Class 8 Science

Microorganisms

Harmful Microorganisms

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

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

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

PathogenDiseaseMode of Transmission
PlasmodiumMalariaSpreads through bite of female Anopheles mosquito.
Dengue virusDengueSpreads through bite of Aedes mosquito.
Common cold virusCommon coldThrough air; when someone coughs or sneezes.
Many bacteriaCholera, diarrhoeaThrough contaminated food and water.
Hepatitis B virusHepatitis BThrough exchange body fluids.
HIVAIDSThrough exchange of body fluids.
Bacillus anthracisAnthrax (in animals)Through contact.



How to prevent Malaria and Dengue:

  • Malaria and dengue spread through mosquitoes. So, breeding of mosquito should be prevented.
  • Don’t allow water to stagnate in 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 bite.

Diseases caused by microbes in plants
MicrobesDiseaseMode of transmission
BacteriaCitrus cankerAir
FungiRust of wheatAir, seeds
VirusYellow vein mosaic of OkraInsects


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

Techniques to prolong the shelf life of a food are called food preservation. We know that microbes proliferate very fast in presence of food, moisture, oxygen and ambient temperature. All 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. Grains are dried in sun before being stored. Many vegetables are also sun-dried so that they can be used in off season.


Chemical Method: Some chemicals prevent 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. Examples: sodium benzoate and sodium metabisulphate.

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. 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 acidic environment and thus vinegar is an effective food preservative.

Heat and Cold Treatment: Most of the organisms can survive within a particular range of temperature. When the temperature becomes too low or too high, organisms cannot survive. Heating a food item or freezing a food items thus helps in preservation. In most of the households, milk is boiled twice or thrice in a day to prolong its shelf life. Milk is pasteurized before being packed. Pasteurization involves heating the milk to 70C for about 15 to 30 minutes and is then quickly cooling down the milk. Pasteurization helps in killing the microbes which may be present in milk.Most of the organisms can survive within a particular range of temperature. When the temperature becomes too low or too high, organisms cannot survive. Heating a food item or freezing a food items thus helps in preservation. In most of the households, milk is boiled twice or thrice in a day to prolong its shelf life. Milk is pasteurized before being packed. Pasteurization involves heating the milk to 70C for about 15 to 30 minutes and is then quickly cooling down the milk. Pasteurization helps in killing the microbes which may be present in milk.

Storage and Packing: Some food items are stored in air-tight packets so that oxygen is not available for 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.


Nitrogen Fixation

You may have read that 78% of atmosphere is composed of nitrogen. But green plants cannot utilize gaseous nitrogen. Gaseous nitrogen needs to be converted into compounds of nitrogen so that plants can utilize nitrogen. The process of converting nitrogen into compounds which can be taken up by green plants is called nitrogen fixation. Following are the main steps of nitrogen fixation:

  • Blue green algae and some bacteria (like Rhizobium) convert gaseous nitrogen into compounds of nitrogen. Such nitrogenous compounds get deposited in soil.
  • During lighting, some of the nitrogen in atmosphere gets converted into compounds of nitrogen. Such nitrogenous compounds reach the soil along with rainwater.
  • Green plants absorb nitrogenous compounds from soil. Plants make protein and various other important molecules from nitrogen. Nitrogen reaches the body of other organisms through food chain.
  • A major portion of nitrogen from animals and plants is removed during excretion. Remaining portion of nitrogen is sent back to the environment when dead animals and plants get decomposed.

Nitrogen Cycle: The cycle of events by which nitrogen is channelized into the living world and back to atmosphere is called nitrogen cycle.