Bacteria

Bacteria are minute microscopic prokaryotes. They are unicellular organisms, which are usually identified by their specific activities, more than their general organisation.

Occurrence

Bacteria are omnipresent, occurring in all environments. They occur in water, in air, in soil, and in or on the bodies of living organisms. There are bacteria which occur in hot springs as well as in extreme cold temperature.

Size

The average size of bacterial cell ranges from 1 mm to 10 mm in length and about 0.7 mm to 1.5 mm in width. The largest bacterium is Beggiatoa mirabilis. It is a few centimeters long and about 30-40 m in diameter.

Shape

The bacteria generally exhibit a fairly constant shape and form. However, some of them are capable of changing their body shape and size in accordance with the environmental needs. Such bacteria are called pleomorphic.

Different Forms of Bacteria

Following are the common forms of bacteria.

Cocci (singular coccus)

They are spherical in shape. They can be further distinguished into

• Monococcus (occur singly)

• Diplococcus (occur in pairs)

• Tetracoccus (occur in groups of four)

• Streptococcus (occur in the form of a filament) and

• Staphylococcus (occur in the form of sheets).

fig. 8.1 - Types of Bacteria

Bacillus (Rl- bacilli)

They are straight, cylindrical, rod shaped bacteria. They may be further distinguished into

• Monobacillus (single)

• Diplobacillus (in pairs)

• Streptobacillus (in filamentous form) and

• Palisade Bacillus (in the form of a stack)

Spirillum

These bacteria are spirally coiled like a corkscrew.

Vibrio

These are elongated, C shaped or comma shaped bacteria.

Flagellation

Most of the bacteria contain one to many locomotor structures called flagella. Cocci do not have flagella. Based on the number and mode of attachment of the flagella, bacteria can be distinguished into the following types:

I . Atrichous : Bacteria without flagella.

II. Trichous: They bear one or more flagella.

a) Monotrichous : Bacteria with a single flagellum, at one end

b) Cephalotrichous : Bacteria with many flagella attached at one end

c) Amphitrichous : Bacteria with a single flagellum at each end

d) Lophotrichous : Bacteria with a group of flagella attached at each end

e) Peritrichous : Bacteria with flagella all over the body

fig. 8.2 - Flagellation in Bacteria

Gram Positive and Gram Negative Bacteria

In 1884 a Danish physician Christian Gram devised a technique of differential staining for bacterial cells, using two simple stains. The procedure involves staining bacteria initially with a weak alkaline solution of gentian (crystalline) violet. All bacterial cells stain blue with this dye. Subsequently these cells are treated with Lugol's iodine solution (Iodine in potassium iodide solution) and washed with alcohol. At this stage if the bacteria retain the purple colour, they are described as Gram positive bacteria and if they lose their purple colour, they are described as Gram negative bacteria. The Gram negative bacteria can be subsequently stained with saffranine for further investigations. The Gram positive and Gram negative bacteria show certain morphological and physiological differences between them.