Microorganisms Friend and Foe Notes || Class 8 Chapter 2 Science ||

Chapter – 2, Science
1.      Microorganisms/microbes are very small which cannot be seen with the help of eyes.
2.      We use microscope or magnifying glass to see them.
3.      E.g. fungus that grows on bread

Classification of Microorganisms
1.      It is classified into four major groups
2.      Bacteria, fungi, protozoa and algae
3.      Viruses are other kind of microorganisms.
4.      They are explained below –
1.      Bacteria are very small, single celled micro-organisms which have cell walls but do not have an organised nucleus & other structure
2.      These are found everywhere like in air, soil, water, every surface around us, on our bodies or inside our body.
3.      Two common examples are Lactobacillus & Rhizobium 
4.      Some are useful & help in making foods (like curd (दही)) & some can cause disease like cholera (हैजा), typhoid (मियादी बुखार), tuberculosis (TB) & food Poisoning (पेट दर्द).

1.      It is a single celled micro-organism.
2.      It is mainly found in ponds (तालाब), lakes, dirty water drains & rivers.
3.      Some common examples of protozoa are Amoeba, paramecium, Plasmodium.
4.      Disease produced by protozoa like dysentery (पेचिश  or दस्त) & malaria.

1.      It is a large group of simple, plant like organisms.
2.      They contain chlorophyll & produce food by photosynthesis just like plants.
3.      Some of the examples are blue-green algae, diatoms & seaweeds (समुद्री शैवाल).
4.      Some are unicellular & most of the algae are multicellular.

1.      Fungi are a large group of organisms which do not have chlorophyll & photosynthesis.
2.      Some examples are Yeast, Moulds (such as bread mould, penicillium), Mushrooms etc.
3.      Some fungi look like plants but they cannot make their own food like the plants.
4.      They food on dead things like remains of dead plants & animals
5.      Fungi need moist & warm conditions to grow.

1.      Viruses are the smallest micro-organisms which can develop only inside the cells of the host organisms (may be animals, plants)
2.      Viruses can reproduce & multiply only inside the cells of other organisms (such as animal cell, plant cell or organisms cell)
3.      Thus, as long as viruses are outside the living cells, they behave as non-living things.
4.      Diseases produced by them are common cold, influenza (ज़ुकाम) (flu), measles (खसरा), polio, chickenpox & smallpox etc

Where do Microorganisms Live?
1.      Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as many algae and fungi.
2.      They can live from cold to hot climate.
3.      They are found everywhere in all types of habitats like air, soil, water bodies, inside the bodies of animals including humans or depend on other organisms.
Microorganisms and Us
Some microorganisms are beneficial & some others are harmful and cause diseases.
Friendly Microorganisms
1.      Microorganisms are used for various purposes.
2.    They are used in the preparation of curd, bread, cake & alcohol.
Making of Curd
1.      Curd contains several microorganisms.
2.      Of these, the Lactobacillus bacterium is multiplies itself & converts milk into curd.
3.      Bacteria are also involved in the making of cheese (पनीर), pickles (अचार) and many other food items.
Commercial Use of Microorganisms
1.      Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar [सिरका]).
2.      Yeast (खमीर) is used for commercial production of alcohol and wine.
3.      Sugar has been converted into alcohol by the use of yeast.
4.      This process of conversion of sugar into alcohol is known as fermentation.
5.    Note – Louis Pasteur discovered fermentation in 1857.
Medicinal Use of Microorganisms
1.      Whenever we fall ill the doctor may give us some antibiotic tablets or injections such as of penicillin.
2.      These medicines kill or stop the growth of the disease-causing microorganisms.
3.      Such medicines are called antibiotics.
4.      Many antibiotics are being produced from bacteria and fungi.
5.      E.g. Streptomycin, tetracycline and erythromycin
Important to Note
1.      In 1929, Alexander Fleming was working on a culture of disease causing bacteria.
2.      Suddenly he found the spores (जीवाणु जो बढ़कर एक नया प्राणी हो जाता है) of a little green mould in one of his culture plates.
3.      He observed that the presence of mould prevented the growth of bacteria.
4.      In fact, it also killed many of these bacteria. From this the mould penicillin was prepared.
Preventions in the Use of Antibiotics
1.      Antibiotics should be taken only on the advice of a qualified doctor.
2.      A person must complete the full course of antibiotics prescribed (नुसखा लिखना) by the doctor.
3.      If you take antibiotics when not needed or in wrong doses, it may make the drug less effective when you might need it in future.
4.      Also antibiotics taken unnecessarily may kill the beneficial bacteria in the body.
5.      Antibiotics are not effective against cold and flu as these are caused by viruses.
1.      A vaccine is a special kind of medicine which provides immunity (मुक्ति) or protection against a particular disease.
2.      Vaccination is a process of giving a vaccine orally (by mouth) or by injection which provides protection against a particular disease like polio, Tetanus, Measles, Rabies cholera, tuberculosis (TB), smallpox and hepatitis.
3.    Edward Jenner discovered the vaccine for smallpox in 1798.
Increasing Soil Fertility
1.      Some of the micro-organisms present in the soil can fix nitrogen gas from the atmosphere to form nitrogen compounds.
2.      These nitrogen compounds mix with the soil & increase the fertility (उपजाऊ) of the soil.
3.      E.g. some bacteria like Rhizobium & blue green algae are able to fix nitrogen gas from atmosphere to enrich the soil with nitrogen compounds & increase its fertility.
4.      These bacteria & blue green algae are commonly called biological nitrogen fixers.

