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Microorganisms-theory Biology-day-6 Target-ssc-cgl-2018

Microorganisms-theory Biology-day-6 Target-ssc-cgl-2018

Microorganisms-theory Biology-day-6 Target-ssc-cgl-2018


Micro means very small, thus micro-organisms are those organisms which cannot be seen by naked eyes. They can be seen by a special apparatus called microscope.

Types of micro-organisms :-

  • Bacteria
  • Viruses
  • Protozoa
  • Fungi
  • Algae

Bacteria : These are single celled micro-organisms which have cell walls, but do not have an organised nucleus. Eg. Lactobacillus and Rhizobium etc.

Viruses : These are the smallest micro-organisms. These are developed inside the cells of the host organisms which may be a plant or an animal. Virus do not respire, feed, grow, excrete or move on their own. Virus can reproduce only inside the cells. Outside the cell they behave as non-living thing.

Protozoa : Protozoa are the single-celled micro-organisms of animal kingdom. E.g. Amoeba, Paramecium, Plasmodium etc.

Algae : Algae have chlorophyll and thus they produce their food on their own. They are different from plants because they don’t have roots, stems and leaves. E.g. Chlamydomonas, Spirogyra, Blue-green algae etc.

Fungi : Fungi look like plants but do not have chlorophyll. E.g. Yeast, Mould, Mushrooms, Toadstools, Puffballs etc. All fungi are made up of fine threads called hyphae, except yeast.



  • Helps in making curd, bread and cake. Lactobacillus turns milk into curd. Yeast is used for making bread. Yeast respire and gives carbon-dioxide which makes the bread rise.
  • Helps in the production of alcohol and wine. Yeast is used for making alcohol. The process of conversion of sugar into alcohol is called fermentation.
  • Helps in the preparation of medicines (antibiotics). First antibiotic penicillin was discovered by Alexander Fleming. Antibiotics are not effective against diseases caused by viruses.
  • Vaccine is a special type of medicine which provides immunity against a particular disease. Diseases that can be prevented by vaccine are… Polio, smallpox, typhoid, cholera, rabies, measles, diphtheria etc.
  • Helps to increase the fertility of soil by fixing the nitrogen present in the atmosphere. Rhizobium fixes the nitrogen gas from the atmosphere and enrich the soil with nitrogen.
  • Helps in cleaning the environment by decomposing dead and decaying matter.


Disease causing micro-organisms are called pathogens. Various diseases caused by micro-organisms  are…

  • Diseases caused by bacteria are : TB, Typhoid, Diptheria, Whooping cough, Food poisoning, Cholera, Citrus canker in plants etc.
  • Diseases caused by viruses are : Common cold, influenza, measles, polio, dengue, chickenpox, smallpox, foot and mouth diseases of animals, Anthrax, Aspergillosis, yellow vein mosaic of  bhindi etc.
  • Diseases caused by protozoa  are : Dysentry, Malaria etc.
  • Diseases caused by fungi are : Ringworm, Athlete’s foot, rust of wheat  etc .



  • Disease caused by eating the food which contains a large number of micro-organisms or toxic substances is called food poisoning.
  • Symptoms of food poisoning are :  Headache, vomiting, diarrhoea, pain in abdomen, fever etc.
  • Bacteria which causes food poisoning are Salmonella, Clostridium botulinum etc. Fungus which causes food poisoning is Aspergillus.



Prevention of food materials is done by various physical or chemical treatment. These methods are…

  • Sun-Drying : Food is dried under the sun to reduce the moisture in the food. In the absence of moisture, micro-organisms can’t grow. The vegetables like Spinach, Methi leaves, Cauliflower and peas are preserved in our homes by the sun-drying method.
  • Heating : Best example is boiling of milk.
  • Cooling : Boiled milk, kneaded dough, cooked food, and fresh fruits and vegetables are kept in a cool place like refrigerator to prevent their spoilage.
  • Deep freezing : This method is used for the preservation of meat, fish, fruits and vegetables.
  • Preservation by common salt : Common salt is used to preserve fruits such as raw mangoes, lemon, amla etc.
  • Preservation by sugar : Food in the form of jam and jellies is stored by using sugar.
  • Preservation by mustard oil and vinegar : Pickles are preserved by using mustard oil.
  • Preservation by using chemicals as preservatives : Sodium metabisulphite and sodium benzoate are used to preserve food.
  • Preservation by air tight packing : This prevents the attack of micro-organisms on food.


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