During the rainy season one may have observed that moist bread gets spoilt and its surface gets covered with greyish white patches. When these  patches are observed through a magnifying glass we see tiny, black rounded structures. These structures are called Microbes or Microorganisms.

  • Microorganisms are tiny microscopic organisms that are not visible to the naked eye.
  • They may be unicellular or multicellular.
  • They can live in all kinds of environment, ranging from ice cold climate to hot springs and deserts to marshy lands.
  • Microorganisms are found in air, water and in the bodies of plants and animals.

Microorganisms are classified into four major groups:

Fig: Algea

Fig: Protozoa

Fig: Fungi

Viruses are also microscopic but are different from other microorganisms.

  • They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.
  • Common ailments like cold, influenza (flu) and most coughs are caused by viruses.
  • Serious diseases like polio and chicken pox are also caused by viruses.

Fig: Viruses

Microorganisms Uses

Beneficial Microorganisms

They are used in:

  • The preparation of curd (Lactobacillus), bread and cake.
  • Some microorganisms decompose the organic waste and dead plants and animals into simple substances and clean up the environment.
  • The preparation of medicines- penicillin, antibiotics (Streptomycin, tetracycline and erythromycin ).
  • The manufacture of vaccines (substance which is used to stimulate the production of antibodies, thereby providing immunity against certain diseases.)
  • In agriculture they are used to increase soil fertility by fixing nitrogen (Cyanobacteria-blus green algae).

Harmful Microorganisms

Disease causing Microorganisms in Humans

Some insects and animals which act as carriers of disease- causing microbes.

      Ex: Housefly – carrier of harmful pathogens

Anophelesmosquito -carries the parasite of malaria (Plasmodium).

Female Aedesmosquito – carrier of dengue virus.

Disease causing Microorganisms in PLANTS


Disease causing Microorganisms in Animals


anthrax – caused by a bacterium.

Foot and mouth disease- caused by a virus.


Food poisoning

  • When food contaminated with pathogens or toxins are consumed, it causes food poisoning.
  • The most common symptom is a pain in the stomach.
  • In severe cases, food poisoning can also cause death.


Food preservation


Chemical methods

  • Chemical preservatives are used in food preservations by major food industries as they are harmless to humans.
  • Salts and edible oils -common chemicals generally used to check the growth of microorganisms.
  • Sodium meta-bisulphate and sodium benzoate are commonly used chemical preservatives.


Preservation by common salt

  • Common salt (sodium chloride) is used as a preserve meat and fish, amla, raw mangoes, tamarind, etc.


Preservation by Sugar

  • Sugar is used for the preservation of jams, jellies and squashes.
  • Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food.


Preservation by oil and Vinegar

  • Many food preparations like pickles are preserved by adding either oil or vinegar to them.
  • Bacteria cannot grow in such medium.
  • Vegetables, fruits, fish and meat can also be preserved.



  • Pasteurization is a process of superheating and cooling the beverages.
  • This prevents the growth of microbes.


Storage and Packing

  • Dry fruits and many vegetables are preserved in airtight/air sealed containers.
  • The absence of air prevents the growth of bacteria or fungi.

Nitrogen cycle

Our atmosphere has 78% nitrogen gas.

Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins.


  • The atmospheric nitrogen cannot be taken directly by plants and animals.
  • Certain bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert it into compounds of nitrogen.
  • Once nitrogen is converted into these usable compounds, it can be utilised by plants from the soil through their root system.
  • Nitrogen is then used for the synthesis of plant proteins and other compounds.
  • Animals feeding on plants get these proteins and other nitrogen compounds
  • When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again.
  • Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere.
  • This ensures that the percentage of nitrogen in the atmosphere remains more or less constant.