Microorganisms – Friend and Foe (Class 8 Science, NCERT)

By / September 5, 2025

πŸ“˜ Microorganisms – Friend and Foe (Class 8 Science, NCERT)

πŸ”Ή 1. Introduction to Microorganisms

  • Microorganisms (microbes) are tiny living organisms that cannot be seen with the naked eye.
  • They are visible only under a microscope.
  • Size: They may be unicellular (made of a single cell, e.g., bacteria) or multicellular (made of many cells, e.g., some fungi).
  • Microbes are present everywhere – in air, water, soil, inside the human body, in animals, and even in extreme environments like hot springs, glaciers, deserts, and marshy areas.
  • Microorganisms are both useful and harmful, which is why the chapter is titled β€œFriend and Foe”.

Types of Microorganisms

  1. Bacteria – single-celled organisms, found everywhere. Example: Lactobacillus, Rhizobium.
  2. Fungi – can be unicellular (yeast) or multicellular (bread mould). Example: Penicillium.
  3. Protozoa – mostly unicellular, cause diseases. Example: Plasmodium, Amoeba.
  4. Algae – photosynthetic organisms. Example: Chlamydomonas, Spirogyra.
  5. Viruses – smallest microbes, can reproduce only inside living cells. Example: HIV, influenza virus.

πŸ”Ή 2. Microorganisms as Friends πŸ‘

Even though many microbes cause diseases, they are also extremely useful to humans.

(a) In the Food Industry

  • Curd formation:
    • Microorganism: Lactobacillus.
    • Function: Converts lactose sugar of milk into lactic acid β†’ milk changes into curd.
    • Benefits: Improves digestion, adds flavour and nutrition.
  • Bread, cake, and pastries:
    • Microorganism: Yeast.
    • Yeast produces carbon dioxide gas during fermentation, which makes the dough rise β†’ soft and fluffy bread.
  • Alcoholic beverages:
    • Microorganism: Yeast (Saccharomyces cerevisiae).
    • Converts sugar into alcohol (process = fermentation).
    • Used in making beer, wine, whisky, rum.

(b) In Medicine

  • Microorganisms produce antibiotics that kill harmful bacteria.
  • Example:
    • Penicillin: Discovered by Alexander Fleming in 1929 from fungus Penicillium notatum.
    • Other antibiotics: Streptomycin, Tetracycline, Erythromycin.
  • Vaccines:
    • Made from dead or weakened microbes.
    • When injected, they trigger the body to produce antibodies, giving immunity.
    • Examples: Polio vaccine, Hepatitis B vaccine, Smallpox vaccine.

(c) In Agriculture

  • Certain bacteria improve soil fertility by fixing nitrogen.
  • Rhizobium bacteria live in root nodules of leguminous plants (peas, beans, gram). They convert nitrogen gas into usable compounds for plants.
  • Blue-Green Algae (Cyanobacteria) like Anabaena also fix nitrogen.
  • These are called biofertilisers.

(d) In Cleaning the Environment

  • Microorganisms act as decomposers.
  • They break down dead plants, animals, and organic waste β†’ recycle nutrients back to the soil.
  • Without microbes, the Earth would be filled with dead bodies and waste.

πŸ”Ή 3. Microorganisms as Foes πŸ‘Ž

Microbes can be harmful as they cause diseases in humans, animals, and plants, and also spoil food.

(a) Diseases in Humans

  • Bacteria:
    • Cholera (Vibrio cholerae)
    • Typhoid (Salmonella typhi)
    • Tuberculosis (Mycobacterium tuberculosis)
  • Viruses:
    • Common Cold
    • Polio
    • Hepatitis B
    • COVID-19 (Coronavirus)
  • Protozoa:
    • Malaria (Plasmodium)
    • Amoebic Dysentery (Entamoeba histolytica)
  • Fungi:
    • Ringworm
    • Skin infections

πŸ‘‰ Carriers of Disease:

  • Some animals/insects spread diseases:
    • Houseflies carry germs of cholera, diarrhoea.
    • Female Anopheles mosquito spreads malaria.
    • Female Aedes mosquito spreads dengue, chikungunya.

(b) Diseases in Animals

  • Anthrax in cattle (bacteria).
  • Foot and Mouth Disease in cattle (virus).

(c) Diseases in Plants

  • Citrus canker (bacteria).
  • Rust of wheat (fungus).
  • Yellow vein mosaic of bhindi (ladyfinger) (virus).
    πŸ‘‰ These reduce crop yield and cause economic loss to farmers.

πŸ”Ή 4. Food Spoilage and Preservation

Food Spoilage

  • Microbes grow on food β†’ change taste, smell, colour.
  • Spoilt food can cause food poisoning.

Food Preservation Methods

  1. Chemical Method – Preservatives like sodium benzoate, sodium metabisulphite.
  2. Common Household Methods:
    • Salt – preserves pickles, fish.
    • Sugar – preserves jams, jellies.
    • Oil & Vinegar – prevent growth of microbes in pickles.
  3. Heat & Cold Treatment:
    • Boiling kills microbes.
    • Refrigeration and deep freezing slow microbial growth.
    • Pasteurisation (by Louis Pasteur): heating milk at 70Β°C then cooling β†’ kills harmful microbes.
  4. Airtight Packing:
    • Chips, dry fruits, milk powder sealed in packets.

πŸ”Ή 5. Nitrogen Cycle 🌍

  • Nitrogen is essential for proteins, vitamins, and DNA.
  • Atmosphere = 78% nitrogen, but plants/animals cannot use it directly.

