Introduction– Those organisms which can only be seen by using microscope are called microorganisms or microbes. e. g-Bacteria, Viruses, Protozoa, Algae, etc. Microbes are useful to us as well as harmful. Many of them cause diseases in plants and animals. Many of the spoil the food, therefore, food preservation methods should be adopted. Many of microbes are useful and help in curd making, fixation of atmospheric nitrogen in the soil, medicine preparation, humus formation etc.  The study of microbes is called microbiology.
Distribution of microbes-The microbes are ubiquitous. They are found in air, pond water, hot spring water, ocean water, ice, soil, plants, spoiled food, old teared shoes, rock cracks, old books, etc. But, they are not found in healthy part of plants, inside healthy person’s body, inside active volcano and inside deep layers of the rock. A type of bacteria called Archebacteria can survive in hot spring water upto 78 degree Celcius temperature. Some bacteria can survive in extreme cold upto – 90 degree Celcius temperature. Let us now do details of these microorganisms –
1. Bacteria (Singular–Bacterium)- They are unicellular, prokaryotic, colourless microorganisms. On the basis of shape, there are four types of bacteria-a) Coccusor Spherical bacteria   b) Bacillus or Rod Shaped bacteria  c) Spirillum or Spiral bacteria and d) Vibrioor Comma shaped bacteria. Bacteria were discovered by Anton van Leeuwenhoek-a Dutch scientist from Netherland.

                                 Types of bacteria on the basis of shape

The bacteria cause a number of diseases in plants and animals. E.g. Cholera, Anthrax, Typhoid, Tetanus, Tuberculosis (CAT3), diarrhoea, Pneumonia, Whooping cough etc. They are useful also. They are found almost everywhere. Rhizobium bacteria are found in root nodules of leguminous plants and help in atmospheric nitrogen fixation. Lactobacillus bacteria convert milk into curd by fermentation. Escherichia coli are also friendly bacteria found in our large intestine. The branch of science which deals with the study of bacteria is called as bacteriology.
2.  Fungi– (Singular – Fungus) – They are non-green organisms that grow on any habitat that contains organic matters. They do not contain chlorophylls. They absorb the nutrients from organic matters. They grow on moist bread, leather, rotten vegetables and fruits, rotten logs, animal dung etc. They are microscopic as well as non-microscopic. They are unicellular as well as multi-cellular. e.g.-Yeast (Unicellular, Eukaryotic cell), Rhizopus (Black bread mould), Penicillium, Aspergillus, Agaricus compestris (Mushroom) etc.

 Examples Of Fungi

Many fungi cause diseases like skin diseases and spoil the food items. Some of the Mushroom varieties are edible. The branch of science which deals about the study of fungi is called mycology.

3.    Algae (Singular – Alga)-Algae are mainly aquatic plants that are found in water and moist surfaces. They are green, red and brown in colour. They are unicellular as well as multi-cellular. They are prokaryotic as well as eukaryotic organisms. The unicellular prokaryotic algae are called blue green algae or Cynobacteria.
Chlamydomonas have two flagella that help in swimming. Volvox is a movable colony. Some are as long as 80 feet. Some algae are edible. Iodine is obtained from brown algae. Nostoc and Anabena helps in nitrogen fixation. Spirogyra has filamentous multi-cellular body and has spiral chloroplast. The study of algae is called algology or phycology.

4. Protozoa (Singular-Protozoan)-They are unicellular, eukaryotic microscopic organisms. They are found in water and body of living organisms. They travel by some special structures of their body like- Cilia (hair like), Flagella (Whip like) or Pseudopodia i.e. false foot. e.g.- Amoeba, Paramecium, Entamoeba, Trypanosomaetc. Many of them cause diseases like Entamoeba histolytica causes amoebiasis due to drinking of contaminated water and Trypanosoma causes sleeping sickness. The study of protozoa is called protozoology. Protozoa are members of Protista Kingdom.  

