Micro-organisms and Us

Micro Organism of Class 8

Microorganisms play an important role in our lives. Some of them are beneficial in many ways whereas some others are harmful and cause diseases. Let us study about them in detail.

Useful Micro-organisms (Friends)

Micro-organisms are used for various purposes, such as

  • Preparation of curd, bread, cake, idli, dosa, cheese.
  • In industry – to produce alcohol, wine and vinegar (acetic acid).
  • In medicines – to produce antibiotics and vaccines.
  • In agriculture – to increase soil fertility.
  • Cleaning the environment.
  • As food.

Preparation of food items

  • Curd: Milk in turned into curd by a bacteria lactobacillus. When a little curd (called starter) containing the bacteria is added to milk, it multiplies and converts the milk into curd. Curd also contains several other micro-organisms.

Curd is an important ingredient of rava idlis and bhaturas. Why?

  • Bread and bakery items:  Yeast plays an important role in preparation of food. This can be better understood as follows:

Take some flour, add some sugar and mix with warm water. Add small amount of yeast powder and knead to make soft dough. When you keep the dough for two hours, you find it rising. Yeast reproduces rapidly and produces carbon-dioxide during respiration (also calledfermentation). Bubbles of the gas fill the dough and increase its volume. This is the basis of the use of yeast in the baking industry for making breads, pastries and cakes.

Other food items:

  • Cheese in produced by the action of bacteria.
  • Idli, dosa, paste (a mixture of dal and rice) in allowed to stand for few hours. As fermentation occurs naturally the mixture rises and becomes sour.
  • During curd formation milk sugar (lactose) is converted into lactic acid by the l actobacillus. These bacteria in curd aid in digestion and also inhibit the growth of disease-causing bacteria.

Industrial use of microbes

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 on natural sugars present in grains like barley, wheat, rice and crushed fruit juices, etc.

You can prepare alcohol by a small activity. Take a 500 mL beaker filled upto ¾ with water. Dissolve 2-3 teaspoons of sugar in it. Add half a spoon of yeast powder to the sugar solution. Keep it covered in a warm place for 4-5 hours. Now smell the solution. Could you get a smell?

It is alcohol, as sugar has been converted into alcohol by yeast. This process of conversion of sugar into alcohol in known as fermentation.

Medicinal use of Micro-organisms:

The various uses of microbes in medicines are as follows:

  • Antibiotics: When sometimes you fall ill, the doctor prescribes some antibiotic tablets, capsule or injections. Micro-organisms are the source of these antibiotics. Antibiotics are the medicines which kill or stop the growth of the disease-causing micro-organisms. These days a number of antibiotics like penicillin, streptomycin, tetracycline, erythromycin, etc. are being produced from bacteria and fungi. The antibiotics are manufactured by growing specific micro-organisms and are used to cure a variety of diseases. Antibiotic mixed with the feed of livestock and poultry can check microbial infection in animals. They are also used to control many plant diseases.
  • The name ‘antibiotics’ comes from two Greek words meaning ‘against life’. It was prepared by Sir Alexander Fleming in 1929 from a mould penicillium and hence he called that particular antibiotic as penicillin. Soon after, different antibiotics were isolated from either bacteria or fungi.

Precautions to be taken while using antibiotics

  • Antibiotics should be taken only on the advice of a qualified doctor.
  • One must complete the course of antibiotics prescribed by the doctor.
  • Antibiotics taken unnecessarily may kill the beneficial bacteria in the body.


  • In your childhood, you must have been given injections or drops to protect your body from several infectious diseases. These injections or drops are called vaccines and micro-organisms are used to make them. Disease such as cholera, typhoid, tuberculosis, hepatitis, chicken pox, measles, polio and small pox can be prevented by vaccination.
  • A vaccine is a suspension of disease-producing microbes which is killed or weakened so that the suspension will not cause disease. When these vaccines are injected into the body of a patient, the body produces antibodies to fight them. The antibodies remain in the body and protect it from any future attack of the disease germs.
  • It is very essential to protect all children against the infectious diseases. Necessary vaccines, for example DPT (for protection against diphtheria, whooping cough and tetanus); BCG (for protection against tuberculosis); and OPV (oral polio vaccine) are available in the nearby hospitals. OPV is given to children orally (through the mouth) at the age of 1.5, 2.5 and 3.5 months.
  • A world wide campaign against small pox has finally led to its eradication from most parts of the world.
  • These days vaccines are made on a large scale from micro-organisms to protect humans and other animals from several diseases.

