Microorganisms: Friend and Foe Revision notes


  • Organisms too small to be seen with our naked eyes are called microorganisms or microbes.  
  • They can only be seen using a microscope.
  • Microorganisms are found in a variety of habitats such as air, soil and water.  
  • They can live and survive in extreme environments, from ice cold regions to hot springs, from deserts to marshy lands.  
  • Some microorganisms are useful to us in many ways. They are called beneficial microbes. Example: Bacterium Rhizobium helps in nitrogen fixation.
  • Some microorganisms cause diseases in plants, animals and human beings. They are called pathogens. Example: Anthrax bacillus causes anthrax in animals.
  • Robert Hooke and Anton Van Leeuwenhoek discovered the existence of microbes during the period 1665 – 1683.

Major Groups of Microorganisms 


  • Found on walls, in fields and in small puddles.
  • May be unicellular or multicellular with a size of 1 µ-1 m. 
  • Oscillatoria, Nostoc and Anabaena are examples of this type of algae.
  • Sexual reproduction occurs by conjugation


  • Grows on stale bread, chapati, pickles, damp clothes and shoes in monsoon.
  • Respires aerobically or anaerobically. 
  • Lives a saprophytic or parasitic life.
  • Mucor, Rhizopus and Aspergillus are examples of fungi.


  • Amoeba, Paramoecium and Plasmodium are examples of protozoa.
  • Amoeba has an irregular shape. 
  • Locomotion occurs through pseudopodia.
  • Paramoecium is slipper shaped.
  • It has numerous cilia for locomotion.
  • Plasmodium is a protozoan living a parasitic life. It is responsible for the spread of malaria.


  • Found in any kind of environment from deserts to Polar Regions.
  • Range in size is from 0.2 to 10 µ.
  • Reproduces sexually and asexually.
  • Respires aerobically or anaerobically.
  • Main types of bacteria according to their shape are Coccus, Bacillus, Vibrio and Spirillium.
  • Rhizobium and Lactobacillus are examples of bacteria


  • Smallest microorganisms which can grow and develop only inside the host cell.
  • In free-living form, viruses are like nonliving particles.
  • Viruses are classified into three types based on the type of host they infect.

Examples: Animal viruses: Foot and Mouth Disease Virus (FMDV) Plant viruses: Tobacco Mosaic Virus (TMV) Bacteriophage: Coliphage infecting Escherichia coli

Conditions Necessary for the Growth of Microorganisms

Conditions Necessary for the Growth of Microorganisms

Useful Microorganisms

Increasing soil fertility

  • Some bacteria and fungi act as decomposers and decompose dead and decaying matter and help in recycling nutrients back to the soil.

Cleaning the environment

  • Microorganisms decompose complex organic matter such as waste of vegetables, fruits, plants and animals, and convert them into simpler substances, thereby cleaning the environment.

Retting of fibers

  • Some bacteria carry out the process of loosening of fibres of plants by rotting, also called retting.

Tanning of leather

  • During tanning, the bacteria attack the skin of animals. The protein structure of the skin undergoes a permanent alteration. This makes them soft and pliable and the skin is converted into leather.

Food industry

  • Yeast helps in the commercial production of beer, wine and other alcoholic drinks by fermenting fruit juices, malted grains or molasses.
  • Yeast is often used in the baking industry for making breads, pastries and cakes.
  • Bacteria find application in the making of curd.
  • Algae are a rich source of wholesome and nutritious food.

Production of antibiotics

  • Antibiotics are manufactured by growing specific microorganisms to treat a wide variety of diseases caused by pathogenic microorganisms.

Production of vaccines

  • Vaccines are made on a large scale from microorganisms to protect human beings and other animals from several diseases.
  • The first antibiotic pencillin was discovered by Alexander Fleming in 1929.
  • Fungus Penicillium notatum produces penicillin.

Some Important Vaccines

Some Important Vaccines

Harmful Microorganisms

Pathogens can successfully cause disease due to the following factors:  

  • Ability to multiply fast.
  • Ability to destroy tissues of the body of other organisms.
  • Ability to produce toxins that affect certain organs of the host.

Communicable Diseases 

  • Microbial diseases which can spread from an infected person to a healthy person through air, water, food, physical contact or through vectors are called communicable diseases. Examples: Tuberculosis, chickenpox.
  • The insects or other animals which transmit disease causing microorganisms to humans without themselves getting infected are called carriers. Examples: Housefly, mosquito.

Important discoveries

  • Alexander Fleming: Discovered penicillin in 1929
  • Louis Pasteur: Discovered fermentation in 1857
  • Edward Jenner: Discovered vaccine for smallpox in 1798
  • Robert Koch: Discovered bacterium Bacillus anthracis in 1876

Some of the Common Human Diseases Caused by Microorganisms 

DiseaseCausative microorganismMode of transmissionPreventive measures
Measles VirusAir Dirty, humid, or overcrowded habitats should be avoided. Vaccination should be given at the appropriate age. 
ChickenpoxVirus Air/Contact
Polio VirusAir/Water
TuberculosisBacteria Air
Hepatitis AVirusWater Drink boiled drinking water.
TyphoidBacteriaWaterPersonal hygiene, cleanliness of the surroundings, and consumption of well-cooked, nutritious food.  Vaccination should be given at the appropriate age.
CholeraBacteria Water/Food
Malaria Protozoa Mosquito biteThe population of mosquitoes should be reduced or eliminated. 

