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The Microbial World
BACTERIA
Some scientists and their contributions to the discovery and knowledge of
microorganisms:
Name of Scientist
Contribution
Anton Von Leeuwenhoek He first saw microbes with his newly developed
microscope.
Louis Pasteur
He was the father of bacteriology. He discovered that
microorganisms make food go bad and he invented a
method of preserving dairy products by pasteurisation.
Robert Rock
Designed many techniques of handling bacteria. He
thoroughly studied the disease anthrax in cattle.
Lister
He found the technique of disinfecting the operating
theatre with carbolic acid.
Pathogens
Disease-causing microbes are called pathogens.
Microbes and their habitat
Microbes are very tiny organisms that cannot be seen with the naked eye but only
with the help of a microscope. Microbes are found everywhere – sea, ponds, snow,
clothes, food, etc. This is their habitat.
Survival of Bacteria in Unfavourable conditions
Many bacteria are good at surviving in unfavourable conditions such as drought, cold,
heat, antiseptics and even toxic substances. They are able to do this because at the
time of the unfavourable conditions, they form a protective coat around the cell called
a cyst.
Useful and Harmful bacteria
1. Food Production (Acetobactor produces vinegar)
2. Antibiotics (Streptococcus produces Streptomycin)
3. Decomposition of dead plants and animals (dead organic matter), thus
cleaning up the environment, adding nutrients in the soil and giving us biogas
fuel.
4. Cellulose digesting bacteria in stomach of cattle.
5. Nitrogen-fixing bacteria (Rhizobium)
1. Plant diseases like bacterial wilt in which the Xylem tissue is blocked and
dries up and dies or water does not reach the tissues.
2. Some bacteria spoil meat, milk, vegetables, etc.
3. Some diseases are caused in humans and animals by bacteria.
EG: Anthrax, Syphilis, Boils, Sore throat, and Typhoid.
VIRUSES
Bacteriophage
Bacteriophage is a virus that attacks bacterial cells. It has a polygonal head made up
of proteins inside which is present genetic material in the form of DNA or RNA. The
head is attached to the tail with the help of a contractible neck. The tail has fibres for
attachment to the host cell.
Activities of virus inside the body for reproduction
The viruses cannot reproduce independently. They need living cells on hosts. Once a
virus attaches itself to its host cell, it injects the genetic material into the cell. This
genetic material instructs the host cell to manufacture more viruses. In this way, the
virus multiplies itself inside our cells. Finally, the host cell dies, releasing the viruses,
which can attack other healthy cells now.
Viruses: On the borderline of the living and non-living world
1. They do not have a cellular structure
2. They cannot grow or reproduce outside a living host cell.
3. They can be crystallized (they are cellulose particles and not cells).
4. They have a protein coat and some genetic material in the form of DNA and
RNA.
5. They are too small to be seen with the help of a light microscope. Their size
range is 0.15 to 0.20 micron.
FUNGI
1. Fungi are plant-like organisms that do not contain chlorophyll.
2. Fungi are either saprophytes or parasites and both types secrete enzymes to
digest external organic matter.
3. The cell wall of fungi is made up of fungal cellulose and within is a living
cytoplasm containing many nuclei.
4. Fungi can thrive in dark, moist places and most of them tolerate a greater
temperature variation than bacteria. Warmth stimulates the growth of many
fungi.
5. Fungi may be unicellular or multicellular. There are four main types of fungi:
- Yeasts are unicellular fungi.
- Mildews are also unicellular fungi
- Moulds are multicellular fungi. They consist of a network of branching
threads called mycelium (or hyphae). They can be seen growing on bread or
rotting fruits.
- Mushrooms are multicellular fungi formed of interwoven hyphae. They can
be seen growing on dead weeds in the rainy season.
Harmful Fungi
Name of disease
Potato Blight
Mildew Attack
Effect
Brown patches on potato leaves
Wet or dark patches on leaves & fruits of flowering plants.
Also affects cereals.
Rust (Puccinia)
Red/brown stripes on leaves of cereals.
