Biology - Microorganisms: Friend and Foe



  • The living organisms (available around us), which we cannot see with our naked eyes, are known as microorganisms or microbes.

  • Microorganisms are classified into the following four major groups −

    • Bacteria

    • Fungi

    • Protozoa

    • Algae


  • Viruses are also microscopic microorganism.

  • Viruses get reproduced only inside the cells of the host organism, which may be a bacterium, plant, or animal.

  • The common ailments, such as cold, influenza (flu), and coughs are caused by viruses.

  • The serious diseases, such as polio and chicken pox are also caused by viruses.

  • The diseases like dysentery and malaria are caused by protozoans.

  • The diseases like typhoid and tuberculosis (TB) are caused by bacteria.

  • The single celled microorganisms are known as bacteria, algae, and protozoa.

  • The multicellular microorganisms are known as fungi and algae.

  • The microorganism can survive in any type of environment ranging from ice cold to hot desert.

  • Microorganisms are also found in the bodies of animals and human beings.

  • Microorganisms, such as amoeba, can live alone; whereas the fungi and bacteria live in colonies.

  • Some of the microorganisms are beneficial to us in many ways whereas some others are harmful and cause diseases to us.

Friendly Microorganisms

  • Microorganisms are used for various purposes, such as preparation of curd, bread, cake; production of alcohol; cleaning up of the environment; preparation of medicines; etc.

  • In agriculture, microorganisms are used to increase soil fertility by nitrogen fixation.

  • The bacterium lactobacillus helps in the formation of curd.

  • The microorganisms, yeast is used for the commercial production of alcohol and wine.

  • For the large scale use of yeast, it is grown on natural sugars present in grains like wheat, barley, rice, crushed fruit juices, etc.

  • The process of conversion of sugar into alcohol (by yeast) is known as fermentation.

  • Streptomycin, tetracycline, and erythromycin are some of the commonly used antibiotics; these are made from fungi and bacteria.

  • These days, antibiotics are mixed with the feed of livestock and poultry that check microbial infection in the animals.

  • Several diseases, such as cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.

  • In 1798, Edward Jenner discovered the vaccine for smallpox.

Harmful Microorganisms

  • The microorganisms that cause diseases to human beings, animals, and plants, are known as pathogens.

  • Pathogens enter into humans’ body through the air while breathing, the water while drinking, or the food while eating.

  • Some pathogens are transmitted by direct contact with an infected person or carried through an animal.

  • The microbial diseases that normally spread from an infected person to a healthy person through air, water, food or physical contact are known as communicable diseases. E.g. cholera, common cold, chicken pox, tuberculosis, etc.

  • Female Anopheles mosquito carries the parasite of malaria and known as carrier.

  • Female Aedes mosquito carries the parasite of dengue virus.

Human Diseases

  • The following table illustrates some Common Human Diseases caused by Microorganisms −

Human Disease Causative Microorganism Mode of Transmission
Tuberculosis Bacteria Air
Measles Virus Air
Chicken Pox Virus Air/Contact
Polio Virus Air/Water
Cholera Bacteria Water/Food
Typhoid Bacteria Water
Hepatitis B Virus Water
Malaria Protozoa Mosquito

Microorganisms causing Disease in Animals

  • In 1876, Robert Köch discovered the bacterium (Bacillus anthracis), which causes anthrax disease.

  • Anthrax, a dangerous disease caused by a bacterium, affects both human and cattle.

  • Foot and mouth disease of cattle is caused by a virus.

  • The following table illustrates some Common Plant Diseases caused by Microorganisms −

Plant Disease Causative Microorganism Mode of Transmission
Citrus canker Bacteria Air
Rust of wheat Fungi Air, seeds
Yellow vein mosaic of bhindi (Okra) Virus Insects

Food Preservation

  • Salts and edible oils are the common chemicals usually used to check the growth of microorganisms, they are known as preservatives.

  • Sodium benzoate and sodium metabisulphite are also used as common preservatives.

  • Common salt is usually used to preserve meat and fish for ages.

  • Sugar reduces the moisture content, which prevents the growth of bacteria; therefore, Jams, jellies, and squashes are preserved by sugar.

  • Use of oil and vinegar averts spoilage of pickles, as bacteria cannot live in such kind of environment.

  • When the milk is heated at about 700C for 15 to 30 seconds and then swiftly chilled and stored; the process prevents the growth of microbes. This process was conceptualized by Louis Pasteur; therefore, it is known as pasteurization.

Nitrogen Cycle

Nitrogen Cycle