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

Microorganisms: Friend and Foe



  • Microorganisms are very tiny and cannot be seen without the help of a microscope.
  • They can survive in various environments, such as cold climates, hot springs, deserts, and marshy lands.
  • Microorganisms are present in the air, water, and the bodies of plants and animals.
  • They can be either single-celled (unicellular) or made up of multiple cells (multicellular).
  • Microorganisms are classified into four major groups: bacteria, fungi, protozoa, and some algae.
  • Viruses, although different from other microorganisms, are also referred to as microbes.
  • Viruses reproduce only inside the cells of their host organisms, which can be bacteria, plants, or animals.
  • Bacteria are single-celled microorganisms that can be found in various environments. They have a wide range of shapes and sizes.
  • These microorganisms can cause a wide range of diseases in humans, animals, and plants.

Friendly Microorganisms

  • Making Curd and Bread:

    • Microorganisms like bacteria (Lactobacillus) are used to convert milk into curd through fermentation.
    • Yeast, a type of fungus, is used in bread-making to ferment the dough, resulting in the production of carbon dioxide, which causes the bread to rise.

    Commercial Use of Microorganisms:

    • Microorganisms are used in various industries for commercial purposes, such as food and beverage production, pharmaceuticals, and agriculture.
    • They are used in the production of products like cheese, yogurt, alcohol, and enzymes used in manufacturing processes.

    Medicinal Use of Microorganisms:

    • Some microorganisms are used in the production of medicines, such as antibiotics, vaccines, and insulin.
    • Antibiotics are used to treat bacterial infections, while vaccines help prevent diseases by stimulating the immune system.
    • Microorganisms like bacteria and yeast are used to produce insulin for diabetic patients.


    • Vaccines are substances made from weakened or killed microorganisms or their components.
    • They are administered to stimulate the immune system and provide immunity against specific diseases, preventing infection and promoting overall health.

    Increasing Soil Fertility:

    • Certain microorganisms, like nitrogen-fixing bacteria, help increase soil fertility by converting atmospheric nitrogen into a usable form for plants.
    • These microorganisms form symbiotic relationships with plants, providing them with essential nutrients for growth.

    Cleaning the Environment:

    • Some microorganisms are capable of degrading or breaking down harmful pollutants in the environment, such as oil spills or industrial waste.
    • They play a crucial role in bioremediation, helping to clean up contaminated sites and restore environmental balance.

    Recycling of Nutrients:

    • Microorganisms are involved in the decomposition process, breaking down organic matter and recycling nutrients back into the ecosystem.
    • They help in the cycling of elements like carbon, nitrogen, and phosphorus, making them available for use by other organisms.

Harmful Microorganisms:

Harmful microorganisms are organisms that can cause diseases and harm to humans, animals, and plants. They include bacteria, viruses, fungi, and parasites that can invade and multiply in our bodies, leading to illness and infections. Examples of harmful microorganisms are Salmonella, E. coli, Influenza virus, Candida fungus, and Giardia parasite.

Disease causing Microorganisms in Humans

Communicable Disease:

  • A communicable disease is an illness that can be spread from one person to another or from animals to humans.
  • These diseases can be transmitted through various routes such as direct contact, airborne droplets, contaminated food and water, or insect bites.
  • Examples of communicable diseases include the common cold, flu, tuberculosis, sexually transmitted infections (STIs), and COVID-19.

Common Diseases Spread by Mosquitoes:

    • Mosquitoes are known to transmit several diseases when they bite and feed on humans.
    • Malaria is a mosquito-borne disease caused by a parasite and characterized by fever, chills, and flu-like symptoms.
    • Dengue is another viral disease transmitted by mosquitoes and causes high fever, severe headache, joint and muscle pain.
  • Foot and mouth disease of cattle is caused by a virus. Anthrax, a bacterial disease is very fatal for human and cattle.

Food Poisoning:

  • Food poisoning occurs when contaminated food is consumed, leading to illness and symptoms such as nausea, vomiting, stomach pain, diarrhea, and fever.
  • Common causes of food poisoning include bacteria (e.g., Salmonella, E. coli), viruses, parasites, and toxins produced by microorganisms.

These are just a few examples of diseases, their modes of transmission, and some preventive measures. It’s important to follow good hygiene practices and seek medical advice for proper prevention and treatment of different diseases.