Ringworm
Ring-like patches on skin of humans; can also cause hairfall.
Athletes’ foot
Itching in the toes of humans.
Some fungi spoil food, leather and wood. Moulds and yeast can grow on food items
and make them unfit for consumption.
Useful Fungi
Name of fungi
Mushrooms &
toadstools
Yeast
Some moulds
Penicillin
Decomposer fungi
Use
Edible
Used for preparation of alcoholic drinks by fermenting the
fruit juice and sugars present in cereals
They are used for making bread fluffy and soft and for
making different flavours of cheese.
Used to make antibiotics to cure diseases
They breakdown dead bodies of plants and animals and
return the nutrients back to the soil.
Reproduction
Yeasts reproduce by budding. Moulds, mildew and mushrooms reproduce by asexual
spore formation. Spores are very tiny single cells that can grow into a new organism.
Yeast
Yeasts are unicellular.
They are round or oval.
Mould
Moulds are multicellular.
Moulds have a filamentous shape and
each is called hyphen.
They can aerobic or anaerobic and Moulds are aerobic.
produce
an
acid
with
alcohol
(fermentation)
Eg: Brewer’s yeast, Baker’s Yeast
Eg: Rhizopus, mucor
The first anti-biotic extracted was penicillin from a blue-green mould called
penicillium by Dr. Alexander Fleming.
Common Cold
A virus causes common cold. When a person having cold, sneezes he gives out a
number of viruses along with mucus. These viruses are present in the surrounding air,
on his handkerchief, clothes, hands, etc. Therefore, when a healthy person breathes
that contaminated air, shakes hands with him or uses his handkerchief, the viruses are
transmitted and the healthy person is infected.
ALGAE
Algae are autotrophs, which means they can produce their own food with the help of
carbon dioxide, water and photosynthetic pigments. We classify algae on the basis of
the colour of their photosynthetic pigment.
Eg: Green Algae
Different types of algae
1. Spirogyra – Green Algae
2. Volvox colony – Unicellular green algae in colony
3. Chlamydomonas – Unicellular brown algae
Habitat
Algae are present in wet or moist places, fresh or salty water. They can be seen on the
surface of water on shores, on moist walls, on moist bark of trees, on wet floors and
on moist rocks, making them slippery. Algae are as abundant in water as grass on
land. Therefore they are also known as grasses of many waters. Algae are the major
producers in water environments. Diatoms are unicellular algae having a special cell
wall containing silicon and manganese.
Reproduction
Algae can reproduce through fission, which means splitting up of cells, eg:
chlamydomonas. It can also occur through fragmentation in which, the body of a
multicellular alga breaks down into many parts and each new part develops into a new
individual.
Eg: Spirogyra
Algae can also reproduce sexually and under unfavourable conditions can also
perform spore formation.
Harmful Effects
1. When algae grow in water, turns green and checks sunlight, so the lower
layers not receiving sunlight will become dark.
2. The places where algae grow become slippery.
3. Excess algal growth reduces the oxygen content in water. It is called
eutrophication. It leads to death of all life in the water.
4. The water containing algae is unfit for drinking for humans and animals.
Uses
1. Algae are the producers of the eater world providing food for animals in water.
2. Some of the algae are used as human food also:
Eg: Kelp – a rich source of iodine
China grass is used for making jellies and puddings
Diatoms provide silica for glass, porcelain, bone china and filters.
3. Some algae provide materials for paints, medicines, dyes, etc.
4. Algae gives out oxygen just like other plants, purifying the water environment.
5. Agar gel extracted from the red algae Agar-agar help in making culture
medicine in laboratories.
Fungi
Heterotroph (saprophyte or parasite)
Does not have synthetic pigment
Types – yeasts, mould, mildews,
mushrooms
Algae
Autotroph
Has photosynthetic pigment
Types – Green alga, red alga, blue-green
alga
PROTOZOA
Protozoa means ‘first animals’. The main features are:
1. They have a single cell in their body.
2. The cell wall is absent.
3. They have loco motorized parts that are organic like pseudopodia in amoeba
and cilia in paramecium, flagella, etc.
