Microorganisms: Friend and Foe

Notes of chapter: Microorganisms: Friend and Foe are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 8.

(1) Microorganism-

A microorganism is a tiny living organism which cannot be seen by naked eyes. Microorganisms can be singled-celled and multi-celled. Single –celled microorganisms are bacteria, protozoa. Multi-celled microorganisms are algae and fungi.

Eg:- Moist bread gets covered greyish white patches. These patches have black round structures on the bread which can be seen by only magnifying glass or by microscope.

(2)Microorganisms can be found in any environment as cold, hot, desert and inside bodies of animals including humans.

(3) Some microorganisms like fungi and bacteria live in colonies while amoeba lives alone.

(4)Types of microorganisms-


Bacteria are single-celled tiny living organisms which cannot be seen by naked eyes. They are responsible for diseases like typhoid and tuberculosis (TB) and fermentation of wine.

Rod Shaped Bacteria of NCERT Microorganisms-Friends and Foe     Spiral Shaped Bacteria of NCERT Microorganisms-Friends and Foe


Fungi are multi-celled tiny living organisms which cannot be seen by naked eyes. Fungi take their food from decay materials. Mushroom, yeast, rust are some examples of it. Fungi are responsible for the infection in mouth and throat, skin itching, ring warm and many other infections.

Penicillium Fungi of NCERT Microorganisms-Friends and Foe                  Bread Mould: Fungi


Protozoa are single-celled tiny living organisms which cannot be seen by naked eyes. Protozoa (Amoeba) live alone. They are responsible for the diseases like malaria, dysentery.

Amoeba: Protozoa of NCERT Microorganisms-Friends and Foe

(iv)Algae –

Algae are multi-celled tiny living organisms. They grow in water or on damp surface. They can perform photosynthesis because of chlorophyll presents in them.

Chlamydomonas: Algae


Viruses are single-celled tiny living organisms. They reproduce only inside the cells of the host organism which may be a bacterium, plant or animal. They are responsible for the diseases like cold, influenza, polio, chicken pox and most cough.

Viruses of NCERT Microorganisms-Friends and Foe

(5) Uses of the microorganism-

(i) Microorganism Lactobacillus, the bacterium is responsible for the formation of curd. It multiplies in the milk and gives curd.

(ii) Bacteria are also helpful in making cheese, pickle and other food items.

(iii) Yeast multiplies very fast and produce carbon- di – oxide which fills the dough of baking items. This process increase the volume of dough which is used in baking food items as bread, cake, pastries etc.

(iv)Microorganisms break the organic waste into harmless and usable substance which is used in fields and clean the environment.

(v) Animal waste which is fermented by microorganisms is used as manure for the plants which increase fertility of soil. Some bacteria and blue green algae are able to fix atmospheric nitrogen and increase the fertility of soil. The microbes which fixe the atmospheric nitrogen are called biological nitrogen fixers. Rhizobium bacteria are involved in the fixation of nitrogen in leguminous plants (Pulses).

(vi) Microorganisms are used for the production of alcohol and vinegar by fermentation of barley, fruit juice, wheat, rice, etc.

(vii) We take antibiotics when fall ill. Bacteria and fungi are the microorganisms which are used in the production of these antibiotics. Streptomycin, tetracycline and penicillin are some examples of antibiotics.

(viii) Vaccine is a substance which produced antigens (antibiotics) when microbes enter in to our body to save our body. Several diseases like cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.

(6)Diseases causing microorganisms-

(i)Diseases causing microorganisms in humans-

(a)Pathogens microorganisms are responsible for the diseases caused by the microorganisms.

(b) Those diseases which can be transferred or communicate by an infected person to a healthy person via air, water, food or with contact are known as communicable diseases.

Eg:- Cold spreads when tiny droplets of infected persons travel at the time of sneezing to healthy person via air.

(c)Carriers are those insects and animals which carry microbes to the healthy persons.

Eg:- Housefly and mosquitoes.

When these carriers sits on garbage or animal excreta pathogens stick to them. When these flies sit on the uncovered food they leave pathogens on food which makes healthy people sick when they eat food.

Anopheles, a female mosquito works as carrier of malaria. It injected the blood of infected person to the healthy person when sucks him.

Female Aedes mosquito acts as a carrier of dengue virus which breeds in water collected anywhere (in coolers, tyres, flower pot etc).

(i) Some common diseases caused by microorganisms in humans-

SN Human Disease Causative Microorganism Mode of transmission Preventive Measures














Chicken Pox




















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












Maintain personal hygiene and food sanitary habits. Consume properly cooked food and boiled drinking water or consume filtered water.

Vaccination should be given at the suitable age.

7. Hepatitis A Virus Water Drink boiled of filtered water.

Vaccination should be given at the suitable age.













Use mosquito net and repellents. Spray insecticides and control breeding of mosquitoes by not allowing water to collect in the surroundings.
10. Food poisoning Virus/Bacteria Oral fecal route Wash food before consumption and cook well

(ii)Diseases causing microorganisms in animals-

SN Animal Diseases Causative Microorganism Mode of transmission
1. Anthrax Bacteria Air
2. Foot and mouth Virus Contact

(iii)Diseases causing microorganisms in plants-

Some common plant diseases caused by microorganisms-

S N Plant Diseases Causative Microorganism Mode of transmission
1. Citrus canker Bacteria Air
2. Rust of wheat Fungi Air, seeds
3. Yellow vein mosaic of bhindi (Okra) Virus Insect

(7)Food preservation –

Food preservation is a process to prevent the growth of microorganisms in the food to store food for long time in usable condition.


S N Method of preservation Preservatives Food preserved Process of preservation
1. Chemical
Sodium benzoate and Sodium metabisulphite Jams, squashes Mix in prepared food.
Common salt Meat and fish,amla, raw mangoes,tamrind etc.
Sugar Jams, squashes and jellies Sugar reduces the moisture of food
Oil and vinegar Pickles, vegetables, fruits etc. Mix with food which does not allow growth of microorganism.
2. Heat and cold treatment  


Heat treatment heat milk Boiling Kills microorganism
Cold treatment cold Milk, food Refrigerate food to slow down the growth of the microorganism


Pasteurisation is a method in which milk is heated to above 700c for 15 to 30 seconds and then immediately cooled down and stored. This method is discovered by Louis Pasteur. We can drink pasteurised milk without boiling.

(iii) Storage food is packed in sealed jars to prevent further attack of microbes.

(8) Nitrogen cycle is a cycle in which atmospheric nitrogen remains more or less same.

Nitrogen can not be taken directly by plants and animals. Plants and animals can use only nitrogen compound. Bacteria and blue green algae of soil convert the atmospheric nitrogen and nitrogen waste of the dead plants and animals into nitrogenous compound. Some other bacteria convert some part of this compound to nitrogen gas which goes back into the atmosphere. Therefore, nitrogen presents in the atmosphere remains same.

Nitrogen cycle is described by a diagram below:-

Nitrogen Cycle of NCERT Chapter Microorganisms-Friends and Foe

Helping Topics

NCERT Solutions Class 8

Worksheet Class 8



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