Harmful Microorganisms

Communicable Disease: A disease which can spread from one person to another is called a communicable disease.

Infectious Disease: A disease which is caused by an attack from microbes is called an infectious disease.

Pathogen: A microbe which causes disease is called a pathogen. Some examples of pathogens are as follows:



Mode of Transmission



Spreads through bite of female anopheles mosquito.

Dengue virus


Spreads through bite of aedes mosquito.

Common cold virus

Common cold

Through air; when someone coughs or sneezes.

Many bacteria

Cholera, diarrhoea

Through contaminated food and water.

Hepatitis B virus

Hepatitis B

Through exchange body fluids.



Through exchange of body fluids.

Bacillus anthracis

Anthrax (in animals)

Through contact.

How to prevent Malaria and Dengue:

  • Malaria and dengue spread through mosquitoes. So, breeding of mosquitoes should be prevented.
  • Don’t allow water to stagnate in the surroundings because mosquitoes lay eggs in stagnant water.
  • Frequently clean the water in cooler.
  • Keep the overhead tank covered.
  • Keep the drains properly covered.
  • Fill any ditch with sand or soil.
  • Use mosquito repellant creams and mosquito nets to prevent mosquito bites.

Diseases caused by microbes in plants



Mode of transmission


Citrus canker



Rust of wheat

Air, seeds


Yellow vein mosaic of Okra



Food Poisoning: Some bacteria produce a toxic substance in food. Consuming a food item with toxic substance can result in food poisoning. Food poisoning is a serious case and needs immediate hospitalization. Lack of timely care in case of food poisoning may prove fatal.

Food Preservation

Microbes in our environment spoil food very fast in the presence of moisture, oxygen and ambient temperature. Thus the techniques to prolong the shelf life of a food is called food preservation. The methods of food preservation are aimed at ruling out the food, moisture, oxygen and ambient temperature for microbes so that they won’t proliferate. Some common methods of food preservation are as follows:

Sun drying: This is a traditional method of food preservation. Sun-drying helps in removing moisture from food.

Chemical method: Presence of some chemicals prevents the growth of microorganisms in food. These chemicals are used in pickles and jams to preserve them. Salt, acids and oil are also used as food preservatives.

Preservation by common salt: When a food item is kept in plenty of salt, water from food comes out because of osmosis. It results in dehydration of the food item. The absence of moisture helps to prevent the growth of microbes. Fish, meat and pickles are preserved by adding salt.

Preservation by sugar: Sugar preserves food by reducing moisture in food. Jam, jelly and squash are preserved by adding sugar.

Preservation by oil and vinegar: The layer of oil on top of food prevents oxygen from entering the food. Some microbes do not survive in absence of oxygen. Some microbes cannot survive in the acidic environment and thus vinegar is an effective food preservative.

Storage and Packing: Some food items are stored in air-tight packets so that oxygen is not available for the proliferation of microbes. Some food items are packed in cans; along with some preservatives. Oily food such as potato chips is packed in air-tight packets which are filled with nitrogen gas. Nitrogen gas prevents the oily food from becoming rancid.

Useful Microorganisms

Microorganisms are used for various purposes. They are used in the preparation of curd, bread and cake. They have been used for the production of alcohol since ages. They are also used in cleaning up the environment. For example, the organic waste (vegetable peels, remains of animals etc.) are broken down into harmless and usable substances by bacteria.

Making curd and Bread

Curd contains several microorganisms. Of these, the bacterium Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd. Bacteria also involved in the making of cheese, pickles and many other ​food items. Yeast reproduces rapidly and carbon dioxide during respiration bubbles of the gas 
Fill dough and increase its volume. This is the basis of the use of yeast in baking industries for making bread, pastries and cakes.

Commercial Use of Microorganism

Microorganisms are used for the large-scale production of alcohol, wine and acetic acid (vinegar). Yeast is used for commercial production of alcohol and wine. For this yeast is grown on natural sugars presents in grains, barley, wheat, rice, crushed fruit juices, etc. This is the smell of alcohol as sugar has been converted into alcohol by yeast. This process of conservation of sugar into alcohol is known as fermentation.

Medicinal use of Microorganism

When we fall ill the doctor may give some antibiotics. The source of this medicine is microorganisms. These medicines kill or stop the growth of the disease-causing microorganism such medicines are called antibiotics. Streptomycin, tetracycline is some of the commonly known antibiotics which are made from fungi and bacteria.


When a disease-carrying microbe enters our body, the body produces antibodies to fight the invader. The body also remembers how to fight the microbe if enters again. So if the dead or weakened microbes are introduced into a healthy body, the body fights and kills them by producing suitable antibodies. This is how a vaccine works.

Increasing Soil Fertility

Some bacteria and blue-green algae are able to fix nitrogen from the atmosphere to enrich the soil with nitrogen and increase its fertility. These microns are commonly called biological nitrogen fixers.

Cleaning the Environment

Microorganisms can be used to degrade the harmful and smell substances and thereby clean up the environment.

So we can say that microorganisms are friends of us in many ways.