5. Viruses (Singular- Virus) – Viruses are ultra microscopic, non-cellular and intensely infectious microorganisms. They are made up of genetic material (D.N.A. /R.N.A.) and Protein.  They have either D.N.A. or R.N.A but never both. They are non-living (dead) outside the body of living organisms but becomes alive, active and start multiplying in the body of living organisms. So, viruses are called connecting link between livings and non-livings. Viruses are like biological puzzle whether to be called living or nonliving. e.g.- Corona Virus, AIDS (Acquired Immune Deficiency Syndrome) Virus or HIV (Human Immunodeficiency Virus) , SARS Virus, Polio Virus, Tobacco mosaic Virus (First discovered virus by Dmitri Ivanovsky in Russia),  Influenza virus  etc.

Microorganisms and the Human Beings– Microorganisms are useful as well as harmful for the human beings. From ancient period useful microbes are used by humans to make, curd, wine, Idli, Dosa etc. Many microbes are harmful and cause diseases in humans.
Useful Activities of microbes
1.     In making of curd and cheese- The curd is made from milk. The process of making curd is called curdling. Lactobacillus bacteria sour the milk and help in curd formation. Streptococcus bacteria are also found in curd. Cheese is also prepared by Lactobacillus bacteria.

2.   In making of bread- Yeasts are used to make bread from flour, and lukewarm water.  The dough of flour is prepared and yeasts are added. Yeasts do fermentation of sugar of dough to produce carbon dioxide gas and ethyl alcohol. The bubbles of carbon dioxide make the dough fluffy and spongy. After baking the dough the bread is prepared.
Fermentation– It is an anaerobic decomposition of carbohydrate (sugar) containing substance by yeasts to form carbon dioxide gas and ethyl alcohol. Fermentation is known to humans many centuries earlier but it was Louis Pasteur who discovered fermentation in 1857. Fermentation is used to prepare wine, beer and toddy like alcoholic beverages. It is used in making of bread, Idli and Dosa.  
3.   In making of Idli and Dosa- Idli and Dosa are prepared from the paste of black gram. This paste is kept overnight or for 8-10 hours for fermentation in air. Streptococcus bacteria help in this process. The carbon dioxide gas released in this process makes the paste fluffy and spongy. After steaming or baking the fermented paste, Idli and Dosa are prepared.  
4. In making of sausages- By fermenting chicken, pork like meat sausages are prepared. They last longer comparatively.
5.   In making of beverages- Ethyl alcohol is found in beverages like wine, beer, rice beer and toddy. Ethyl alcohol is formed due to fermentation of sugar containing substances by yeast. Fermentation is an anaerobic process i.e. no oxygen is required in it. Sugarcane, grapes, rice, molasses, other fruits are used to prepare wine. Barley is used to prepare beer. Toddy is prepared by fermenting toddy palm and coconut water.
6.   Agar Agar- Agar Agar is obtained from seaweeds i.e. marine algae. It is used as health supplement. It is used for growing of bacteria and fungi in laboratories. It is used in making of puddings, custards, sauces etc. 

