Bacteria:

Bacteria are discovered by Anton Van Leeuwenhoek in 1676. They are found everywhere in air, water, soil, and bodies of organisms. There is no natural death of bacteria. They are killed either using chemicals or by infecting with virus.

Morphology:

Size of the bacteria varies from 2-5µm and 0.3-0.8µm width in rod shaped bacteria. In spherical bacteria the size varies from 0.5 – 1µm in diameter.

• Spherical bacteria: the bacteria having spherical in shape are called coccus bacteria. The coccus bacteria may be found singly and freely are called monococcus or micrococcus. When they are found in the group of two is called diploccus. Sometimes they are found in the group of four are called tetracoccus. Some of them are found in chain are called streptococcus and some are found in large group like bunch of grape called staphylococcus.
• Rod like Bacteria: the bacteria having shape of the cell is if rod like called bacillus. The bacillus bacteria are found in singly and freely are called monobacillus. Some of them are found in the group of two are called diplobacillus. They are also found in chain are called streptobacillus.
• Helical Bacteria: The bacteria having shape of the cell is coiled called helical bacteria. These are of two types:
• Vibrio: The bacteria having the shape are comma like are called Vibrio. eg. Vibrio cholera.
• Spirillum: The bacteria having shape is helically coiled are called Spirillum.

Cell Structure:

Capsule:

The capsule is the outer most covering of gelatinous substance is called capsule. It is made up of polysaccharides. The capsule may be absent in some bacteria. They are called non-capsulated bacteria and other having capsule are called capsulated bacteria. Usually the capsulated bacteria are poisonous or virulent that causes diseases and the non-capsulated bacteria are non-virulent. The function of the capsule is to protect cell, protects from desiccation and secrets poisons.

Cell wall:

It is found under the capsule in capsulated bacteria and the outermost layer in non- capsulated bacteria. It is rigid and is composed of a kind of complex substance called peptidoglycan or Murine or Mucopeptide. The peptidoglycan is composed of complex carbohydrates called N-acetyle muramic acid and N acetyl glucosamine and aminoacid ie. L alanine, D alsnine and D glutanic acid. Polysaccharides and lipids are also present in cell wall.

On the basis of cell wall structure and stain retaining property bacteria are two types:

Function of cell wall: It provides rigidity to the cell, it maintains cell shape; it protects protoplasm of the cell.

Conjugation: The process in which two bacterial cells conjugate with the help of pili is called conjugation. During the process one bacterial cell transfer the chromosomal segments to another bacterial cell.

Beneficial Activities:

In Agriculture, the bacteria are mostly important for following reasons

1. Nitrogen fixation:  Plants cannot trap nitrogen from the atmosphere but bacteria can fix nitrogen and change it into nitrogenous compounds. The phenomenon is called nitrogen fixation. The bacteria, which fix atmospheric nitrogen, are called nitrogen-fixing bacteria. Some of the nitrogen fixing bacteria are found freely in soil are called free-living bacteria, eg. Clostridium and Azotobacter. Some of them are found in root nodules making association with leguminous plants called symbiotic bacteria, eg. Rhizobium.
2. Nitrification: Some bacteria convert ammonium compounds into nitrates in the soil. The process is called nitrification. The bacteria that take part in this process are called nitrifying bacteria, eg. Nitrobacter and Nitrosomonas. = Ammonia---(by Nitrosomonas)---> Nitrites---- (by Nitrobacter) --> Nitrates
3. Ammonification: Some bacteria utilize proteins of dead bodies and convert it into amino acids. These amino acids are converted into ammonia by some bacteria. The process is called ammonification and the bacteria involve in the process are called ammonifying bacteria. Then the ammonia reacts with CO2 and H2O and gives ammonium carbonate which is absorbed by plants.
4. Bacteria decompose dead bodies. They convert complex organic compounds into simple inorganic compounds. Therefore they're called natural scavengers.
5. Bacteria make the milk sour and produce flavor. They are responsible for coagulation of milk. E.g. Lacto bacillus.
6. Bacteria convert sugary substances into alcohol, acids, acetones; etc the process is called fermentation.
7. Bacteria also help to produce different types of enzymes like Amylase secreted from Bacillus, Protease from Bacillus, Streptokinase from Streptomyces.
8. Bacteria are useful for vitamin production like Vitamin B (Cabalmin) is secreted from Pseudomonas, Vitamin B (Riboflavin) is secreted from Clostridium.
9. They are important for antibiotic production like Terramycin from Streptomyces rimosus, Streptomycin from S. griseus, Neomycin from S. fradiae.
10. They are also important to produce hydrogen commercially. During the process they ferment carbohydrate and hydrogen gas is produced.
11. Bacteria decompose waste products.

Harmful activities:

• Some of the species cause food poisoning. They secret some toxic chemical substances on out food stuff which cause food poisoning, eg Staphylococcus and Clostridium.
• Some are responsible for human diseases
• Cholera: Vibrio cholera, Pneumonia: Staphylococcus pneumoniae, Diarrohea:   Escherechia coli, Tuberclosis: Mycobacterium tuberclosis, Leprosy: M. leprae, Meningitis: Nisseria meningitides.
• Some bacteria are responsible for plant diseases.
• Red stripe in sugarcane, Leaf streak in rice, black rot in cabbage and yellow rot in wheat are caused by Xanthomonas and Pseudomonas.