NCERT Standard 8 : Microorganisms Friend and Foe Class 8 Notes

Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae. Virus reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal. Common ailments like cold, influenza (flu) and most coughs are caused by viruses. Serious diseases like polio and chicken pox are also caused by viruses. Diseases like dysentery and malaria are caused by protozoans whereas typhoid and tuberculosis (TB) are bacterial diseases.


Example for Bacteria: Rod shaped bacteria,Spiral bacteria
Example for Protozoa: Amoeba, Paramecium
Example for Fungi: Penicillium, Aspergillus, Bread mould
Example for Algae: Chlamydomonas, Spirogyra

Bacterium Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd. Bacteria are also involved in the making of cheese, pickles and many other food items.


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. The process of conversion of sugar into alcohol by Yeast is known as fermentation. Louis Pasteur discovered fermentation in 1857.

In 1929, Alexander Fleming was working on a culture of disease causing bacteria. Suddenly he found the spores of a little green mould in one of his culture plates. He observed that
the presence of mould prevented the growth of bacteria. In fact, it also killed many of these bacteria. From this the mould penicillin was prepared.

Female Anopheles mosquito which carries the parasite of malaria. Female Aedes mosquito acts as carrier of dengue virus.

Measles, Chicken Pox, Polio,  Hepatitis B are caused by Virus
Tuberculosis, Cholera, Typhoid are caused by Bacteria

Anthrax is a dangerous human and cattle disease caused by a bacterium.

Curd contains several microorganisms. Of these, the bacterium Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd.

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 to check microbial infection in animals. They are also used to control many plant diseases.

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. Bacterium Rhizobium lives in the root nodules of leguminous plants, such as beans and peas, with which it has a symbiotic relationship. Sometimes nitrogen gets fixed through the action of lightning

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 metabisulphite are common preservatives. These are also used in the jams and squashes to check their spoilage.

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