7.  Spirulina – Spirulina is a biomass of Cynobacteria (earlier called as blue green algae). They are rich in protein; therefore, Spirulina is used as protein supplement by humans and some aquatic animals.
8.    Cleaning environment and humus formation- Humus is dark colored organic matter formed by decomposition of dead and decaying plant and animal debris. The humus is rich in nutrients and when mixed with soil, increase the water holding capacity, aeration, and improve soil texture. The bacteria and fungi decompose the dead plants and animals. They are called decomposers.
9.   Nitrogen Fixation and increasing soil fertility by some bacteria and blue green algae- Nitrogen is most essential soil nutrient for plants. Some bacteria like Rhizobium shows symbiotic association within root nodules of leguminous plants like pea, gram, mustard, Linseed, soybeans etc.  These bacteria help in fixing atmospheric nitrogen in the soil which is taken by plants. Blue green algae (Cynobacteria) like Anabena and Nostoc also fix atmospheric nitrogen in soil. Blue green algae work as biofertilizers.
10.  In making of Antibiotics- Antibiotics are medicines to cure bacterial infections and diseases. The first antibiotic ‘Penicillin’ was discovered by Alexander Fleming in 1929 at London. Penicillin was called ‘wonder drug’ because of curing T.B. (Tuberculosis) patients at that time. Antibiotics are prepared from bacteria and fungi. E.g. Penicillin, Tetracycline, Streptomycin, Erythromycin, Chloramphenicol, Aureomycin etc. Antibiotics are given to patient in form of tablet, capsule or injections.  
Precautions while taking antibiotics-
a.  Antibiotics should be taken only on the prescription of a qualified doctor.
b. The course of taking antibiotics must be completed as prescribed by the doctor.
c. When unnecessarily or wrongly taken the efficacy of antibiotics becomes less when needed next time.
d. They have side effects and kills beneficial bacteria of body if wrongly and unnecessarily taken.
11.   Vaccines– when a pathogen (disease causing microbe) enters inside the body, the body prepare antibodies to fight the pathogen. The pathogen is called antigen. If there is victory of antibodies over antigen, then body remembers how to fight against the same pathogen if it enters the body once again. The antibodies remain inside the body. This is how vaccines work.
In vaccine a low strength (usually heat killed) pathogen is administered orally of by injection in the body. So, the body becomes immunized by vaccination. If the same pathogen infects the body, there will be no disease. Vaccination causes immunization.
Edward Jenner (An English Doctor) discovered vaccine for smallpox disease in 1798.
Harmful activities of microbesMany microorganisms are harmful to humans, animals and plants. The disease causing microbes are called pathogens. Microbes spoil the food, clothes and leather. Let us study in details-
1.   Disease causing microbes in humans– The pathogens enter in the human body through air, water, or food. Pathogens (germs) also enter the human body by contact of infected humans, animal, soil or any other source of infection. Those diseases which spread from an infected person to a healthy person by air, water, food or physical contact are called communicable diseases. e.g.- Cholera, Common cold, Chicken pox, tuberculosis etc. The modes of communication of diseases are different.
The animals and insects act as carrier of diseases. They help in spreading of diseases. Carriers of diseases are also called as vectors.
Example 1-Flies like house flies sit on garbage and animal excreta and pathogens present in that stick to their bodies. When houseflies sit on food items, the pathogens come to food. On eating such contaminated food the person becomes sick. We should avoid eating of uncovered foods.
Table 2.1  Some common Human Diseases caused by Microorganisms
Contaminated Water/ Food
Contaminated Water
Maintain personal hygiene and sanitary habits. Eat well cooked food and boiled drinking water. Vaccination
Chicken Pox
Air/Physical contact
Air/ Contaminated water
Complete isolation of patient, Keeping away of personal belongings of patients. Vaccination
Contaminated water
Drinking boiled water. Vaccination
use mosquito repellent, No water logging and spray Insecticide in surrounding
Example 2- Female Anopheles mosquito is the carrier of malaria parasite (Plasmodium vivex). Plasmodium vivex is a protozoan. When it bites the healthy person, the malaria parasite enters the body and causes malaria disease.
Example 3- Female Aedes mosquito is the vector of dengue virus.
2.  Disease causing microbes in animals– A number of diseases are caused by microbes in humans. e.g.- Anthrax is a bacterial disease caused in humans and cattle. Robert Koch (1876) discovered the Bacillus anthracis bacterium which causes anthrax disease.  Foot and mouth disease of cattle is caused due to a virus.
3.     Disease causing microbes in plants– The crop plants, cereals, pulses, vegetables, fruits are infected by plant pathogens. Due to this crop productivity reduces. Mainly bacteria, fungi and viruses cause diseases in plants. By using fungicides, insecticides, other chemicals and methods disease instances can be reduced and checked.
Plant Disease
Mode of Transmission
Tobacco mosaic
Contact by body and clothes
Citrus canker
Rust of wheat
Air, Seed
Late blight of potato
Air, Water

4.    Food poisoning– The uncovered and long time prepared food often get infected by microbes like bacteria and fungi. These microbes release toxic substances which make the food poisonous. On consumption of such foods the person becomes seriously ill and even death may be caused. So, the preservation of food to prevent it from being spoil is essential.
FOOD PRESERVATION METHODS– The unpreserved food is attacked by bacteria and fungi. The spoiled food has foul smell, colour change and change in taste. It is not safe to eat such soiled food. But, the pickles do not spoil easily because some preservatives are added in it. Let us discuss some of food preservation methods.
1.   Preservation by Chemicals – Those salts which are used to prevent growth of microbes in food items are called food preservatives. e.g. Sodium benzoate  and Sodium matabisulphite. They are added in pickles, jams, squashes and other canned foods. 
2.   Preservation by common edible salt – Salts control the growth of bacteria and fungi. Meat, fish, amla (richest source of vitamin C), raw mango, pickles, tamarind, etc. are used to preserve by salting.
3.    Preservation by sugar- Sugar helps in reducing the moisture content in the food. So, the growth of bacteria and fungi is inhibited and food does not spoil. E.g. Jams, Jellies and Squashes are commonly preserved by sugar. Murraba (made up of amla fruits) and sweets have huge amount of sugar to prevent their spoilage.
4.      Preservation by oil and Vinegar- Edible oils checks the contact of oxygen with air and makes the environment in such way that prevents growth of bacteria. 5 % acetic acid is called vinegar. Vinegar is used to prevent growth of bacteria and fungi. So, oils and vinegar are used to control spoilage of pickles, vegetables, fruits, fish and meat.