Some Common Human Diseases Caused by Microorganisms
Human Disease Causative Microorganism Mode of Transmission Preventive Measures
Tuberculosis Bacteria (Mycobacterium tuberculosis) Air Keep the patient in complete isolation, vaccination
Measles Virus Air Vaccination
Chicken Pox Virus Air/Contact Maintain personal hygiene, vaccination
Polio Virus Air/Water Vaccination, use boiled drinking water
Cholera Bacteria (Vibrio cholerae) Water/Food Consume properly cooked food, drink boiled drinking water
Typhoid Bacteria (Salmonella typhi) Water Vaccination, maintain personal hygiene
Hepatitis A Virus Water Vaccination, drink boiled drinking water
Malaria Protozoa (Plasmodium) Mosquito Use mosquito nets and repellents, control mosquito breeding
Some of the Common Plant Diseases
Plant Disease Microorganism Mode of Transmission
Citrus Canker Bacteria Air
Rust of Wheat Fungi Insects and Seeds
Yellow Vein Mosaic of Bhindi Virus Insect

Food Preservation:

Food Preservation

  • Here are the common methods of preserving food in our homes. :
    1. Chemical Preservatives:

      • Salts and edible oils, such as sodium benzoate and sodium metabisulphite, are commonly used to prevent the growth of microorganisms in food.
      • These chemicals are called preservatives and are added to pickles, jams, squashes, etc., to inhibit the spoilage caused by microbes.
      • Salt or acid preservatives are used in pickles, while sodium benzoate and sodium metabisulphite are used in jams and squashes.
    2. Preservation by Common Salt:

      • Common salt has been used for ages to preserve meat, fish, amla, raw mangoes, tamarind, etc.
      • Meat and fish are covered with dry salt to prevent the growth of bacteria and preserve them for longer periods.
    3. Preservation by Sugar:

      • Jams, jellies, and squashes are preserved using sugar.
      • Sugar reduces the moisture content, inhibiting the growth of bacteria that spoil the food.
    4. Preservation by Oil and Vinegar:

      • Oil and vinegar are used to preserve pickles because bacteria cannot survive in such an environment.
      • Vegetables, fruits, fish, and meat can be preserved using this method.
    5. Heat and Cold Treatments:

      • Boiling is a common method to kill many microorganisms. It is often done before storing or using milk.
      • Low temperatures, such as refrigeration, inhibit the growth of microbes, which is why we store food in the refrigerator.
      • Pasteurization is another heat treatment method where milk is heated to about 70°C for a short time, rapidly chilled, and stored. This process kills harmful microbes and extends the shelf life of milk.
    6. Storage and Packing:

      • Sealed, airtight packaging is used to prevent the attack of microbes on dry fruits, vegetables, and other food items.
      • Packaging food in sealed packets helps maintain its freshness and quality by keeping out bacteria and other contaminants.

    These methods, such as using chemicals, heat treatments, and proper storage, help prevent spoilage and extend the shelf life of various food items.

Nitrogen Fixation:

  • Nitrogen fixation is the process by which atmospheric nitrogen is converted into a form that plants and other organisms can use.
  • Nitrogen fixation is essential for the growth and development of plants, as nitrogen is a crucial component of proteins, nucleic acids, and other vital molecules.
  • Some plants, like legumes (e.g., peas, beans, and clover), have a mutualistic relationship with nitrogen-fixing bacteria, where the bacteria live in their root nodules and provide them with a source of fixed nitrogen.

Nitrogen Cycle:

  • Here’s the nitrogen cycle explained in points:

    1. Nitrogen gas (N2) makes up about 78% of the Earth’s atmosphere.
    2. Nitrogen is an essential component of proteins, chlorophyll, nucleic acids, and vitamins in living organisms.
    3. Atmospheric nitrogen cannot be directly used by plants and animals.
    4. Certain bacteria and blue-green algae present in the soil perform nitrogen fixation, converting atmospheric nitrogen into nitrogen compounds.
    5. Nitrogen-fixing bacteria and blue-green algae convert atmospheric nitrogen into ammonia (NH3) or nitrate (NO3-).
    6. Plants can absorb and utilize these nitrogen compounds from the soil through their root systems.
    7. The absorbed nitrogen is then used by plants to synthesize proteins and other compounds.
    8. Animals obtain these proteins and nitrogen compounds by consuming plants.
    9. When plants and animals die, nitrogenous wastes are broken down by bacteria and fungi in the soil.
    10. The nitrogenous compounds are converted back into ammonia or nitrate, which can be used by plants again.
    11. Some bacteria convert a portion of the nitrogenous compounds into nitrogen gas (N2), which is released back into the atmosphere.
    12. This completes the nitrogen cycle, where nitrogen moves between the atmosphere, soil, plants, and animals, maintaining a relatively constant percentage of nitrogen in the atmosphere.

    The nitrogen cycle ensures the availability of nitrogen for living organisms and plays a crucial role in the functioning of ecosystems.

1 thought on “Microorganisms: Friend and Foe | Class 8 Notes | Science Chapter 2”

Leave a Comment

Verified by MonsterInsights