Cleaning the Environment
1.      Some micro-organisms (Like some bacteria & fungi) decompose (सड़ जाना) the harmful and smelly dead remains of plants and animals, and animal wastes (like faeces [मल], dung [गोबर], and urine) into harmless materials which mix up with soil.
2.      These harmless materials or simpler substances contain plant nutrients which are used by the new plants for their growth.
3.      In this way, it cleans the environment.
Harmful Microorganisms
1.       Microorganisms are harmful (हानिकारक) & can cause diseases in human beings, plants and animals.
2.       Such disease-causing microorganisms are called pathogens.
3.       Pathogens can be bacteria, viruses, protozoa, or fungi etc.
4.       Microorganisms cause disease such as cholera (हैजा), common cold, chicken pox, tuberculosis (TB), tetanus, food poisoning etc.
5.       Some microorganisms spoil (खराब करना) food, clothing and leather.
Disease causing Microorganisms in Humans
1.       Pathogens enter our body through the air we breathe, the water we drink or the food we eat.
2.       They can also get transmitted by direct contact with an infected person or carried by an animal.
3.       Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases.
4.       Examples cholera, common cold, chicken pox and tuberculosis.
5.       When a person suffering from common cold sneezes (छींकना), fine droplets of moisture carrying thousands of viruses are spread in the air.
6.       The virus may enter the body of a healthy person while breathing and cause infection.
Prevention of communicable diseases
1.       A person suffering from common cold should always cover his mouth and nose with a handkerchief while sneezing.
2.       We should keep our food covered to protect it from getting infected by flies (मक्खी).
3.       We should protect ourselves from mosquito bites by using mosquito nets over our beds.
4.       We should avoid physical contact with an infected person & not use his towel, clothes.
Carriers of Disease-causing Micro-Organisms
1.       There are some insects and animals which act as carriers of disease-causing microbes.
2.       The two most common insects – housefly & mosquito
Role of Housefly in spreading diseases
1.       The houseflies feed on garbage (कूडा), animal excreta (मल-मूत्र), dead organic matter and uncovered human food.
2.       The body and legs of housefly have a lot of fine hair.
3.       When the flies sit on a garbage heap (ढेर) or refuse (kachra) area, then millions of Disease-causing Micro-Organisms (like bacteria) transfer to other places like our uncovered food.
4.       Then, these bacteria or germs enter into our body as we eat this uncovered food.
5.       Some of the dangerous diseases spread by houseflies like cholera, Tuberculosis (TB), Typhoid & Diarrhea.

