacterial chlorophyll by which they can perform photosynthesis. Here, the energy used in building up processes is derived from light. As an example of such bacteria is the green sulphur bacteria.
Light Energy
6CO H2S C6H12O6 + 6 H2O + 12 S
Green sulphur bacteria

5. Autotrophic Bacteria
B. Chemoautotrophs. Here the cells lack chlorophll. Accordingly the source of energy should be something else than light. The energy used here is released from chemical reactions carried out by such bacteria. Most of such reactions are oxidative.

6. Autotrophic Bacteria (Chemoautotrophs)
Three examples we have :
1) Nitrosomonas, oxidizes ammonia or ammonium salts to nitrites with a release of energy :
2NH­3 + 3O HNO2 + 2H2o + Energy
2) Nitrobacter, oxidizes nitrites to nitrates with a release of energy :
2HNO2 + O HNO3 + Energy
3)Thiobacillus thiooxidans, oxidizes sulphur with a release of energy :
2S +3O2 + 2H2O H2SO4 + Energy
The energy released from any of these oxidation reactions is utilized by the specific organism in building up organic carbohydrates from the simple inorganic sources, CO2 and water.

7. II. Heterotrophic Bacteria
Heterotrophs live either as
Saprophytes on
dead organic matter.
Parasites on
plants, animals and humans causing serious diseases.
Symbionts
with other living organisms
sharing benefit, e.g. Rhizobium (nodule bacteria) which lives in symbiosis with the roots of legumes forming root nodules.
It can fix nitrogen and build proteins and other nitrogenous compounds to be partially delivered, to the higher plant and get in return the carbohydrates from the latter.