CBSE Class 8 Science Notes Chapter 2 - Microorganisms: Friends and Foes

A microorganism is a living organism that is very minute. It can only be seen with the aid of a microscope. Consider that the largest human cells are about the diameter of human hair. Bacteria cells are one- hundredth the size of a human cell and viruses are much smaller again. A bacterium next to a human cell is like a tiny boat next to a big ship.

1.0Habitat of Microorganisms

• Microorganisms are present everywhere in air, soil, water, foodstuffs, sewage waste as well as inside animal bodies. 
• They can survive in all types of environment even in extremely harsh environmental conditions like hot springs, desert soil, saline water, ice-cold water and marshy lands. 
• They are also present in dead and decaying organic matter. 
• Some can live alone while others like bacteria and fungi may live in colonies.
•  Some microbes live as parasites either outside or inside the body of other organisms and cause diseases.

2.0Major Groups of Microorganisms 

There are five major groups of microorganisms i.e. bacteria, virus, protozoan, algae and fungus.

1. Bacteria 

• These are single celled organisms, found in a wide range of habitats ranging from glaciers to deserts and hot springs. e.g. Lactobacillus (curd bacteria). 
• They are generally heterotroph but some bacteria also show autotrophic nutrition. 

2. Fungi 

• These are multicellular, heterotrophic organisms, lack chlorophyll and are generally found in colonies. 
• Yeast is the only exception which is unicellular in nature. e.g. Penicillium, Aspergillus. 

3. Protozoa 

• These are unicellular, heterotrophic microorganisms. 
• Usually found in moist & aquatic habitats. e.g. Amoeba, Paramecium

4. Algae 

• These are unicellular or multicellular eukaryotic autotrophic organisms, contain chlorophyll pigment and carry out photosynthesis. 
• Cell wall of algae is mainly made up of cellulose. e.g. Chlamydomonas, Spirogyra, Chlorella.

5. Viruses 

• Viruses are acellular structures because they are not built up of cells. 
• They are much smaller than cells. These are ultramicroscopic organisms, which require cells of host organisms to reproduce. e.g. Influenza virus, polio virus, AIDS etc. Structure of the virus consists of nucleic acid and a protein coat. Nucleic acid can either be DNA or RNA. 

Microorganisms - As Friends 

1. Preparation of curd, bread, cake, idli, dosa and cheese 

• Formation of curd (dahi) : Making of curd is a process in which the lactose (milk sugar) of the milk is converted into lactic acid by the action of certain bacteria (mainly Lactobacillus). This curd is further used in the preparation of idli-dosas and bhaturas. 
• In bread making: Yeast is added to the uncooked dough to make the dough "rise". The dough rises due to the production of carbon dioxide (because of respiration of yeast) in the dough. The bread is then baked. Heat of baking drives off the carbon dioxide, making the bread porous and light. 
• Add flavor : Some fungi (Aspergillus and Penicillium) are added to cheese to give a special flavor.

2. Commercial use of microorganisms 

• In industry The wine and beer (brewery) industries are important industries which make use of the fungus yeast. 
• Yeast is grown on natural sugars present in fruit juices and grains like barley and rice. Yeast brings about conversion of sugars into alcohol and carbon dioxide. 

3. Agricultural use of microorganisms 

• All plants and animals need nitrogen, the gas that makes up 78% of the air. Some bacteria and Blue green algae are the only organisms that can take nitrogen directly from air. 
• These microbes convert atmospheric nitrogen into compounds of nitrogen and so called as biological nitrogen fixers. Thus, they are very useful to agriculture, as they enrich the soil with nitrogen. 

4. Nitrogen cycle 

• Nitrogen is an essential element for all living organisms because it is a principal component of proteins and nucleic acids. 
• The circulation of nitrogen through the living and nonliving components of the biosphere (air, soil, water, plants and animals) is called the nitrogen cycle.

Steps involved in nitrogen cycle 

• Atmospheric nitrogen is fixed into nitrogen compounds by Rhizobium bacteria, Blue green algae. This is called nitrogen fixation. 
• The plants absorb nitrogen compounds from the soil and convert them into plant proteins. 
• The plants are eaten up by animals and thus plant proteins are used for making animal proteins. 
• When the plants and animals die, putrefying bacteria and fungi present in the soil decompose the proteins of dead plants and animals into ammonia. This process is called ammonification.
• Ammonia thus formed is converted first into nitrites and then to nitrates by the action of Nitrosomonas and Nitrobacter bacteria respectively. The process is called nitrification. These nitrates are absorbed by plants. 
• The soil contains denitrifying bacteria called Pseudomonas which convert nitrate form of nitrogen into free nitrogen which goes back into the atmosphere. The process is called denitrification.

