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2. Microorganisms – Friends and Foe

This chapter talks about small living organisms called microorganisms and how they interact with us

The living organisms that are so small, they cannot be seen with the unaided eye are called Microorganisms or Microbes.

They can be classified into 4 categories

Category Cell Structure Diseases
Bacteria Single Celled
  • Cholera
  • Tuberculosis(TB)
  • Typhoid
Fungi Multi Cellular Skin fungal infections
Protozoa Single Celled
  • Dysentery
  • Malaria
Algae Some algae are Single Celled while others are Multi Cellular
  • Various forms of Shellfish poisoning ( we are mainly exposed to harmful algae by consuming contaminated sea food like oysters, prawns etc


 Besides these microorganisms, there is another type of microorganisms called Viruses. Viruses cause some common ailments like

  • Chicken pox
  • Common cold
  • Hepatitis B
  • Influenza (flu)
  • Measles
  • Polio

 2.1      Where do they live?

Microorganisms live & survive in varied environmental conditions ranging from ice cold climate to hot springs & deserts to marshy lands. But the most conductive environment for them to grow & multiply is the humid & moist environment. Some microorganisms grow on other organisms while others exist freely. Large numbers of microorganism are found on & inside human body.

 2.2      Friendly Microorganisms

It is difficult to imagine our lives without the friendly microorganisms. Some of the uses of these microorganisms from our daily life are

  1. Preparation of curd, bread & cakes
  2. Commercial production of alcohol, wine & vinegar
  3. Preparation of medicines
  4. Used in agriculture to increase soil fertility
  5. In cleaning up of the environment

 2.2.1      Commercial Uses

Microorganisms are used in Industries for manufacturing various substances. For example

  1. Curd
  2. Breads and Cakes
  3. Alcohol and Wine
  4. Acetic Acid (Vinegar)

Curd is prepared by adding a small amount of curd to milk. This process is called Curding. Curd contains bacteria called Lactobacillus, which helps promotes curding of milk & thus turn milk into curd

Breads and Cakes are made by adding a small amount of Baker’s Yeast to the dough. The yeast (a fungi) reproduces rapidly in moist and warm conditions and releases carbon dioxide (CO2) into the dough thus making the bread porous & puffy.

The process of conversion of sugar into alcohol is known as Fermentation. This is done in the presence of Yeast. Fermentation was discovered by Louis Pasteur in 1857.

Vinegar is made by fermentation/oxidation of ethanol by acetic acid bacteria.

 2.2.2      Medicinal Uses

Microorganisms are used in the field of medicine for preparing various types of medicines. For examples

  1. Antibiotics
  2. Vaccines

Antibiotics are the medicines that kill or stop the growth of disease causing microorganisms. These medicines are manufactured by growing specific types of bacteria and fungi which have the capability to prevent the growth of other microorganisms. Antibiotics are effective against bacteria but are not very effective against viruses.

Antibiotics are also used in crop production and animal husbandry to control microbial infection in plants and animals respectively

Antibiotics should only be taken on the advice of a doctor and in prescribed doses only. They should be continued for the prescribed duration to ensure that microbes do not get immune to these antibiotic medicines; otherwise they will not be effective when you need it next time.

Penicillin – the first antibiotic was discovered by Alexander Fleming in 1929.

Vaccines are a type of medicines, which help prevent the occurrence of a disease, rather than curing them when they occur. They prevent us from diseases like

  • Tetanus
  • Polio
  • Chicken Pox

How do vaccines work?

Human body has a natural defence system. When a microbe enters the body, the body produces antibodies to fight the intruder. As part of vaccination, weakened or dead cells of infection causing microbes are introduced in the body in a controlled manner. The body fights & kills these microbes by producing suitable antibodies. Even after the infection is cured, these antibodies remain in the body and hence the body remembers how to fight such microbes during future infections.

Vaccine for small pox was discovered by Edward Jenner in 1798.

2.2.3      Agricultural Uses

Some microorganisms can convert nitrogen present in air into Ammonia (NH3), Nitrites (NO2) or Nitrates (NO3) which can be absorbed by the soil. This process is called nitrogen fixing. It increases the fertility of the soil and leads to increased agricultural yield

Examples of such microorganisms are

  1. Rhizobium bacteria
  2. Blue green algae (They were traditionally considered algae but are now considered as bacteria)

2.2.4      Environmental Uses

Microorganisms decompose the organic waste like vegetable peels, wastes from plants & animals & convert them into manure. It is also important to remember that plastics are not bio-degradable and do not get decomposed. They are harmful to the environment, so we should discourage the use of plastics.

