Bacteria : Definition, Characteristics, Structure and Size

Bacteria : Definition, Characteristics, Structure and Size
Bacteria : Definition, Characteristics, Structure and Size

Bacteria are microscopic, smallest prokaryotic most primitive unicellular organisms. Lack true chlorophyll so called unicellular plant. Bacterial reproduction done by binary, fission..

These are very small in size and thus little can be know about them by conventional light microscope. Their fine structure has been revealed with the help of electron microscope. All bacteria show typical prokaryotic structure.

History of Bacteria or Bacteriology
Study of bacteria is called Bacteriology. Which scientist study of bacteria is called Bacteriologists.

1. Antony van Leeuwenhoek (1672 - 1723) - He discovered bacteria in sewage water, saliva and debris peeled off from the teeth, under the crude microscope prepared by him self. He found numerous 'little animalcules'.

2. Ehrenberg (1829) - First time called Bacteria (singular bacterium). These are very small, relatively simple, single-celled organisms whose genetic material is not enclosed in a nuclear membrane, for this reason bacteria are called prokaryotes. 

3. Louis Pasteur (1822-1895) - He worked on fermentation and reported that it takes place by bacteria. He further proved these organisms are responsible for rotting of substances and cause of various type of diseases in man, animals and plants. On the basis of his work, Pasteur (1865) proposed "The Germ Theory of Disease".

4. Robert Koch (1843-1910) - He is German physician, his major work in field of Bacteriology. Robert Koch is called the Father of Bacteriology.

General Characters of Bacteria
  1. The bacteria are of extremely small size, simplest, unicellular, prokaryotic, primitive and living microorganisms.
  2. They occur in water, soil, air, animals and plants.
  3. They are found in snow as well as in hot water streams.
  4. They may be spherical, elongated, rod-like and some are variously spiral.
  5. They commonly occur singly but in few species, individuals are attached to form a chain like structure.
  6. The bacteria have rigid cell wall (a characteristic feature of plants) composed of a substance called Peptidoglycan (by contrast, cellulose in the main substance of plant and algal cell walls).
  7. Some bacteria have one or many hair or whip-flagella. The flagella provide the power of movement to the bacteria.
  8. The bacterial cells a well defined nucleus is not found. The nucleus does not have any nuclear membrane, nucleolus and chromosomes. Eeukaryotic cell and histone proteins do not remain attached with the DNA helix.
  9. In the bacterial cell in the centre of cytoplasm, there is a clear portion, which is called nucleoid or genophore. In the nucleoid, a long, ringed DNA filament, folded at some places is found.
  10. The nuclear material found in bacteria is termed as incipient nucleus.
  11. Mitochondria are absent and their function is carried out by complex localized infoldings in the cell membrane, known as mesosoines.
  12. Cell organelles like plastids, mitochondria, Golgi complex, endoplasmic reticulum, etc., are absent in bacteria.
  13. Ribosomes (70s type) are found freely distributed in the protoplasm. They are generally in the form of polysomes.
  14. Bacteria generally reproduce by binary fission. Some bacteria conjugation type of sexual reproduction takes place.
  15. The great majority of bacteria are useful to the living communities, as they are responsible for the decay and recycling of organic materials in soil.

Size and Shapes of Bacteria

(A) Size of bacteria - Theses are very small microscopic organisms. An average bacterial cell ranges from 0.5 - 2.0 µm in diameter. A single drop of water may contain as many as 5 x 10⁶ bacteria. In one gram top soil their number may vary from 1 × 10³ to 1 × 10⁹ Their size varies with the shape.

For example, Radius of spherical or coccus bacteria ranges from 0.5 to 2.5 μm and rod-shaped or bacillus bacteria are larger than cocci and are 0.3 to 15 μm in width. Thiophysa volutans, a sulphur bacterium, measuring about 18 um in diameter is perhaps the largest amongst all bacteria.

(B) Shapes of Bacteria - Bacterial cells generally appear in several shapes. Individual bacteria may from pairs, chains, clusters or other grouping. On the basis of shapes and sizes the bacteria may be classified as follows -

1. Bacillus or rod-shaped bacteria - These bacteria are rod-shaped, cylindrical or elongate and are motile or non-motile cells. They are of following types -
  • Single bacillus - When only one rod-like structure represents the bacterium. This is most common type bacteria, example - Rhodospirillum sodamense.
  • Diplobacilli - When they are found in pairs, they are called diplobacillus example - Bacillus subtilis.
  • Streptobacilli - When they are found in a chain, they are called streptobacillus. Example - Bacillus, tuberculosis.
    Bacteria : Definition, Characteristics, Structure and Size

2. Spherical or Cocci - The spherical or ellipsoidal bacteria are called cocci (singular coccus) They measure 0.5 - 2.25 µm in diameter. They are non-motile. The cocci are classified into the following six groups.
  • Micrococci - They are live alone. They are called as micrococci. example - Micrococcus luteus.
  • Diplococci - When cocci occur in pairs. They are called diplococcic. example - Diplococcus pneumoniae.
  • Streptococci - Spherical bacteria occur in long chains. They are called as Streptococci. example - Streptococcus lactis, S. pyogenes.
  • Tetracocci - When they form groups of four cells. Example - Pedicoccus cerevisiae, Neisseria.
  • Staphylococci - An irregular group of many spherical bacteria. Example- Staphylococcus albus, S aureus.
  • Sarcinae - When spherical bacteria divide in three planes in a regular pattern producing a cuboidal arrangement of cells. They are called 'sarcinae'. example- Sarcinae lutea, S. verticuli.
Bacteria : Definition, Characteristics, Structure and Size

3. Spiral or helical bacteria - These are slightly larger and elongated spiral rods. These length is 10 - 15μm and 0.5 to 3.0 µm in width. These bacteria have one or more flagella at each pole. They usually occur singly or in small chains, but are seldom found in groups. These following shapes are-
  • Vibrios - These are slightly curved rods of half turn. They resemble with the sign of comma (,) and are commonly known as 'comma bacteria'. Examples - Vibrio cholerae the agent of cholera and Vibrio comma.
  • Filamentous - Theses bacteria such as Beggiatoa and Thiothrix are filamentous.
  • Pleomorphic - These bacteria are capable of changing their shape and size temporarily in response to changes in the surrounding environment. As such a single bacterium may occur in more than one shape in its life cycle. For example, Acetobacter may occur as bacillus (single rods) or streptobacillus (chain of rods).

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