Prevention of Diseases spread by houseflies
1.       We should not leave household garbage (कूडा) here & there and it should be put in the garbage bins (कूड़ादान) which is properly covered.
2.       Food always is kept covered.
3.       Avoid eating uncovered food.
4.       The flies should be killed by using insecticide (कीटनाशक) spray.
5.       Some diseases spread by houseflies can be prevented by vaccinations.
 Role of Mosquitoes in spreading diseases
1.       Mosquito spread diseases from one person to another.
2.       Note here that housefly carries the diseases-causing microbes (जीवाणु) on the hair (outside its body) but the mosquito carries microbes inside its body.
3.       Mosquito breed (नस्ल बढाना) in stagnant (गतिहीन) water of ponds, dirty drains (नाली), pools, shallow (कम गहरा) lakes etc.
4.       The most common disease spread is ‘malaria’
5.       Actually, it is the female Anopheles mosquito, which carries the parasite (परजीवी) of malaria
6.       The malarial parasite (called plasmodium) causes malaria diseases.
7.       Another disease spread by mosquito is ‘dengue’ & is caused by virus.
8.       The female Aedes mosquito acts as a carrier of dengue virus & spread the dengue disease from person to person.

Some Common Human Diseases caused by Microorganisms

Human Disease
Causative Microorganism
Mode of Transmission
Preventive Measures (General)
Measles (खसरा)
Keep the patient in complete isolation (एकांत). 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.
Drink boiled drinking water. Vaccination.
Use mosquito net and repellents (दूर भगाने वाला). Spray insecticides and control breeding of mosquitoes by not allowing water to collect in the surroundings.

Disease causing Microorganisms in Animals
1.       Anthrax is a dangerous human and cattle disease caused by a bacterium.
2.       Foot and mouth disease of cattle is caused by a virus.
Disease causing Microorganisms in Plants
1.       Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and others.
2.       The diseases reduce the yield of crops.
3.       They can be controlled by the use of certain chemicals which kill the microbes.

Plant Diseases
Micro- organism
Mode of Transmission
Citrus canker
Rust of wheat
Air seed
Yellow vein mosaic of bhindi (Okra)

Food Poisoning
1.       If the food is not covered properly, stored properly or preserved properly, then it gets spoiled (खराब हो जाना) by the micro-organisms (like bacteria & fungi) on it.
2.       Micro-organisms that grow on our food sometimes produce toxic (जहरीला) substance.
3.       The diseases caused due to the presence of a large number of micro-organisms in the food are called food poisoning.
4.       Major symptoms – Vomiting, diarrhea (loose motion), pain in abdomen (पेट), headache, fever
Preservation of Food
1.      The process in which the food materials are given a suitable physical or chemical treatment to prevent (रोकना) their spoilage (खराब होना) is called food preservation.
2.      There are different methods to preserve the food
Chemical Method
1.       Salts and edible oils are the common chemicals generally used to check the growth of microorganisms.
2.       Therefore, they are called preservatives.
3.       We add salt preservatives to pickles (अचार) to prevent the attack of microbes.
4.       Sodium benzoate and sodium metabisulphite are common preservatives.
5.      These are also used in jams and squashes (शरबत) to check their spoilage.
Preservation by Common Salt
1.       Common salt has been used to preserve (सुरक्षित रखना) meat and fish for ages.
2.       Meat and fish are covered with dry salt to check the growth of bacteria.
3.      Salting is also used to preserve amla, raw (कच्चा) mangoes, tamarind (इमली) etc.
Preservation by Sugar
1.       Jams, jellies and squashes are preserved (सुरक्षित रखना) by sugar.
2.      Sugar reduces the moisture content which prevents (रोकना) the growth of bacteria which spoil food.
Preservation by Oil and Vinegar
1.       Use of oil and vinegar (सिरका) prevents spoilage of pickles because bacteria cannot live in such an environment.
2.       Vegetables, fruits, fish and meat are often preserved by this method.
Heat and Cold Treatments
1.       Boiling kills many microorganisms.
2.       Similarly, we keep our food in the refrigerator.
3.       Low temperature prevents the growth of microbes.
1.       Pasteurised milk can be consumed without boiling as it is free from harmful microbes.
2.       The milk is heated to about 700C for 15 to 30 seconds and then suddenly chilled (ठंडा) and stored.
3.       By doing so, it prevents the growth of microbes.
4.       This process was discovered by Louis Pasteur & called pasteurisation.
Storage and Packing
Dry fruits and even vegetables are sold in sealed air tight packets to prevent the attack of microbes.
Nitrogen Fixation
1.      The process of converting nitrogen gas of atmosphere into compounds of nitrogen (which is used by plants) is called nitrogen fixation.

2.      The nitrogen gas of atmosphere can be fixed (converted into nitrogen compound) – by certain nitrogen-fixing bacteria present in the soil, by Rhizobium bacteria present in the roots, by blue-green algae


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