Structure of Bacterial Cell
Bacterial is very small, simple unicellular organisms with a length varying from 2 to 5. On the basis of shape bacteria are either coccus (spherical), bacillus (rod shape), spirillum (spiral) or vibro (comma) types. On the basis of arrangement of flagella, these may be atrichous (flagellum absent) monotrichous (one flagellum at one end), leptotrichous (bunch of flagella on one side), amphitrichous (flagella on whole surface).
Bacterial cell is surrounded by a prominent cell wall constituted by polysaccharides, lipids and proteins. The cell wall is permeable to water and ions of small molecules.
Slime layer and capsule:
Some bacterial cells are completely enveloped by a slimy layer, which is relatively thick to form the capsule. Capsulated bacteria are more harmful. Capsule protects the cell from antibodies and desiccation.
Mesosomes take part in aerobic respiration and it is found in gram positive bacteria. The protoplasm is either transparent or granular.
Below the cell wall, the plasma membrane is present. Plasma membrane at certain points forms coiled invaginations called Mesosomes.
Cytoplasm is composed of complex proteins, lipids and mineral, nucleic acid and water. Glycogen is the reserve food material. It contains 70S type of ribosomes. Other organelles like endoplasmic reticulum, mitochondria; Golgi complex, etc. are absent. However, in photosynthetic bacteria some pigments are present (bacterio chlorophyll).
Bacteria are prokaryotes, there is no well-organised nucleus. Nuclear membrane and nucleolus are absent. At the center there is a clear zone called nucleoid where only a single naked chromosome (without histone protein, only DNA) is present in a very much coiled form.
In addition to the DNA in some bacterial cells an additional circular DNA is present in the cytoplasm. It is called as episome or plasmid.
Mode of Nutrition:
Nutrition, bacteria are either autotrophs or heterotrophs. Autotrophic bacteria are again divided into two groups;
(i) Photoautorophs trap solar energy for utilization CO2 and use H2S in place of H2O. Thus, O2 is never released.
(ii) Chemoautotrophs obtain energy from chemical by oxidation process for food synthesis and carbon from CO2. This type include nitrifying bacteria, sulphur bacteria, iron bacteria etc. Heterotrophs are also of two types- Photoheterotrophs and chemoheterotrophs. Photoheterotrophs use light as energy source and an organic compound as source of carbon, e.g. Green and purple bacteria. Chemoheterotrophs use chemical as energy and carbon may also be obtained from the same organic substances. Most of the bacteria belong to this type.
Economic Importance of Bacteria
Bacteria are both friend and foe to the human race.
1. Soil fertility:
It participates in nitrogen fixation and increases the soil fertility, e.g. Azotobacter, Clostridium, etc.
2. Nitrifying bacteria:
These bacteria convert ammonia to nitrate, e.g. Nitrosomonas and Nitrobacter.
These bacteria convert protein into ammonia. Such bacteria are therefore, also called bacteria of decay or putrefying bacteria, e.g. Bacillus sps.
4. In dairy:
These bacteria convert lactose sugar of the milk into lactic acid. The preparation of cheese and curd are based on these principles, e.g. Lactobacillus.
5. In Industries:
Many useful products of economic importance are manufactured by using bacteria.
Antibiotics cure disease competitive inhibition which are obtained mostly from bacteria.
7. Symbiosis in human intestine:
The bacteria like E. Coli found inside human intestine is mostly helpful in digestion.
8. Preparation of alcohol:
Ethyl alcohol and butyl alcohol are manufactured by the activity of Clostridium acetobutylicum in sugar solution.
9. Preparation of vinegar:
By the activity of Acetobacter in sugarcane juice.
10. Curing of tea, coffee, tobacco:
Curing and flavouring of tea, coffee and cocoa are done by Bacillus bacteria.
11. Preparation of leather and tanning:
The hair and fats are removed from the skin by the activity of Clostridium.
12. Sewage disposal:
Faecal matter collected in sewage digested by a number of aerobic bacteria.
(1) Food Poisoning:
Some bacteria excrete toxic substance and cause food poisoning e.g. Clostridium, Salmonella, etc.
Bacteria reduce soil fertility by releasing free nitrogen, e.g. thiobacillus.
(3) Human disease:
They cause various diseases, infect the human body, e.g. bacillus vibrio etc.
(4) Plant diseases:
Many serious plants diseases are caused by bacteria, e.g. Xanthomonas, Pseudomonas, etc.