MICROORGANISMS :: FRIEND AND FOE: Class 8 Science NCERT Chapter 2 Note

Microorganisms or microbes: The living organisms around us which we normally cannot see are called microorganisms or microbes.
Example: In rainy season moist bread gets spoilt and its surface gets covered with greyish white patches. These tiny, black rounded structures are microorganisms or microbes.

2.1 Microorganisms

Microorganisms are classified into four major groups. These groups are bacteria, fungi, protozoa and some algae.

• Viruses are also microscopic but are different from other microorganisms.
• They, however, reproduce only inside the cells of the host organism, which may be a bacterium, plant or animal.
• Common eliments like cold, influenza (flu) and most coughs are caused by viruses.
• Serious diseases like polio and chicken pox are also caused by viruses.

Diseases like dysentery and malaria are caused by protozoa(protozoans) whereas typhoid and Tuberculosis (TB) are bacterial diseases.

2.2 Where do Microorganisms Live?

• Microorganisms may be single-celled like bacteria, algae and protozoa, or multicellular, algae and fungi.
• They live in all types of environment, ranging from ice cold climate to hot springs, eserts to marshy lands.
• They are also found inside the bodies of animals including humans.

2.3 Microorganisms and Us

Microorganisms play an important role in our lives. Some of them are beneficial in many ways whereas some others are harmful and cause diseases.

Friendly Microorganisms

• Microorganisms are used in the preparation of curd, bread and cake.
• Microorganisms have been used for the production of alcohol since ages.
• Microorganisms are also used in cleaning up of the environment.
• Bacteria are also used in the preparation of medicines.
• In agriculture they are used to increase soil fertility by fixing nitrogen.

Making of Curd and Bread

• Curd contains several microorganisms.
• The bacterium, Lactobacillus promotes the formation of curd.
• It multiplies in milk and converts it into curd.
• Bacteria are also involved in the making of cheese, pickles and many other food items.
• An important ingredient of rava (sooji ) idlis and bhaturas is curd.
• Bacteria and yeast are also helpful for fermentation of rice idlis and dosa batter.

• Yeast reproduces rapidly and produces carbon dioxide during respiration.
• This is the basis of the use of yeast in the baking industry for making breads, pastries and cakes.

Commercial Use of Microorganisms

• 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.
• Yeast is grown on natural sugars present in grains like barley, wheat, ice, crushed fruit juices, etc.

This is the smell of alcohol as sugar has been converted into alcohol by yeast. This process of conversion of sugar into alcohol is known as fermentation.

Medicinal Use of Microorganisms

• Antibiotic tablets, capsules or injections such as of penicillin are the source of these medicines is microorganisms.
• These medicines kill or stop the growth of the disease-causing microorganisms. Such medicines are called antibiotics.
• These days a number of antibiotics are being produced from bacteria and fungi.
• Streptomycin, tetracycline and erythromycin are some of the commonly known antibiotics which are made from fungi and bacteria.

The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases.

Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals. They are also used to control many plant diseases.

Antibiotics are not effective against cold and flu as these are caused by viruses.

Vaccine

• 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 it enters again.
• If dead or weakened microbes are introduced into a healthy body, the body fights and kills the invading bacteria by producing suitable antibodies.

Several diseases, including cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.

Several diseases, including cholera, tuberculosis, smallpox and hepatitis can be prevented by vaccination.

Increasing Soil Fertility

Some bacteria are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility. These microbes are commonly called biological nitrogen fixers.

Cleaning the Environment

• The microorganisms decompose dead organic waste of plants and animals converting them into simple substances.
• These substances are again used by other plants and animals.
• Microorganisms can be used to degrade the harmful and smelly substances and thereby clean up the environment.

2.4 Harmful Microorganisms

The microorganisms cause diseases in human beings, plants and animals. Such disease- ausing microorganisms are called pathogens.

Disease causing Microorganisms in Humans

Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases.

Examples: cholera, common cold, chicken pox and tuberculosis.

There are some insects and animals which act as carriers of diseasecausing microbes. Example:

• Housefly
• Female Anopheles mosquito, which carries the parasite of malaria.
• Female Aedes mosquito acts as carrier of dengue virus.

All mosquitoes breed in water. Hence, one should not let water collect anywhere, in coolers, tyres, flower pot, etc. By keeping the surroundings clean and dry we can prevent mosquitoes from breeding.

Disease causing Microorganisms in Animals

For example, anthrax is a dangerous human and cattle disease caused by a bacterium. Foot and mouth disease of cattle is caused by a virus.

Disease causing Microorganisms in Plants

Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple and others. The diseases reduce the yield of crops.

Food Poisoning

• Food poisoning could be due to the consumption of food spoilt by some microorganisms.
• Microorganisms that grow on our food sometimes produce toxic substances.

2.5 Food Preservation

Chemical Method

• Salts and edible oils are the common chemicals generally used to check the growth of microorganisms. Therefore they are called preservatives.
• Sodium benzoate and sodium metabisulphite are common preservatives. These are also used in jams and squashes to check their spoilage.

Preservation by Common Salt

• Common salt has been used to preserve meat and fish for ages.
• Meat and fish are covered with dry salt to check the growth of bacteria.
• Salting is also used to preserve amla, raw mangoes, tamarind, etc.

Preservation by Sugar

Jams, jellies and squashes are preserved by sugar. Sugar reduces the moisture content which inhibits the growth of bacteria which spoil food.

Preservation by Oil and Vinegar

• Use of oil and vinegar prevents spoilage of pickles because bacteria cannot live in such an environment.
• Vegetables, fruits, fish and meat are often preserved by this method.

Heat and Cold Treatments

• Boiling kills many microorganisms.
• We keep our food in the refrigerator. Low temperature inhibits the growth of microbes.

• Pasteurised milk can be consumed without boiling as it is free from harmful microbes.
• The milk is heated to about 700C for 15 to 30 seconds and then suddenly chilled and stored.
• By doing so, it prevents the growth of microbes. This process was discovered by Louis Pasteur. It is called pasteurisation.

Storage and Packing

Now a days dry fruits and even vegetables are sold in sealed air tight packets to prevent the attack of microbes.

2.6 Nitrogen Fixation

• Rhizobium is involve in the fixation of nitrogen in leguminous plants (pulses).
• Rhizobium lives in the root nodules of leguminous plants, such as beans and peas, with which it has a symbiotic relationship.
• Sometimes nitrogen gets fixed through the action of lightning.

2.7 Nitrogen cycle

• Our atmosphere has 78% nitrogen gas.
• Nitrogen is one of the essential constituents of all living organisms as part of proteins, chlorophyll, nucleic acids and vitamins.
• The atmospheric nitrogen cannot be taken directly by plants and animals.
• Certain bacteria and blue green algae present in the soil to fix nitrogen from the atmosphere and convert it into compounds of nitrogen.
• Once nitrogen is converted into these usable compounds, it can be utilised by plants from the soil through their root system.
• Nitrogen is then used for the synthesis of plant proteins and other compounds.
• Animals feeding on plants get these proteins and other nitrogen compounds.

• When plants and animals die, bacteria and fungi present in the soil convert the nitrogenous wastes into nitrogenous compounds to be used by plants again.
• Certain other bacteria convert some part of them to nitrogen gas which goes back into the atmosphere.

As a result, the percentage of nitrogen in the atmosphere remains more or less constant.

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