Steps of Nitrogen Cycle:

  1. Nitrogen Fixation – Lightning and bacteria (Rhizobium, Cyanobacteria) fix atmospheric nitrogen into nitrates.
  2. Nitrification – Soil bacteria convert ammonia into nitrates/nitrites.
  3. Assimilation – Plants absorb nitrates β†’ used to make proteins. Animals eat plants to get proteins.
  4. Decomposition – Dead plants/animals decompose, releasing nitrogen compounds into soil.
  5. Denitrification – Certain bacteria convert nitrates back to nitrogen gas β†’ released into atmosphere.

πŸ‘‰ This cycle maintains balance of nitrogen in nature.

πŸ”Ή 6. Key Scientists

  • Louis Pasteur: Discovered pasteurisation, proved microbes cause fermentation & disease.
  • Alexander Fleming: Discovered Penicillin (first antibiotic).

✨ Extended Notes on Microorganisms

πŸ”Ή Economic Importance of Microorganisms

Microorganisms are deeply connected with our daily life and economy, and NCERT gives multiple examples of their uses.

  1. In Dairy Products πŸ₯›
    • Lactobacillus bacteria convert milk into curd.
    • These bacteria multiply in warm milk and make it thick and sour.
    • Curd improves nutritional quality of milk by adding Vitamin B12.
    • Curd is also used to make buttermilk, paneer, cheese, and yoghurt.
  2. In Alcohol and Bakery 🍞
    • Yeast is used for fermentation.
    • Produces alcohol (beer, wine, industrial alcohol).
    • Produces carbon dioxide that makes dough rise in breads and cakes (baking industry).
  3. In Medicines πŸ’Š
    • Some microorganisms produce antibiotics (life-saving drugs that kill harmful bacteria).
    • Example: Penicillin, discovered by Alexander Fleming, obtained from fungus Penicillium.
    • Other examples: Streptomycin, Tetracycline, Erythromycin.
    • These medicines have saved millions of lives from bacterial infections.
  4. In Vaccine Development πŸ’‰
    • Microorganisms are used to prepare vaccines that protect from future infections.
    • Vaccines available for: Smallpox, Polio, Hepatitis, Chickenpox, COVID-19.
    • When vaccines are injected, our body develops immunity against disease-causing microbes.
  5. In Agriculture 🌱
    • Rhizobium bacteria (present in root nodules of pulses like gram, peas, beans) fix nitrogen from air into the soil.
    • This improves soil fertility naturally.
    • Blue-green algae (Cyanobacteria like Anabaena, Nostoc) also help in nitrogen fixation.
    • Such microorganisms are called biofertilisers.
  6. In Cleaning Environment 🌍
    • Microorganisms decompose dead plants and animals.
    • This process recycles nutrients into the soil, maintaining balance in nature.

πŸ”Ή Harmful Effects of Microorganisms

Even though microorganisms are very useful, some are harmful and cause diseases.

(a) Diseases in Humans

  • Bacteria β†’ Tuberculosis, Cholera, Typhoid.
  • Viruses β†’ Polio, Hepatitis, Influenza, COVID-19.
  • Protozoa β†’ Malaria (Plasmodium), Amoebic dysentery (Entamoeba histolytica).
  • Fungi β†’ Ringworm, skin infections.

πŸ‘‰ Such diseases that spread from one person to another are called Communicable Diseases.

  • Spread by: air, water, food, direct contact, insects (mosquitoes, houseflies).

(b) Diseases in Animals

  • Anthrax (bacteria) β†’ dangerous disease in cattle and humans.
  • Foot and Mouth Disease (virus) β†’ affects cattle.

(c) Diseases in Plants 🌿

  • Citrus canker (bacteria).
  • Rust of wheat (fungus).
  • Yellow vein mosaic of bhindi (virus).
    πŸ‘‰ These reduce yield and cause heavy loss to farmers.

πŸ”Ή Food Spoilage and Preservation

  • Microorganisms grow on food β†’ spoil it (bad smell, colour change, toxic substances).
  • Food Preservation Methods (as per NCERT):
    1. Chemical method – Sodium benzoate, Sodium metabisulphite.
    2. Salt and sugar – high concentration prevents growth of microbes.
    3. Oil and vinegar – pickles.
    4. Heat and cold treatments – boiling, refrigeration, deep freezing.
    5. Pasteurisation – invented by Louis Pasteur; milk heated to 70Β°C for 15–30 sec then cooled.
    6. Airtight packing – prevents entry of microbes (chips, milk powder).

πŸ”Ή Nitrogen Cycle (NCERT Special)

Microorganisms are essential in maintaining the nitrogen cycle in nature:

  1. Nitrogen fixation β†’ by Rhizobium and blue-green algae.
  2. Nitrification β†’ conversion into nitrates.
  3. Assimilation β†’ plants absorb nitrates; animals get nitrogen from plants.
  4. Decomposition β†’ dead organisms return nitrogen to soil.
  5. Denitrification β†’ some bacteria convert nitrates back to nitrogen gas.

πŸ‘‰ This cycle ensures nitrogen balance in the atmosphere.

πŸ“Œ Important Notes (NCERT-Style Summary)

  1. Microorganisms β†’ Bacteria, Fungi, Protozoa, Algae, Viruses.
  2. Friends β†’ food industry, medicines, vaccines, agriculture, cleaning environment.
  3. Foes β†’ diseases in humans, animals, plants; food spoilage.
  4. Preservation methods β†’ salt, sugar, oil, vinegar, chemicals, refrigeration, pasteurisation, airtight packing.
  5. Nitrogen cycle β†’ maintains balance of nitrogen in nature.
  6. Examples to remember:
    • Curd β†’ Lactobacillus.
    • Antibiotic β†’ Penicillin.
    • Biofertiliser β†’ Rhizobium.
    • Malaria β†’ Plasmodium.
    • Amoebic dysentery β†’ Entamoeba histolytica.

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