Common Animal Diseases caused by Microorganisms 

DiseaseAnimal affectedCausative microorganism
AnthraxCattle, sheep, elephantBacteria
DiphtheriaGuinea pigs, kittens, rabbitsBacteria
DysenteryMonkeys Bacteria 
Plague  Rats, squirrels, rabbits, guinea pigsBacteria 
TuberculosisMonkeys, dogs, parrot, cattleBacteria
Lung necrosisAdult cattleFungi 
Foot and mouth diseaseSplit hoofed animalsVirus 
Swine fluPigs Virus 

Some Common Plant Diseases Caused by Microorganisms 

DiseasePlant affectedCausative microorganism
Angular leaf spotCottonBacteria  
Blight Canker PaddyBacteria  
Rust Wheat  Fungi
Smut Barley, wheat, oats, sugarcane, forage grasses Fungi
ErgotCorn, rye, wheat, bajra, barleyFungi
Tikka diseaseGroundnutFungi
Blast Rice, ragi Fungi
Late blight diseasePotato, tomato, black pepper, vanillaFungi
Sheath blight diseasePaddy Fungi
Ringspot diseaseSugarcaneFungi
Red rot disease SugarcaneFungi
Yellow vein mosaic diseaseOkraVirus 

Prevention of Communicable Diseases

  • Mouth and nose should be covered with a handkerchief while sneezing or coughing.
  • Uncovered roadside foods should not be consumed.
  • Food should be kept covered to protect it from getting infected by flies.
  • We should protect ourselves from mosquito bites by using mosquito nets or applying mosquito repellant creams.
  • Only boiled water must be consumed.
  • One must avoid sharing towels and combs.
  • There should not be stagnant water or open garbage dumps in the locality.

Food Poisoning

  • Spoilage of food by the action of microorganisms is called food poisoning
  • It mostly occurs due to consumption of food spoilt by fungi and bacteria.
  • When these organisms come in contact with food, they decompose it and release chemical toxins. This contaminates or spoils food. 

Food Preservation

Food preservation is the process of treating and handling food in order to stop or slow down its spoilage while maintaining its nutritional value, texture, and flavor.

Advantages of food preservation

  • Reduces wastage of food
  • Helps to transport food to distant places
  • Helps to store food for use in seasons when they are not available

 Principles of food preservation

  • Keeping out food from the reach of microorganisms
  • Removing microorganisms
  • Inhibiting the growth of microorganisms
  • Killing of microorganisms

Methods of food preservation

  • Boiling
  • Salting
  • Adding sugar
  • Dehydration (Drying)
  • Irradiation
  • Pasteurisation
  • Refrigeration
  • Chemical preservatives

Methods of Food Preservation


  • Boiling water kills all bacteria. Higher temperatures (110°C) kill spores


  • Biodegradation is prevented under normal conditions

Preservation by sugar

  • Reduces the moisture content which inhibits the growth of bacteria

Dehydration (Drying)

  • Inhibits growth of microbes


  • Radioactive or ultraviolet radiation kills microorganisms


  • Heating to a temperature of about 60°C for 30 min and then chilling kills bacteria


  • Inhibits growth and multiplication of microbes

Chemical preservatives

  • Sodium benzoate increases the concentration of solutes causing plasmolysis and death of bacteria and mould.

Nitrogen Fixation

  • The process of converting nitrogen gas of the atmosphere into compounds of nitrogen which can be used by plants is called nitrogen fixation
  • By nitrogen-fixing bacteria: Ammonia is converted to nitrates by nitrifying bacteria.
  • By Rhizobium bacteria: Rhizobia live in the root nodules of leguminous plants. They convert free nitrogen from the atmosphere into soluble nitrates.
  • By blue-green algae: They fix nitrogen gas into ammonia, nitrates and nitrites. 
  • By lightening: During lightning, nitrogen and oxygen from the atmosphere combine to form nitrogen oxides which react with rainwater and form dilute nitric acid. Nitric acid reacts with the minerals present in the soil and forms nitrate.

Nitrogen Cycle 

  • The process of circulation of nitrogen between the atmosphere, soil, plants and animals is called the nitrogen cycle.
  • Atmospheric nitrogen is fixed by biological nitrogen fixers as well as through physical processes such as lightning. 

Nitrogen Fixation during Lightning

Nitrogen Fixation during Lightning
  • Oxides of nitrogen formed during lightening mix with rain water to form nitric acid. Nitric acid when mixes with minerals in the soil for nitrates.
  • Plants take up these nitrates and convert them into proteins. Animals consume plants.
  • When plants and animals die, nitrogenous compounds present in their bodies are broken down into ammonia by ammonification.
  • Nitrifying bacteria such as Nitrosomonas convert this ammonia into nitrates by nitrifcation. Nitrates are stored in humus.
  • Sometimes nitrates are converted into molecular nitrogen by denitrifying bacteria Pseudomonas.
  • Due to constant recycling of nitrogen, the percentage of nitrogen in the atmosphere remains more or less constant.

Biological Nitrogen Fixation 

Biological Nitrogen
  • Bacteria such as Rhizobium which live in the root nodules of leguminous plants convert free nitrogen into soluble nitrates.
  • This process makes the soil with nitrogen and increases its fertility. In return these bacteria get shelter from plants.
  • Some bacteria such as Azotobacter convert inorganic molecular nitrogen into amino acids and proteins.
  • Nitrogenous wastes from dead plants are converted back into ammonia by bacteria such as Clostridium spp.
  • Then ammonia is converted into nitrites by Nitrosomonas bacteria, and then into nitrates by Nitrobacter.
  • Nitrification is followed by denitrification, wherein nitrates are converted into nitrogen gas (N2 ) by the action of denitrifying bacteria. The nitrogen gas is then released into the atmosphere.