CHAPTER - 2 MICROORGANISMS : FRIEND AND FOE

1
CHAPTER - 2MICROORGANISMS FRIEND AND FOE
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 (Ladys finger) Virus
  Water

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
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.