Diversity in Size  and Shapes of Bacteria

Diversity in Size of Bacteria

The smallest known bacteria belong to the group of Mycoplasmias, which infect animals.They are spherical bacteria and have approximate diameter of 0.1 to 0.2 micrometers. Escherichia coil, have rod shape and are about 7 micrometers long and 1.8 micrometer in diameter. Some Spirochaets reach 500 Micrometer in length. One of the largest bacteria belong to Epulopiscium. Lt is visible to naked eye 9600 micrometer long, 80 micrometer thick).

Diversity in shapes of Bacteria

Bacteria have three main shapes : 

Spherical
Rod shaped 
Spiral 

shapes of Bacteria cell

However it has recently been shown that certain bacteria are capable of dramatically changing shape, for example Helicobacter pylori exists as both a helix-shaped from and a spherical form. Such bacteria are called pleomorphic.

Cocci (Spherical shape Bacteria): 

Cocci are spherical bacteria. 

Cocci generally appear in group which can be distinguished on the basis of plane of cell division and number of cells.

Single (vertical) plane of cell division:

bacteria appear in pairs, called diplococci.

In chain of many cells, called streptococci.

Two plane of cell divisions :

(first vertical then again vertical but at right angle to the first), bacteria appear in a square of four cells, called tetrad. 

Three plane of cell divisions :

(first vertical then again vertical but at right angle to the first and then horizontal), bacteria appear in a cubical arrangement of eight cells, called sarcinae.

If plane of cell division becomes irregular and many bacteria are produced which arrange like bunches of grapes, called staphylococci. 

Examples of cocci are : Streptococcus pneumoniae.

Arrangement of cocci

Bacilli (Rod Shaped Bacteria): 

Bacilli are straight or rod shaped organisms. 

They always divide in vertical plane so they are found either in pairs, called diplobacilli .

In chains of many cells, called streptobacilli. 

Some rod shape bacteria have spherical end like cocci. Such bacteria are called coccobacilli. 

Example of rod shaped bacteria are Escherichia coil.

Spirilli ( Spiral Shape Bacteria):

Spirilli are spiral shaped bacteria. 

They usually occur singly, seldom form colonies. 

Thin and flexible spiral shaped bacteria are called Spirochetes. E.g., Treponema pallidum. Relatively, thick and rigid spiral shaped bacteria are called spirillum e.g., Spirillum minus, whereas, curved or comma shaped bacteria which are intermediate to spiral and rod shaped, are called vibrio e.g., Vibrio cholerae.

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