CBSE Class 8 Science Chapter 2 Revision Notes

Chapter 2: Microorganisms-Friend And Foe Revision Notes

  • Microorganisms are too small to be seen by our eyes.

  • They cannot be seen without a microscope, except for fungus that grows on bread.

  • Microorganisms are divided into four categories: Bacteria, fungus, protozoa, and certain algae are among these groupings.

  • Viruses are tiny as well. They, on the other hand, only multiply within the cells of the host organism, which can be bacteria, a plant, or an animal.


cold, influenza (flu), polio, and chicken pox Viruses
dysentery and malaria protozoa(protozoans)
typhoid and tuberculosis (TB) bacterial diseases

Microorganisms can thrive in a variety of environments, including arctic temperatures, hot springs, deserts, and marshy regions.

Microorganisms are extremely crucial in our life: Both beneficial and harmful


  • Making curd (Lactobacillus), bread (yeast), and cake
  • Decomposition of organic wastes into simple substances that can be used again.
  • Improve soil fertility by fixing nitrogen: biological nitrogen fixers such as certain bacteria and blue-green algae
  • Production of alcohol, wine and acetic acid (vinegar)
  • Fermentation is the term for the process of turning sugar into alcohol carried out by yeast.


  • Production of antibiotics such as Streptomycin, tetracycline, and erythromycin made from fungi and bacteria.
  • To produce vaccines against polio, cholera, tuberculosis, smallpox, and hepatitis by helping the body produce antibodies.


  • Humans, plants, and animals are all harmed by some of the microbes. Pathogens are microbes that cause disease.
  • Communicable illnesses are microbial infections that can transmit from an infected person to a healthy person by the air, water, food, or personal contact.


  • Insects and animals that help carry disease-causing microbes are called carriers (Anopheles mosquito for malaria).
  • Anthrax (bacteria) and foot and mouth (virus) are disease-causing microorganisms in animals.
  • Citrus canker (bacteria), and rust of wheat (fungi) are common plant diseases caused by microorganisms
  • Food poisoning could be caused by eating food that has been spoiled by bacteria due to the release of toxic chemicals.


  1. Chemical method: Preservatives such as salts, edible oils, sodium benzoate and sodium metabisulphite are used in the jams and squashes.

  2. Common salt: salting to preserve meat, fish, amla, and raw mangoes.


  1. Sugar: Lowers moisture content in jams, jellies, and squashes.

  2. Oil and vinegar: Vegetables, fruits, fish, pickles, and meat.

  3. Pasteurisation: heat and cold treatments are seen in pasteurised milk (70 degrees C for 15 to 30 seconds and then suddenly chilled and stored).

  4. Storage and packing: dry fruits and vegetables are sealed in airtight packages to prevent microbial contamination.


Rhizobium is a symbiotic bacteria that resides in the root nodules of leguminous plants like beans and peas and helps fix nitrogen.

nitrogen cycleSource

Source: Chapter-2.pmd (


Similar Posts