Types of Bacteria

Bacteria are the smallest organisms having cellular structure, their average diameter being 1um.
They can be classified on the basis of their shape, nutrition and mode of respiration.

1) On the basis of Shape

The bacterial cells show considerable variation in their shape. In general, bacteria exhibit the following four basic cell forms.

a) Bacillus or rod-shaped bacteria

Bacilli are the most common form of bacteria. These are rod-shaped, cylindrical or elongate and are flagellated or non-flagellate.

b) Coccus or spherical bacteria

Spherical or ellipsoidal bacteria known as cocci. Almost all spherical bacteria lack flagellated and are non-motile.

c) Spiral or helical bacteria

These are slightly larger and elongated spiral rods. A spirillum has more than one turn of a helical and is about 1.5um in diameter and upto 15um in length.

d) Comma-shaped vibrium

Vibrio is an another form of bacteria, which resemble with the sign of comma ( , ).

2) On the basis of Nutrition

Bacteria, like all living organisms, require a source of energy. On the basis of their mode of nutrition, they are usually classified into three categories.

a) Photoautotrophic bacteria

Like higher plants, Photoautotrophic bacteria are capable of converting radiant energy into chemical energy, but cannot 'split' water to obtain reducing power. Thus no oxygen is evolved. This process is therefore called anoxygenic photosynthesis. In place of water, these bacteria obtain reducing power from hydrogen sulphide, thiosulphate, hydrogen or even some organic compounds.
Depending on their colour, they may be of the following three types.
I) Green sulphur bacteria
ii) Purple sulphur bacteria
iii) Non-sulphur purple bacteria

b) Chemosynthetic bacteria

These are non-photosynthetic but autotrophic bacteria. They derive energy from ammonia, nitrate, ferrous iron, hydrogen sulphide and other inorganic substance for the synthesis of their food.
On the basis of substrate specificity, Chemosynthetic can be classified into the four group.
I) Sulphur bacteria
ii) Iron bacteria
iii) Hydrogen bacteria
iv) Nitrifying bacteria

c) Heterotrophic bacteria

These bacteria obtain their readymade food from an organic source. 
The heterotrophic bacteria can be distinguished into three major nutritional categories.
I) Parasitic bacteria
ii) saprophytic bacteria
iii) Symbiotic bacteria

3) On the basis of mode of respiration

On the basis of mode of respiration bacteria can be categorised into two types. When oxygen is essential for their life, then these bacteria are called strict or obligate aerobes and, if they cannot grow in oxygen, they are known as strict or obligate anaerobes. Some bacteria can live in both environment and are called facultative.

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