5.   Heat and cold treatment- Heating and boiling kills many microbes. The milk which is heated at 70 degree C temperature for 15 to 30 seconds and suddenly chilled and stored is called pasteurised milk. Such milk can be consumed even without boiling as it is free from germs. Pasteurisation was discovered by Louis Pasteur. In refrigerators the temperature is reduced. So, the bacteria and fungi becomes almost inactive and do not release enzymes for spoiling the food.
6.    Preservation by storing and packing- The air tight packets of food like dry fruits and vegetables prevent their spoilage. The nitrogen gas filled in chips packets also prevent its’ spoilage.  


7.   Preservation by smoke and sun drying- Smoke and sun drying reduces the moisture content in the food. So, the food and seeds last longer. E.g. Fish, meat, mushroom, rice, are commonly preserved by these methods.
NITROGEN FIXATION– There are some free living soil bacteria and bacteria that form symbiotic association with roots of leguminous plants help in fixation of atmospheric nitrogen in the soil. Rhizobium bacteria is found in the root nodules if leguminous plants. They fix nitrogen and in turn get the food. That part of root nodules has a pigment called leghaemoglobin. Cynobacteria (blue green algae) like Nostoc, Anabena, etc. fix the atmospheric nitrogen in soil. They are called biofertilizers.


NITROGEN CYCLE– The percentage of nitrogen in the atmosphere is 78 %. Nitrogen is very essential for living organisms as it is a constituent of proteins, vitamins, Chlorophylls, nucleic acids like D.N.A and R.N.A. The atmospheric nitrogen cannot be taken directly by plants and animals. Certain bacteria and Cynobacteria (blue green algae) fix the atmospheric nitrogen in the soil in form of nitrogenous compounds. In factories when chemical fertilizers are prepared and also during lightening, nitrogen is fixed in nitrogenous compounds.

The nitrogenous compound present in the soil is taken by plants along with water absorption. Nitrogen is then used to prepare vitamins, chlorophylls, nucleic acids and proteins. The nitrogen comes to animals when they eat them and their products. When plants and animals die, decomposers like bacteria and fungi release the nitrogenous compounds in the soil. Another kind of bacteria called denitrifying bacteria release the nitrogen from the nitrogenous compounds. Thus, the percentage of nitrogen remains almost constant in the atmosphere.
For more information visit the following websites- www.microorganisms.com

CHECK YOUR PROGRESS                     Time- 1: 15 Minute    Total Marks =32
Q.1 What are antibiotics? What precautions must be taken while taking antibiotics?-5
Q.2 What are carriers? Give two suitable examples of carriers and diseases carried by them.-5
Q.3 Name the organisms that fix atmospheric nitrogen.-2              
Q.4 Write a short note on usefulness of microbes.- 5
Q.5 Write a short note harms caused by microbes.-5
Q.6 Explain any five methods of food preservation.-5
Q.7 Explain the nitrogen cycle with a suitable diagram.-5
Q.1 Fill in the blanks.
(a) Microorganisms can be seen with the help of a …………………
(b) Blue green algae fix ………………….. directly from air to enhance fertility of soil.
(c) Alcohol is produced with the help of …………………….
(d) Cholera is caused by ……………………….
Answer- (a) microscope   (b) nitrogen   (c) yeast   (d) bacteria
Q.2Tick the correct answer:
(a) Yeast is used in production of –
 (i) Sugar                                             (ii) Alcohol
(iii) Hydrochloric acid                        (iv) Yeast
(b) Which of the following is an antibiotic-?
(i) Sodium bicarbonate                        (ii) Streptomycin 
(iii) Alcohol                                         (iv) Yeast
(c) Carrier of malaria causing protozoan is-
(i) Female Anopheles mosquito          (ii) Cockroach
(iii) Housefly                                       (iv) Butterfly
(d) The most common carrier of communicable disease is-
(i) Ant                                                   (ii) Housefly
(iii) Dragonfly                                      (iv) Spider
(e) The bread or idli dough rises because of-
(i) heat                                                 (ii) grinding
(iii) growth of yeast cells                    (iv) kneading
(f) The process of conversion of sugar into alcohol is called –
(i) nitrogen fixation                           (ii) moulding
(iii) fermentation                               (iv) infection
Answers- (a) Alcohol       (b) streptomycin                (c) female Anopheles mosquito  
                  (d) housefly    (e) growth of yeast cells    (f) fermentation

