Micro-Organisms: Friend And Foe - Class 8 : Notes

(i) Organisms which are too small to be seen by the naked eye, especially a single celled organism is called microorganisms or microbes.
(ii) Microorganisms are classified into four major groups- bacteria, fungi, protozoa and some algae.



Red, Brown and Green Alga

(iii) Viruses are also microscopic. They reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.



Where do Microorganisms Live:
(i) Microorganisms may be single celled (bacteria, some algae and protozoa) or multi cellular (algae and fungi) can survive under all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands.
(ii) They are also found inside the bodies of other organisms including humans.
(iii) Growth of some microorganisms depends on other organisms while other organisms exist freely.
(iv) Microorganisms like amoeba can live alone, while fungi and bacteria may live in colonies.

Microorganisms and Us:
Some of Microorganisms are beneficial in many ways while some others are harmful and cause diseases.

Friendly Microorganisms:
Microorganisms are used for various purposes.

(i) Some microorganisms are used in the production of curd, bread and cake.
(ii) Some microorganisms have been used for the production of alcohol since ages.
(iii) They are also used in cleaning up of the environment as composers. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria.
(iv) In agriculture fields, microorganisms are used to increase soil fertility by fixing nitrogen.

Commercial Use of Microorganisms:
(i) For large scale production of alcohol, wine and acetic acid (Vinegar) Microorganisms are used

(ii) For commercial production of alcohol and wine yeast is grown on natural sugars present in grains like barley, wheat, rice and crushed fruit juices, etc.

Medicinal Use of Microorganisms:
(i) Whenever we fall ill the doctor may give us some antibiotic tablets, capsules or injections like peenicillin which are made up of microorganism.
(ii) These days the medicines produced from Bactria and fungi kill or stop the growth of the disease-causing microorganisms. Such medicines are called antibiotics.
(iii) Streptomycin, tetracycline and erythromycin medicines which are made from fungi and bacteria are some of the commonly known antibiotics.
(iv) The antibiotics are made by growing specific microorganisms and are used to cure a variety of diseases. Antibiotics should be taken only on the advice of a qualified doctor.
(v) Antibiotics are even added with the feed of livestock and poultry for checking microbial infection in animals.
(vi) Microorganisms are also used to control many plant diseases.

(i) When a disease-carrying microbe enters our body, the antibodies produced by our body fight with the invader. If microbes enter again, the body also remembers that how to fight with the microbes.
(ii) If dead or weakened microbes are injected in a healthy body, the body fights and kills them by producing suitable antibodies.
(iii) The antibodies remain in the body for protecting from the disease causing microbes. This is how a vaccine works.
(iv) Several diseases like cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.

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

Cleaning the Environment:
At the time of making manure, we collect wastes of plants, vegetables and fruits from nearby houses and gardens. They put them in a pit meant for waste disposal. After some time, it decomposed by microorganisms and gets converted to manure. By this method environment is cleaned.

Harmful Microorganisms:
(i) Some of microorganisms are harmful in many ways.
(ii) In human beings, plants and animals, some of the microorganisms cause diseases. Such disease-causing microorganisms are called pathogens.
(iii) Food, clothing and leather are spoiled due to some harmful microorganisms.

Disease— causing Microorganisms in Humans:
(i) Pathogens enter our body through the different way like air when we breathe, the water when we drink or the food when we eat. They can also get transmitted by direct contact with an infected person or carried through an animal.
(ii) Microbial diseases such as cholera, common cold, chicken pox and tuberculosis that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases.
(iii) There are some insects and animals which act as carriers of disease causing microbes. For example housefly is one such carrier that flies sit on the garbage and animal excreta etc. Pathogens stick to their bodies. When these flies sit on uncovered food they transfer the pathogens. Whoever eats the contaminated food is likely to get sick. Some more examples of carrier microbes are the female Anopheles mosquito, which carries the parasite of malaria. Female Aedes mosquito acts as carrier of dengue virus.
(iv) All mosquitoes breed in water. So, one should not let water collect anywhere like in coolers, tyres, flower pot etc. By keeping the surroundings clean and dry we can prevent mosquitoes from breeding.

