Beneficial Activities of Bacteria:
There are many kinds of bacteria without which we could not live. They are absolutely essential to the presence of life on earth. They make possible the continued existence of green plants and therefore of animals because the plants are the only source of food for animals.


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Diagrammatic life cycle of an endospore forming bacterium.

Bacteria enrich the soil with various nutrients such as nitrogen, phosphorous and improve the quality and texture of the soil.
For Example,
1. Rhizobium, a symbiotic bacterium lives in the root nodules of leguminous plants and converts atmospheric nitrogen into organic compounds.
2. Azotobacter and Azospirillum are free-living bacteria which absorbs free nitrogen present in soil, air and convert them into amino acids and enrich the soil.
3. Blue Green Algae such as Anabaena, Nostoc add organic matter to the soil and enrich it.


Ammonifying Bacteria:
The saprophytic bacteria break down the proteins and other nitrogen containing remains of the plant and animal origin in the soil to amino acids by secreting enzymes.
The amino acids are then converted into ammonia by a group of bacteria called the ammonifying bacteria. The liberated ammonia may combine with carbon dioxide and water in the soil to form ammonium carbonate.
A few plants such as the common cereals can make use of ammonium compounds as a source of nitrogen. The majority of plants, however, cannot absorb ammonium compounds as a source of nitrogen.

Nitrifying Bacteria:
Ammonia is very soluble. It moves in the soil rapidly and is acted upon by microorganisms of the category of chemosynthetic autotrophs in the soil. They are the nitrifying bacteria such as Nitrosomonas and Nitrobacter. They form nitrates from ammonium compounds.
Nitrosomonas oxidizes ammonium carbonate to nitrous acid liberating energy. The nitrous acid then combines with bases in the soil forming potassium nitrite. Nitrobacter oxidizes nitrites to nitrates again liberating energy.
Neither the ammonifying nor the nitrifying bacteria add to the total quantity of combined nitrogen in the soil. The ammonifying bacteria convert amino acids into ammonia. The process is called Ammonification.
The nitrifying bacteria convert nitrogen from the unavailable form of ammonium salts to the available nitrates. This process converting unavailable ammonium salts into available nitrates is called Nitrification.
Nitrogen-fixing Bacteria:
A considerable amount of nitrogen is lost by denitrification and through drainage. The loss must be made good by equal gains if the soil fertility is to be maintained.
The electric discharges in the atmosphere bring about the formation of traces of nitrogen compounds which are washed to the soil by rain water. The largest additions, however, come from a biological fixation process through the activity of two types of nitrogen-fixing bacteria.
Some of them live free in the soil and others in the root nodules of leguminous plants. They are able to make use of the atmospheric nitrogen and change it into nitrogenous compounds.
The nitrogen-fixing bacteria are thus unique in tapping a source of nitrogen not available to most other plants. This process of nitrogen transformation is called Nitrogen fixation.


Fermentation bacteria are anaerobic, but use organic molecules as their final electron acceptor to produce fermentation end products. Streptococcus, Lactobacillus, and Bacillus, for example, produce lactic acid, while Escherichia and Salmonella produce ethanol, lactic acid, succinic acid, acetic acid, CO2, and H2.
Fermenting bacteria have characteristic sugar fermenting patterns, i.e., they can metabolise some sugars but not others.For example, Neisseria meningitidis ferments glucose and maltose, but not sucrose and lactose, while Neisseria gonorrhoea ferments glucose, but not maltose, sucrose or lactose. Such fermenttion patterns can be used to identify and classify bacteria.
During the 1860s, the french microbiologists Louis Pasteur studied fermenting bacteria. He demonstrated that fermenting bacteria could contaminate wine and beer during manufacturing, turning the alcohol produced by yeast into acetic acid (vinegar). Pasteur also showed that heating the beer and wine to kill the bacteria preserved the flavor of these beverages. The process of heating, now called pasteurization in his honour, is still used to kill bacteria in some alcoholic beverages, as well as milk.
(a) Source of Antibiotics:

(b) Preparation of Serums and Vaccines:



1) Food poisoning:
Of course, all activities of bacteria are not beneficial. Some saprophytic bacteria cause decay of our food and make it unpalatable. The activities of certain bacteria produce powerful toxins in the food.
These toxins are powerful enough to cause food poisoning which results in serious illness and even death. Some species of Staphylococcus are the common offenders.
There is another dangerous food poisoning bacteria known as Clostridium botulinium. It causes botulism—a fatal form of food poisoning.

2) Disease:
Many parasitic bacteria are the causative agents of bacterial diseases. They cause diseases of our economic plants; domesticated animals and man. T.J. Burrill in 1878 first gave the information that bacteria causes plant disease.
There are more than 170 species of bacteria which cause plant diseases. Usually they are rod like and non spore forming. Many of them also have flagella.
The bacteria gain entry into the host through wounds or natural openings such as stomata, lenticels, hydathodes and through the thin epidermis.
3) Denitrification:
There are sometimes a group of bacteria in the soil which reverse the nitrifying processes. They injure the soil by causing the loss of a part of its combined nitrogen.
They do this by breaking down nitrates into nitrites and nitrites into ammonia compounds or to free nitrogen.
NO3 ? NO2 ? NH3 ? N gas ?
The free nitrogen pass into the atmosphere and is lost in the soil. The result is that the soil fertility is lowered. This process is called denitrification. The bacteria which bring about denitrification are called the denitrifying bacteria.
They are mostly active in the soil containing excess of nitrogen compounds such as the heavily manured soils. Soils deficient in oxygen are favourable for the activity of this type of bacteria.
Denitrification is checked if the soil is well aerated by ploughing and digging and drained. It is uneconomic to use natural and artificial nitrate manures simultaneously.
The denitrifying bacteria found in the faeces containe the manures that tend to destroy nitrates.