5. Medicinal use of microorganisms 

• Microbes are being employed for the production of both antibiotics and vaccines. 
• Antibiotics : Antibiotics are produced by a number of microorganisms and inhibit the growth of other microorganisms even at very low concentrations. Fungi and bacteria are important microbes producing a variety of antibiotics. 
• Antibiotics obtained from bacteria : Streptomycin, aureomycin and chloromycetin. 
• Antibiotics obtained from fungi : Penicillin and Griseofulvin. These antibiotics are used to cure a variety of diseases in humans, plants and animals

6. Cleaning in the environment 

• Microbes, like bacteria and fungi, bring about the decay of dead plants and animals, both on land and in water. 
• They act on dead plants and animals and convert various complex organic compounds into simple inorganic substances. Thus, microbes also help in recycling of nutrients. 
• They also help in sewage disposal by putrefaction (incomplete decomposition of dead organic matter).

Microorganisms - As Foe 

Microorganisms can cause harm in many ways. Many microbes are the causative agents of various diseases that affect plants and animals. While others causes are damage of clothes, leather and spoilage of food.

1. Disease causing microorganisms 

(a) In human - Modes of Transmission of some human diseases caused by microbes

(b) In animals : Microbes not only cause disease in humans but also in animals. The following table shows the list of microorganisms affecting various animals. 

(c) In plants : There are several microbes causing diseases in plants. The list of microorganisms affecting plants is shown below in the table. 

3.0Microbes Causing food spoilage

The contamination of food by microorganisms results in food spoilage. The consumption of this spoiled food by humans or animals results in food poisoning. Microorganisms like bacteria and fungi are responsible for food poisoning. These microbes when grown on food substances produce toxins. These make the food poisonous causing vomiting, serious illness and even death. Therefore, there is a need to preserve food from being spoiled. 

Food preservation

The food that we eat is either in raw or cooked form. All types of food can be targets of microbes which result in food spoilage.

Methods of Food Preservation 

• The process of treating and handling food with an aim to stop or slow down its spoilage while maintaining its nutritional value, texture, and flavor. 

Heat and cold treatment

• Boiling of milk and liquid food products kills many microorganisms. Similarly low temperature inhibits the growth of microbes. 
• Pasteurisation : This method was developed by scientist Louis Pasteur. It is used to store milk and liquid food products. In this the food is heated up to 60°C for 30 minutes or 70°C for 15 to 30 seconds and then is suddenly chilled and stored. 
• Low temperature Storage : Microorganisms present in food products cannot be destroyed by low temperature but their metabolic activities can be arrested. So that they can't act on food products at –10°C to –18°C temperature. Fruits, eggs and meat can be stored for a long time. 
• Refrigeration : Fresh fruits, vegetables, fish, meat, eggs, etc. can be stored by keeping the temperature below 5°C.

Chemical Method 

• Chemicals which are used to check the growth of microbes are called preservatives. 
• The commonly used preservatives are sodium benzoate, sodium metabisulfite and potassium metabisulphite. 
• This method is employed in the preservation of fruit juices, jams, squashes, fruits, etc.

Preservation by common salt 

• Common salt has been used to preserve meat and fish for ages. 
• Concentrated solutions of common salt can kill microorganisms due to dehydration. Salting is also used to preserve amla (Indian gooseberry) and raw mangoes.

Preservation by sugar 

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

Preservation by oil and vinegar 

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

Preservation by radiation 

• In this method air tight packed food substances are passed through a radiation beam, microorganisms present in food substances get destroyed due to radiation. 
• This method is new and cheaper. 

Storage and Packing 

• Dry fruits and vegetables are sold in sealed airtight packets to prevent the attack of microbes.

Advantages of food preservation 

• This prevents the food from being spoiled by the action of microorganisms.  
• It increases the storage period of food materials.  
• It helps in obtaining the off-season food materials. 
• It makes the transportation of food materials easier.