Bacteria-Discovery,characters,shape and size

Discovery of bacteria

Bacteria were 1st discovered by a Dutch botanist,Antony von Leeuwenhoek in 1676. He observed them in a rain drop with the help of  a simple microscope designed by him He also gave illustrated account of these microscopic organism from his own teeth scum. These were called 'animalcules' by the early microbiologists. It was Linnaeus (1758) who placed them under the genus vermes.The name Bacteria was given  by a German biologist Christian G. Ehrenberg (1828).However Bacteria as a group were recognised by German botanist, Karl Wilhelm von Nageli (1857) who proposed the name Schizomycetes for this group.

Antony von Leeuwenhoek

Louis Pasteur, a French chemist, observed that bacteria are the cause of   some diseases. This fact was called 'Germ Theory of  Disease". He also discovered bacteria causing chicken cholera. Almost at the same time Robert Koch discovered  bacteria causing anthux and tuberculosis.

General characters of Bacteria

1. Bacteria are the  simplest and the most primitive organisms. They are cosmopolitan and are found in all the places wherever life is possible.
2. Bacteria are considered to be the plants because they have a rigid cell wall.
3. Bacteria are prokaryotes like blue-green algae.
4. Bacteria are strictly unicellular and generally occur singly.Some of them are founds in groups.
5. Bacteria also lack membrane bound organelles like endoplasmic reticulum, golgi bodies,mitochondria,plastids etc.
6. Bacteria lack nuclear membrane and ,therefore, distinct nucleus and nucleoli absent.
7. Most of the bacteria lack chlorophyll,hence they are heterotrophic. However, few bacteria possess chlorophyll and are autotrophic.The chlorophyll  remains scattered in the cytoplasm.
8. Genetic material is represented by histone free DNA.
9. They commonly reproduce vegetatively by  fission.
10. Variations in the genetic characters are due to genetic recombinations brought about by conjugation, transformation and transduction.Flagellated gametes are, however not produced.

Shape of  Bacteria

There are have more than 2000 species of bacteria. and there have 4 fundamental shape are present.

[I]  Bacillus or rod-shaped

These are small stick or rod-shaped bacteria. This shape is the commonest of all the bacterial shapes. Bacilli may occurs in two form.

1. Diplobacillus- In this form two or more bacilli occur in a pair.    e.g,  Corynebacterium diphtheriae.
2. Streptobacillus-In this type many  bacilli occurs in a chain,     e.g,  Bacillus tuberculosis, Bacillus anthracis, etc.

[II]  Coccus or spherical

These  bacteria are  oval or spherical in shape .Cocci are generally non-motile and are the smallest bacteria.Cocci occur in 6 forms.

1. Micrococci-In this type each coccus is a free individual;  e.g,  Micrococcus luteus.
2. Diplococcus-Here cocci remain attached, mostly in pairs,  e.g,  Diplococcus pneumoniae.
3. Streptococcus-In this type cocci form chain;  e.g,  Streptococcus lactis.
4. Tetracoccus-Here four cocci form a characteristic group,  e.g,  Neisseria.
5. Staphylococcus-Cells form irregular groups or bunches, e.g,  Staphylococcus aureus.
6. Sarcinae-This is a type where 8-64 cocci are arranged in cubes, e.g,  Sarcina lutea.

 [III]   Spiral or helical

These bacteria are helical or cork screw shaped or coiled.  e.g,  Spirillum volutans.

[IV]Some other shapes

1. Comma shaped-These are short, incomplete spirals and look like comma (,). They are also known as Vibrio.  e.g,  Vibrio cholerae.
2. Stalked road- These are rod-shaped with a distinct stalk,  e.g,  Caulobacter.
3. Branched filament- The bacterium is long and thread-like,  e.g,  Streptomyces.






4. Pleomorphic-The shape of many bacteria changes according to time, habitat and medium. This characters of existance of different shapes in the same species is called pleomorphism, e.g,  Azotobacter.

Size of Bacteria

Bacteria vary considerably in size. The average size of a bacteria cell  is 0.5-1.0 ×2.0-5.μm Beggiatoa mirabilis is the largest bacterium.(16-45  μm). Bacillus butschlii, with a length of approximately 80  μm is considered to be the longest bacterium. Dialister pneumosintes is probably the smallest bacterium (length  0.15-0.31  μm).