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
2) TYPES OF MICROORGANISMS :-
There are four main types of microorganisms. They are :- bacteria, fungi, algae and
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.
E.g. :- Algae – Chlamydomonas, Spirogyra etc.
Fungi – Bread mould, Penicillium, Aspergilles etc.
Protozoans – Amoeba, Paramaecium etc.
3) FRIENDLY MICROORGANISMS (USEFUL MICROORGANISMS) :-
i) Microorganisms 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
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 microorganisms. Such medicines are called antibiotics. E.g. :-
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.
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
E.g. :- cholera, common cold, chicken pox, tuberculosis etc.
Some insects and animals act as carriers of disease causing microbes.
E.g. :- House fly, Female anopheles mosquito (malaria), Female Aedes mosquito (dengue
SOME COMMON HUMAN DISEASES CAUSED BY
Human disease Causative Mode of
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
Citrus canker Bacteria Air
Rust of wheat Fungi Air / Seed
Yellow mosaic of
Okra (Lady’s finger) Virus Water
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
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
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
v) By heat and cold treatments :- Heating food items kills microbes.
Similarly storing food items at low temperatures prevents growth of
Pasteurization :- 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 pasteurization. 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.