Q.3Match the organisms in Column A with their action in Column B.
             Column A                    Column B
(i)                  Bacteria                        (a) Fixing nitrogen
(ii)                Rhizobium                    (b) Setting of curd
(iii)               Lactobacillus                (c) Baking of bread
(iv)              Yeast                              (d) Causing malaria
(v)                A protozoan                   (e) Causing cholera
(vi)              A virus                           (f) causing AIDS
                                         (g) Producing antibodies
Answer- (i) –e        (ii) – a       (iii) – b    (iv) – c  (iv) – d  (v) – d    (vi) – f

Q.4 Can microorganisms be seen with naked eyes? If not, how can they be seen?
Ans- No, microorganisms cannot be seen with naked eyes. Microorganisms are so tiny that they cannot be seen by naked eyes. They can only be seen with the help of microscope.

Q.5 What are the major groups of microorganisms?
Ans- The microorganisms are broadly classified in to five major groups-Bacteria, Viruses, Algae, Fungi and Protozoa.

Q.6 Name the microorganism which can fix atmospheric nitrogen in the soil.
Ans- The Cynobacteria (Blue green algae), Rhizobium and some free living soil bacteria fix atmospheric nitrogen in the soil.

Q.7 Write 10 lines on the usefulness of microorganisms in our lives.
Ans- The usefulness of microorganisms are as following-
a.)    In making of curd and cheese the Lactobacillus bacteria are used.
b.)    In making of bread the yeast are used.
c.)     In making of Idli and Dosa yeast are used.
d.)    In making of sausages and fruit vinegar microbes are used.
e.)    Cleaning environment and humus formation occurs by bacteria and fungi.
f.)    Nitrogen Fixation and increasing soil fertility occurs by free living soil bacteria, Cynobacteria and Rhizobium bacteria.
g.)    In making of antibiotics some bacteria and fungi are used.
h.)  Vaccines are prepared from microbes. Vaccines contain antigen that develops immunity against diseases.
i.)  Spirulina is an alga which is rich source of protein. This is eaten as health supplement.
j.)    Agar Agar is obtained from marine algae. It is used as stabilizing and thickening agent for puddings, custard, sauces etc. Agar is also used as culture medium for growing bacteria, fungi in laboratories.
Q.8 Write a short paragraph on the harms caused by microorganisms.
Ans- Microbes are not only useful to plants, animals and humans but very harmful to these. Some microbes spoil food, clothes and leather. Microbes cause a number of diseases in plants, animals and humans.

Q.9 What are antibiotics? What precautions must be taken while taking antibiotics?
Ans- Antibiotics are medicines to cure bacterial infections and diseases. e.g. Cholera, Anthrax, Typhoid, Tetanus, Tuberculosis, Pneumonia, whooping cough etc.The following precautions should be taken while taking antibiotics-
a.   Antibiotics should be taken only on the prescription of a qualified doctor.
b.  The course of taking antibiotics must be completed as prescribed by the doctor.
c.  When unnecessarily or wrongly taken the efficacy of antibiotics becomes less when needed next time.
d. They have side effects and kills beneficial bacteria of body if wrongly and unnecessarily taken.
Disclaimer– The contents written in this chapter are based on various sources. Any similarity will be considered as a coincidence.
Suggestions for further improvement are most welcomed.

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