CHAPTER - 2 MICROORGANISMS : FRIEND AND FOE. Microorganisms (Microbes) :-. Very small organisms which cannot be seen with the naked eye. may be unicellular or multicellular may exist alone like amoeba or in colonies like Fungi & Bacteria
They are different from other microorganisms as they reproduce only in the body of host organisms like bacteria, plants or animals.
Some Bacteria and Blue Green Algae are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility.
These microbes are called Biological Nitrogen Fixers.
Micro organisms decompose dead organic
waste of plants and animals and convert them
into simple substances which are again used by
other plants and animals.
i) Some micro organisms cause diseases in plants and animals. Such disease causing microorganisms are called pathogens.
ii) Some micro organisms grow in food substances and produce toxic substances and makes the food poisonous. Food poisoning causes illness and even death.
iii) Some microorganisms spoil materials like clothing, leather, wood etc.
Some insects and animals act as carriers of disease causing microbes.
Eg :- House fly, Female anopheles mosquito (malaria), Female aedes mosquito (dengue fever) etc.
Microorganisms cause diseases in plants like rice, wheat, potato, sugarcane, apple, orange etc. These diseases reduce crop yield. They can be controlled by using chemicals which kill the microbes.
Human disease Causative Mode of
Chicken pox Virus Air / Contact
Polio Virus Air / Water
Measles Virus Air
Hepatitis – B Virus Water
Tuberculosis Bacteria Air
Typhoid Bacteria Water
Cholera Bacteria Water / Food
Malaria Protozoa Mosquito
Some common plant diseases caused by microorganisms :-
Plant disease Causative Mode of
Citrus canker Bacteria Air
Rust of wheat Fungi Air / Seed
Yellow mosaic of
Okra (Lady’s finger) Virus Water
By using Oil & Vinegar
storage and packing
Pasteurisation :-The process of heating milk to about 70oC for 15 to 30 seconds and then suddenly chilling it to prevent the growth of microbes is called pasteurisation. This process was discovered by Louis Pasteur.
vi) By storage and packing :- Many food items are stored in air tight containers to protect them from microbes.
The nitrogen in the atmosphere is converted into nitrogen compounds in the soil by nitrogen fixing bacteria and blue green algae. Lightning also converted into nitrogen compounds in the soil. The nitrogen compounds in the soil is used by plants for the synthesis of proteins and other compounds. Animals feeding on plants get these proteins and other compounds. When plants and animals die, bacteria and fungi in the soil converts the nitrogenous waste into nitrogen compounds in the soil which are again use by plants. Some other bacteria converts some nitrogen compounds in the soil into nitrogen gas which goes back into the atmosphere. Hence the nitrogen in the atmosphere almost remains constant.