In Agriculture-Increasing Soil Fertility

All plants and animals need nitrogen, the gas that makes up 78% of the air. Some bacteria and blue green algae are able to fix nitrogen (convert the elemental nitrogen to compounds of nitrogen) from the atmosphere. These microbes commonly called biological nitrogen fixers enrich the soil with nitrogen compounds and increase its fertility.

The bacterium Rhizobium, live in the root nodules (swollen structures in the root) of plants like beans, peas, etc. fix nitrogen.

Cleaing the environment:

Microbes, like bacteria and fungi, bring about the decay of dead plants and animals. In particular the waste of plants, vegetables and fruits get decomposed and is converted to manure.

Take two pots and fill each pot half with soil. Mark them A and B. Put plant waste in pot A and things like polythene bags, empty glass bottles and broken plastic toys in pot B. Put the pots aside. Observe them after 3–4 weeks. You will find that plant waste in pot A, has been decomposed into manure by the action of microbes. The microbes could not ‘act’ on the things of pot B and decompose them.

Micro-organisms decompose dead organic waste of plants and animals into simple substances. Without microbes the earth would soon be covered with foul smell from dead organisms. The simple substances thus formed by decomposition are then restored to the soil, water and air in the form that can be used by the plants and animals again.

Microbes thus help in cleaning the environment and recycling of nutrients.

As Food: Microbes are even used as food.

  • Some fungi, such as mushrooms are eaten raw or cooked.
  • Some sea weeds are used as food in China and Japan.
  • Some substances obtained from algae are used to thicken food, such as ice creams and jellies.

Harmful microorganisms (Foe)

Many microorganisms are harmful and are disease causing. Some spoil food. Some microbes even spoil, clothing and leather.

Disease causing microorganisms

Microbes can cause disease in plants, animals, and human beings. These disease causing microorganisms are called pathogens or germs

  • In human being: Pathogens enter our body through the air we breathe, the water we drink, the food we eat or by direct contact. Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases
  • Spread through air: When a person suffering fromcommon cold sneezes, fine droplets carrying thousands of viruses are spread in the air. The virus may enter the body of a healthy person while breathing.
  • Spread through carriers: Some insects, and animals like houseflies, mosquitoes, cockroaches and rats act as carriers of disease causing microbes. These carriers carry germs of diseases from the infected person’s excreta or the garbage to the food and drinking water of a healthy person. When a healthy person eats contaminated food, he may fall sick and in this manner diseases get spread from one infected person to healthy person through carriers. Another example of a carrier is the female Anopheles mosquito which carries the parasite of malaria.

Human Disease

Causative Microorganism

Mode of Transmission

Preventive measures (General)



Chicken Pox








Air / Contact

Air / Water

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



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.


All mosquitoes breed in water. Hence, we should not allow any stagnant water collecting for e.g. in coolers, tyres, flower pot, etc. Spraying of kerosene should be done on the surface of water to form a thin film which blocks oxygen and kills larvae of mosquitoes.

  • In animals: Microorganisms not only cause diseases in human and plants but also in other animals as follows:




Foot and mouth diseases (in cattle)


Anthrax in cattle


Sleeping sickness (in cattle, pigs and horses).

  • In plants: Several microorganisms cause diseases in plants like wheat, rice, orange, sugarcane, etc.

Plant Disease


Mode of Transmission


Citrus canker



citrus canker

Rust of wheat


Air seeds

rust of wheat

Yellow vein mosaic of Bhindi (Okra)




Tobacco mosaic


Sap(by direct transfer of sap by contact of a wounded plant with healthy one)

tobacco mosaic

These diseases reduce the yield of crops. They can be controlled by the use of certain insecticide, pesticide and herbicides which kill the microbes.

Food spoiling microorganisms

Bacteria and fungi are responsible for spoilage of food. The souring of milk, putrefaction of meat, rotting of potatoes, vegetables and fruits are due to the action of bacteria. Microorganisms that grow on our food sometimes produce toxic substances. Consuming such food can cause food poison leading to serious illness and sometimes even death. So, it is very important that we preserve food to prevent it from being spoilt. NCERT solutions for class 8 Science prepared by Physics Wallah will help you to solve your NCERT text book exercise. 

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