2.3      Harmful Microorganisms

Harmful microorganisms affect us in many ways. Some harmful microorganisms cause diseases while others spoil food, clothes & leather etc.

2.3.1      Disease causing Microorganisms

Disease causing microorganisms are called Pathogens. They enter our body through air, water & the food that we eat. They also get transmitted by direct contact with the infected person or can be carried through mosquitoes, flies and animals.

The microbial diseases that spread from an infected person to the healthy person through air, water, food or direct contact are called Communicable Diseases. For example

  • Cholera
  • Chicken Pox
  • TB

Diseases which get transmitted through air are called airborne diseases. Similarly, the diseases transmitted through water are called waterborne diseases.

How do we get airborne diseases like common cold?

Common cold is a viral infection of the upper respiratory tract. It can be caused by upto 200 different types of viruses. The virus grows mainly in the nose and produces nasal secretions. When an infected person sneezes, these secretions are blown into the air in the form of tiny droplets. They can straightaway land on another person and cause infection. They can also land on other objects and surfaces in the room where they can survive for upto three hours. If another person touches the contaminated surface, he could also catch cold

There are some insects & animals which can transfer disease causing microbes to human body. They are called Carriers. For example

  1. Housefly is a carrier of Cholera
  2. Female Anopheles mosquito is a carrier of Malaria
  3. Female Aedes mosquito is a carrier of Dengue

They can transfer pathogens either by contaminating our food/water or by biting & transmitting the pathogens to our blood

Microorganisms can cause diseases in plants as well. Almost all the crop yielding plants get affected by diseases caused by microorganisms, thereby affecting the yield of crop. For example

  1. Citrus canker affecting citrus fruits is caused by Bacteria
  2. Rust of wheat affecting wheat crops is caused by Fungi
  3. Yellow veing mosaic of ocra (bhindi) is caused by Virus

This can be controlled by use of chemicals to kill the microbes.

Similarly, Microorganisms can cause disease in animals as well. What affect us most are the diseases affecting cattle, livestock and poultry

  1. “Bird flu” affects poultry and is caused by a virus
  2. “Foot and Mouth” diseases affects cattle and is caused by bacteria

2.3.2      Microorganisms which spoil our Food, clothes and Leather

The microorganisms that grow on food, produce toxic substances making the food poisonous & unfit for consumption. If this food is consumed it causes illness. This is called ‘Food Poisoning’

In order to prevent ‘Food Poisoning’, we need to preserve our food so that it can be consumed for a longer time. There are various methods to preserve food

Preservation Method How does it work Food that can be preserved using this method
Preservative Chemicals like Sodium Benzoate and Sodium Metasulphite Block the growth of microbes by interrupting the normal functioning of their cells Pickles, Jams and Squashes
Common Salt Absorb water from food. Bacteria need water to live and reproduce Meat, Fish, Raw Mangoes, Goose berries
Sugar Absorb water from food. Bacteria need water to live and reproduce Jams and Jellies
Pickling using Oil & Vinegar High pH (acidity) inhibits the functioning of bacterial cells Vegetables, Fish and Meat
Hot & Cold Treatment Heating kills most bacteria. Suddenly chilling prevents growth of bacteria Milk
Storage & Packing Cut off air and water Dry Fruits

Pasteurization is a method where milk is heated to about 70ºC for 10 to 30 seconds & then instantly chilled & stored. With this heat & cold treatment, most of the microbes get killed. This process was discovered by Louis Pasteur.

Fungi cannot make their own food. They obtain food by decomposing other organic materials around them. They excrete enzymes on the host and the nutrients generated are absorbed by the fungi.

Fungi that obtain nutrients from a non-living organic host are called saprobes, while those who obtain it from a living host are called parasites.

Moulds are a type of fungi that grows wherever there is food, water and air. There are many types of moulds. One of the most common one is called penicillium (that’s where antibiotic penicillin comes from) and is responsible for rotting the Oranges. Moulds grow slower in colder temperature, that’s why we keep our food in refrigerator. Similarly, other types of moulds grow on damp clothes (organic matter) and leather

2.4      Nitrogen Cycle

Atmospheric air has 78% nitrogen gas. Nitrogen forms the important constituent of all living organisms as part of proteins, chlorophyll, nucleic Acids & vitamins. The amount of nitrogen in the atmosphere remains constant. The nitrogen cycle explains how one form of nitrogen gets converted into the other form, but still ensures that the percentage of nitrogen in atmosphere remains more or less constant. So, the total atmospheric nitrogen remains constant.