1. CHAPTER - 2 MICROORGANISMS : FRIEND AND FOE
VIKAS YADAV

2. 1) Microorganisms (Microbes) :-
i) Microorganisms are very small organisms which cannot be seen with the unaided eye. They can be seen only with a magnifying glass or microscope.
ii) Microorganisms may be unicellular or multicellular.
iii) Microorganisms may exist alone or in colonies.
iv) Microorganisms are found in all kinds of environment like ice cold climate, hot springs, deserts, marshy lands etc. They are also found inside the bodies of other organisms.

3. 2) Types of microorganisms :-
There are four main types of microorganisms. They are :- bacteria, fungi, algae and protozoans.
Viruses are also considered as microorganisms. They are different from other microorganisms because they reproduce only in the body of host organisms like bacteria, plants or animals.
Eg :- Algae – Chlamydomonas, Spirogyra etc.
Fungi – Bread mould, Penicillium, Aspergilles etc.
Protozoans – Amoeba, Paramaecium etc.

4. Virus

5. 3) Friendly microorganisms (Useful microorganisms) :-
i) Microrganisms like bacteria decomposes organic waste into manure and increases the fertility of the soil.
ii) Microorganisms help in fixing atmospheric nitrogen into nitrogen compounds in the soil and increases the fertility of the soil.
iii) Microorganisms are used for making curd from milk, for making cheese, pickles, bread, cakes, pastries, alcohol, wine, vinegar (acetic acid) etc.
iv) Microorganisms are used for making medicines.
a) Making curd from milk :- The bacterium called Lactobacillus reproduces in milk and helps to convert milk into curd.
b) Making bread :- The fungus called yeast reproduces in flour dough and produces carbon dioxide during respiration which makes the dough soft and helps in making bread, cakes, biscuits, pastries etc.
c) Making alcohol :- The fungus called yeast reproduces in sugar solution and converts it into alcohol. This process is called fermentation.
d) Making medicines :- Some bacteria and fungi are used to make medicines which kill or stops the growth of disease causing microrganisms. Such medicines are called antibiotics. Eg :- streptomycin, tetracycline, erythromycin etc.
Vaccines are dead or weakened microbes introduced into the body to produce antibodies. These antibodies protect the body from disease causing microbes. Diseases like polio, cholera, typhoid, small pox, hepatitis etc. can be prevented by taking vaccines.

6. 4) Harmful microorganisms :-
i) Some microorganisms cause diseases in plants and animals. Such disease causing microorganisms are called pathogens.
ii) Some microorganisms grow in food substances and produce toxic substances and makes the food poisonous. Food poisoning causes illness and even death.
iii) Some microorganisms spoil materials like clothing, leather, wood etc.
5) Disease causing microorganisms in Humans :-
Disease causing microorganisms enter our body through air, water, food, contact or insects. These microorganisms are called pathogens.
Diseases which can spread from an infected person to a healthy person are called communicable diseases.
Eg :- cholera, common cold, chicken pox, tuberculosis etc.
Some insects and animals act as carriers of disease causing microbes.
Eg :- House fly, Female anopheles mosquito (malaria), Female aedes mosquito (dengue fever) etc.

7. Female anopheles mosquito
Aedes mosquito

8. 6) Disease causing microorganisms in plants :-
Microorganisms cause diseases in plants like rice, wheat, potato, sugarcane, apple, orange etc. These diseases reduce crop yield. They can be controlled by using chemicals which kill the microbes.

9. Some common human diseases caused by microorganisms:-
Human disease Causative Mode of
microorganism transmission
Chicken pox Virus Air / Contact
Polio Virus Air / Water
Measles Virus Air
Hepatitis – B Virus Water
Tuberculosis Bacteria Air
Typhoid Bacteria Water
Cholera Bacteria Water / Food
Malaria Protozoa Mosquito
Some common plant diseases caused by microorganisms :-
Plant disease Causative Mode of
microorganism transmission
Citrus canker Bacteria Air
Rust of wheat Fungi Air / Seed
Yellow mosaic of
Okra (Lady’s finger) Virus Water
bhindi

10. 7) Food preservation :-
Food is spoilt by microorganisms. Spoilt food has bad smell and bad taste and causes food poisoning. Food can be preserved by protecting it from microorganisms. Food can be preserved by different methods. They are :-
i) Chemical method :- Food like pickles can be preserved by using chemicals like salts and edible oils. Jams and squashes can be preserved by using sodium benzoate, sodium metabisulphite. These chemicals are called preservatives.
ii) By using common salt :- Food items like fish, meat, amla, raw mangoes, tamarind etc can be preserved by using common salt. It prevents the growth of microbes.
iii) By using sugar :- Food items like jams, jellies, squashes etc. can be preserved by using sugar solution. It prevents the growth of microbes.
iv) By using oil and vinegar :- Food items like pickles, vegetables, fish, meat etc. are preserved by using oil and vinegar. It prevents the growth of microbes.
v) By heat and cold treatments :- Heating food items kills microbes. Similarly storing food items at low temperatures prevents growth of microbes.

11. Pasteurisation :- The process of heating milk to about 70oC for 15 to 30 seconds and then suddenly chilling it to prevent the growth of microbes is called pasteurisation. This process was discovered by Louis Pasteur.
vi) By storage and packing :- Many food items are stored in air tight containers to protect them from microbes.

12. NITROGEN FIXING BACTERIA FIX ATMOSPHERIC NITROGEN
8) Nitrogen cycle :-
ATMOSPHERIC
NITROGEN
LIGHTNING FIXES
NITROGEN
BACTERIA TURN
COMPOUNDS OF
NITROGEN INTO
GASEOUS NITROGEN
NITROGEN FIXING BACTERIA
AND BLUE GREEN ALGAE
FIX ATMOSPHERIC NITROGEN
UPTAKE BY
PLANTS
ANIMAL EATS
PLANTS
NITROGENOUS
WASTE FROM
EXCRETION
AND DEATH
COMPOUNDS OF
NITROGEN IN THE SOIL

13. Nitrogen cycle :-
The nitrogen in the atmosphere is converted into nitrogen compounds in the soil by nitrogen fixing bacteria and blue green algae. Lightning also converted into nitrogen compounds in the soil. The nitrogen compounds in the soil is used by plants for the synthesis of proteins and other compounds. Animals feeding on plants get these proteins and other compounds. When plants and animals die, bacteria and fungi in the soil converts the nitrogenous waste into nitrogen compounds in the soil which are again use by plants. Some other bacteria converts some nitrogen compounds in the soil into nitrogen gas which goes back into the atmosphere. Hence the nitrogen in the atmosphere almost remains constant.