Bacteria: Definition, Size,Distribution and Classification

Bacteria: Definition, Size,Distribution and Classification

Bacteria were known as Planet’s initial life forms. A gram of dirt contains around 40 million bacterial cells whereas a milliliter of fresh-water contains one million on earth. there are around  5x 10^30bacteria on earth. Bacteria exist practically in any situation i.e. they are cosmopolitan. The construction of bacteria is simple mainly unicellular but the behavior of bacteria is complex because it shows diverse metabolic activities.

Distribution of Bacteria

Bacteria are found in all-natural ecosystems and have a worldwide distribution. Their distribution is because they can withstand great extremes of temperature, moisture, acidity, and salinity and are adapted to a wide variety of energy sources. They occur in the atmosphere to a height of about six kilometers and on the seafloor five kilometers below the mean sea level. they exist in hot springs and can also survive below freezing points in the Atlantic ice. Bacteria live at the pressure of one atmosphere but they can tolerate the pressure of 3000-6000 atmospheres. some bacteria can live in presence of oxygen while some live without oxygen. the great success of bacteria is due to their small size, rapid reproductive rate, ability to survive under adverse conditions, and metabolic adaptations.


                           Size of Bacteria

The bacterial cell ranges from 0.5 to 2.0m in diameter. A single drop of water may contain as many as 5x10x6 bacteria. In one gram of topsoil, their number varies from 1x10x3 to 1x10x9. their size also varies with the shape.

Classification of bacteria

Cohen recognized four basic forms of bacteria

 1. Coccus  2. Bacillus  3. Spiral  4. Vibrio

1. Coccus: the spherical or ellipsoidal bacteria are called cocci. they range in size from 0.5 to 2.25 meters in diameter. They are non-motile atrichous and often occur in chains or clusters of variable sizes and shapes. Based on the arrangement and the number of cells in a cluster, The cocci are classified into the following groups

A.Monococcus: When the cocci occur singly they are known as monococcus or micrococci e,g Micrococcus cerolyticus, M. cyrophilus, M. luteus.   

B.Diplococcus: When the cocci occur in pairs they are known as Diplococcus. E,g Diplococcus pneumonia.

C.Streptococcus: when the cocci occur in long chains they are known as Streptococcus e,g S. lactis, S. pyrogenes.

D.Tetracoccus: When the cocci occur in groups they are known as Tetracoccus. E,g  Pedicocci cerevisiae, Neisseria.

E.Staphylococcus: When the cocci occur in irregular benches or the form of grapes. It is known as Staphylococcus,g S. Albus, S. aureus.

F.Sarcinae: When the cocci occur in cuboidal manner it is called Sarcinae. E,g S. lutea, S. verticuli.

2. Bacillus or rod-shaped bacteria: It is the most common form. these are motile or non-motile cells that are rod-shaped, cylindrical, or elongate. The rods may have rounded or blunt ends. Some are extremely short and nearly undetectable from spherical bacteria while others are long and thin. A typical bacillus is 1.5 meters long and 0.5 meters wide

The Bacillus is classified into the following groups

A. Monobacillus: When the bacillus bacteria occur singly it is called monobacillus eg Bacillus cereus

B. Diplobacillus: When bacillus bacteria occur in pairs they are called diplobacillus eg Corynebacterium diphtheria

C. Streptobacillus: When the bacillus bacteria occur in long chains they have known streptobacillus eg Bacillus tuberculosis 3. Spiral bacteria or Helical bacteria: these are slightly larger and elongated spiral rods. It has more than one turn of the helix. It is about 15m in diameter and up to 15 m in length. they are frequently seen singly or in small groups but seldom in groups eg Spirillum undulum,      S. voluntans, etc

 4. Vibrios: these are half-turn rods that are slightly bent they resemble the sign of comma and are commonly known as comma bacteria. A vibrio bears a single flagellum at its tip and is about 10m in length and 15-17m in with width e,g vibrio coli, V. cholera 

5. Pleomorphic bacteria: some bacteria are capable of changing their shape and size temporarily in response to changes in their surrounding environment.

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