The bacteria constitute a unique group of one-celled organisms. During last few decades, much has been known on the structure, reproduction and importance of bacteria.

The organism has become a prolific source of research of biologists, biotechnologists, microbiologists, chemists and medical professionals. Common man is influenced by the organism in many ways.


Bacteria are ubiquitous in nature. They are among the most numerous of the living beings present in all conceivable environments. They are present in aquatic habitats. Every moment, we breathe in or breathe out a number of bacteria who are present in enormous numbers in the atmosphere.


They are present in our foodstuffs. They can thrive in all parts of our body, right from the mouth parts to the terminal part of the digestive system.

They can withstand extreme conditions of temperature. Bacteria are being detected from the hot springs, saline waters and from the tops of the icy mountains. They are also found in huge numbers from all types of soils. The reasons for their universal distribution are –

(i) Highly simple structure.

(ii) Faster metabolism under suitable conditions.


(iii) Resistance of the vegetative cells to adverse environmental conditions.

(iv) Formation of heat resistant endospores.

(v) Diversity of the mode of nutrition.

Size and Shape


Bacteria are not visible to the naked eyes. They can only be seen under, microscope. Dimension of bacteria are measured in microns.

The bacteria are much variable in their shape. They ordinarily occur in three- forms – rods, spheres and helices. Again, under these, there are a variety of intermediate forms.

1) Rod shaped bacteria

These types of bacteria may be cylindrical, straight or slightly curved with rounded and blunt ends. These are called bacillus forms or bacilli. These forms of bacteria are commonest and they range from diameter and in length.


Some are flagellated also. The bacilli may occur singly or in groups of twos (diplobacillus). When bacilli occur in chains, we call it streptobacillus. Occasionally, bacilli may occur in parallel or palisade arrangement.

2) Spherical bacteria

This group of bacteria with spherical or ellipsoidal cells varying from 0.5 to 1.25 micron in diameter. These are called coccal forms or cocci. Almost all coccal bacteria are no flagellated. These forms may occur singly. But when they occurring twos, fours and
in chains arc respectively called diplococcus, tetracoccus and streptococcus. The coccal bacteria may occur in loose masses of variable size and irregular in shape which arc called staphylococci.

3) Helical Bactria


Bacteria cells twisted spirally with 1 to 5 complete turns are called spirilla. When bacterial cell is short and looks like a curved rod are called vibrios. Vibrios have flagella at one pole only.

4) Filament

When bacterial cells divide and joined to one another, they form the filaments. The numbers of filamentous forms are very rare.

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