Some Common Human Diseases caused by Microorganisms:

Human Disease

Causative Microorganism

Mode of Transmission

Preventive measures (General)



Chicken Pox










Keep the patient in complete isolation. Keep the personal belongings of the patient away from those of the others. Vaccination to be given at suitable age







Maintain personal hygiene and good sanitary habits. Consume properly cooked food and boiled drinking water. Vaccination

Hepatitis B Virus Water

Drink boiled drinking water. Vaccination.




Use mosquito net and repellents. Spray insecticides and control breeding of mosquitoes by not allowing water to collect in the surroundings

Disease-causing Microorganisms in Animals:
Some of microorganisms not only cause diseases in humans and plants, but also in other animals. For example: anthrax is an example of 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:
Some of microorganisms cause diseases in plants such as wheat, rice, potato, sugarcane, orange, apple and others. These diseases reduce the yield of crops. They can be controlled by the use of certain chemicals or pesticides which kill the microbes, which affect the yield the crops.

Some Common Plant Diseases caused by Microorganisms

Food Poisoning:
Food poisoning in humans could be due to the consumption spoiled food by some microorganisms. Microorganisms that grow on our food produce toxic substances and make the food poisonous causing serious diseases. So, it is very important that we preserve food to prevent it from being spoilt by the microorganisms.

Food Preservation:
Microorganisms spoil our food. Spoiled food emits bad smell and has a bad taste and changed colour. Here some common methods to preserve food in our homes.

a. Chemical Method:
(i) Preservatives like Salts and edible oils are the common chemicals generally used to check the growth of microorganisms.
(ii) We add salt or acid preservatives to pickles to prevent the attack of microbes. Sodium benzoate and sodium meta bisulphite are common preservatives which are used in the jams and squashes to check their spoilage.

b. Preservation by Common Salt:
(i) To preserve meat and fish for ages, common salt is used. Meat and fish are covered with dry salt to check the growth of bacteria.
(ii) Salting is also used to preserve amla, raw mangoes, tamarind, etc.

c. Preservation by Sugar:
(i) Sugar is used for preserving jams, jellies and squashes are preserved.
(ii) It reduces the moisture content which inhibits the growth of bacteria which spoil food.

d. Preservation by Oil and Vinegar:
(i) Oil and vinegar are used to prevent spoilage of pickles because bacteria cannot live in such an environment.
(ii) Vegetables, fruits, fish and meat are often preserved by this method.

e. Heat and Cold Treatments:
(i) Boiling of milk kills many microorganisms, after that it is stored or used.
(ii) We keep our food in the refrigerator. Low temperature inhibits the growth of microbes.
(iii) Pasteurized milk can be taken without boiling as it is free from harmful microbes. The milk is heated to about 700C for 15 to 30 seconds and then suddenly chilled and stored. By doing so, this process prevents the growth of microbes. This process was discovered by Louis Pasteur. It is called pasteurization.

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

Nitrogen Fixation:
(i) Rhizobium is involved in the fixation of nitrogen in leguminous plants (pulses).
(ii) Nitrogen also gets fixed through the action of lightning. But the amount of nitrogen remains constant in the atmosphere.

Nitrogen Cycle:
(i) Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins, available 78% in our environment. Bacteria and blue green algae present in the soil for fixing nitrogen from the atmosphere and then convert into compounds of nitrogen.
(ii) After this, usable compounds can be utilised by plants from the soil through their root system. These compounds are then used for the synthesis of plant proteins and other compounds. Animals feeding on plants get these proteins and other nitrogen compounds.
(iii) When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again. Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere. As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

Nitrogen cycle
Nitrogen Cycle


Leave a Reply