Class 7 Science Micro Organisms Chapter Notes

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Micro Organisms

Micro-Organisms or Microbes
There are many living organisms around us, present in soil, water and air. Some of them are so small that we cannot see them with our naked eyes. They are called microorganisms. To see them we require a special device called microscope. The science which deals with the study of micro-organisms is known as microbiology (micro means small; bios means life).

Occurrence of Microbes
Micro-organisms are present everywhere in air, soil, water, foodstuffs, sewage, waste as well as inside animal bodies. It is almost impossible to think of any place without them. They are present at great depth in the oceans, in streams, in the snow of polar regions, in hot springs and even in sulphur spring. Soil may contain as many as 50 millions types of bacteria excluding other microbes. The maximum number of microbes are found in natural substrate, rich in organic food, having a suitable temperature and moisture for their growth and multiplication.
The air we breathe, the water we drink and the food we eat, also have microbes. Several microbes remain on the surface of our body, in our intestines, nose, mouth and other natural openings. The most microbes found in our body are harmless. Some of them are even beneficial.

Microorganisms and Us
Microorganisms play an important role in our lives. Some of them are beneficial in many ways whereas some others are harmful and cause diseases.

Major Group of Microorganisms
There are five major groups of microorganisms. These are: Bacteria, Fungi, Algae, Protozoa and Viruses.

Commercial Use of Microorganisms
Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar). Yeast is used for commercial production of alcohol and wine. For this purpose yeast is grown in natural sugars present in grains like barley, wheat, rice and crushed fruit juices, etc.

Medicinal Use of Microorganisms
Whenever we fall ill the doctor may give us some antibiotic tablets, capsules or injections such as of penicillin. The source of these medicines is microorganisms. These medicines kill or stop the growth of the disease - causing microorganisms. Such medicines are called antibiotics. These days number of antibiotics are being produced from bacteria and fungi. Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria. The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. Antibiotics are even mixed with the feed of livestock and poultry of check microbial infection in animals. They are also used to control many plant diseases.

Vaccine
When a disease - carrying microbe enters our body, the body produces antibodies to fight the invader. The body also remembers how to fight the microbe if it enters again. So, if dead or weakened microbes are introduced in a healthy body, the body fights and kills them by producing suitable antibodies. The antibodies remain in the body and we are protected from the disease causing microbes. This is have a vaccine works. Several diseases, including cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.

Increasing Soil Fertility
Some bacteria and blue green algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility. These microbes are commonly called biological nitrogen fixers.

Commensals
Some microorganisms live inside our body. They help in certain functions of our body systems. For example some bacteria in our gastrointestinal tract help in digestion. These type of organisms which reside in our body and are beneficial as well are called commensals.

Harmfull Microorganisms
Microorganisms are harmful in many ways. Some of the microorganisms cause disease in human beings, plants and animals. Such disease causing microorganisms are called pathogens. Some microorganisms spoil food, clothing and leather.

Disease Causing Microorganisms in Human
Pathogens enter our body through the air we breathe, the water we drink or the food we eat. They can also get transmitted by direct contact with an infected person or carried through an animal. Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases. Example of such diseases include cholera, common cold chicken pox and tuberculosis. There are some insects and animals which act as carriers of disease causing microbes. Housefly is one such carrier. The flies sit on the garbage and animal excreta. Pathogens stick to their bodies. When these flies sit on uncovered food they may transfer the pathogens. Another example of a carriers the female Anophiles mosquito, which carries the parasite of malaria. Female Aedes mosquito acts as a carrier of dengue virus.

Disease causing microorganisms in Animals
Several microorganisms not only cause diseases in humans and plants, but also in other animals. For example anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.

Disease causing Microorganisms in Plants
Several microorganisms cause disease in plants like wheat, rice, potato, sugarcane, orange, apple and others. The diseases reduce the yield of crops. They can be controlled by the use to certain chemicals which kill the microbes.

Food Poisoning
Food poisoning could be due to the consumption of food spoil by some microorganisms. Microorganisms that grow on our food someties produce toxic substances. These make the food poisonous causing serious illness and even death. So, it is very important that we preserve food to prevents it from being spoil.

Food Preservation
Microorganisms spoil our food. Spoiled food emits bad smell and has a bad taste and changed colour.

Common methods to preserve food in our Homes

Chemical Method: Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore, they are called preservatives. We add salt or acid preservatives to pickles to prevent the attack of microbes. Sodium benzoate and sodium metabisulphate are common preservatives. These are also used in the Jams and Squashes.

Preservation by Common Salt: Common salt has been used to preserve meat and fish for ages. Meat and fish are covered with dry salt to check the growth of bacteria. Salting is also used to preserve amla, raw mangoes, tamarind etc.

Preservation by Sugar: Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture content which inhabits the growth of bacteria which spoil food. 

Preservation by Oil and Vinegar: Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment. Vegetables, fruits, fish and meat are often preserved by this method.

Heat and Cold Treatments: Low temperature inhibits the growth of microbes. Pasteurized milk can be consumed without boiling as it is free from harmful microbes. The milk is heated to about 700°C for 15 to 30 seconds and then suddenly chilled and stored. By doing so, it prevents the growth,.of microbes. This process was discovered by Louis Pasteur. It is called pasteurization.

Storage and Packing: Dry fruits and even vegetables are sold in sealed air tight packets to prevent the attack of mirobes.

Nitrogen fixation: Rhizobium is involved in the fixation of nitrogen in leguminous plants. Sometimes nitrogen get fixed through the action of lightning. But the amount of nitrogen in the atmosphere remains constant.