4. Most of them are heterotrophs only some are autotrophs.
5. They live in moist or wet places.
6. They reproduce through binary fission.
Eg: Amoeba, paramecium, and multiple fission, like plasmodium.
Under unfavourable conditions, they reproduce through spore formation.
Diseases caused by protozoa
Protozoa
Disease
Plasmodium
Malaria
Trypanosome
Sleeping sickness
Leishmania
Kala-azar
Giardia
Giardiasis
Entamoeba hystoletica
Amoebiasis
Cause/Vector
Female Anopheles
Tsetse fly
Sand fly
Contaminated
Food & Water
Useful Protozoa
1. Most animals do not possess enzymes to digest the cellulose in the walls of
plant cells. However, some species of protozoa do. These live in guts of
animals and digest the cellulose in food.
2. The blue whale is the largest animal that has ever lived. It feeds on shoals of
fish as well as minute protozoa called zooplankton
COMMUNICABLE DISEASES
These diseases are transmitted from an infected person to a healthy person through the
following methods:
1. Air: There are called airborne diseases.
Eg: Mumps, pneumonia, influenza, scarlet fever, and diphtheria.
2. Water and food: They are called waterborne or foodborne diseases.
Eg: Cholera, diarrhoea, jaundice, typhoid, and dysentery.
3. Direct contact and clothes: They are contagious diseases.
Eg: Fungal infections, ringworm, athlete’s foot, scabies, and leprosy.
4. By cuts, wounds, and blood: They are caused by cuts and wounds.
Eg: Tetanus, staphylococcus infection, hepatitis, and AIDS.
5. By animals and insects: An animal or insect carrying pathogens of a specific
disease is known as a vector.
Disease
Malaria
Dengue
Plague
Sleeping Sickness
Rabies
Vector
Female Anopheles
Female Aides
Rat, rat flea
Tsetse fly
Dogs and other animals
6. Through Sexual activity: They are sexually transmitted diseases.
Eg: AIDS, Hepatitis B, Syphilis, Gonorrhoea, sleeping sickness.
How microbes cause diseases
1. By tissue destruction: For example, the tuberculosis bacteria multiply in the
lung tissues and destroy the cells. The poliovirus attacks and destroys the cell
bodies of motor neurons in the spinal cord.
2. By producing toxins: The poisons produced by germs are absorbed by the
blood stream and affects other parts of the body, example: the tetanus
organism living in a wound in the foot produces a poison that causes paralysis
in the upper region of the body.
Prevention of diseases
1. Control of germs and vectors:
(a) Maintaining personal and domestic hygiene.
(b) Checking the growth and breeding of mosquitoes, flies and cockroaches,
etc. By using insecticides, disinfectants, antiseptics, etc.
(c) Keeping the food covered and properly preserved.
(d) Keeping the neighbourhood and surrounding clean by avoiding stagnation
of water and accumulation of garbage.
(e) Enhancing the natural immunity of the body. Our bodies are capable of
fighting infections due to presence of WBC. They can make antibodies
that can destroy or ingest the foreign particles or germs (antigens). Due to
this antigen-antibody reaction, our immune system can prevent diseases in
our body. The capacity of a living body to fight infections is known as
immunity. It can be enhanced by taking a balanced diet, exercising and
relaxing, living in a clean environment and regular vaccination.
2. Vaccination:
Vaccination or immunization is a technique to develop resistance to a
particular disease in a living body. A vaccine is a small quantity of dead or
weak germs or toxins produced by these germs, on being given through
injection or orally, it helps in developing immunity against a particular
disease.
Eg: DPT – Diphtheria, Pertussis, Tetanus
MMR – Mumps, Measles, Rubella
BCG – Tuberculosis
Typhoid, chickenpox, hepatitis B, polio
Storage and preservation
1. Sun Drying: This is an old method of preserving food and clothes. Usually, all
clothes, shoes and books are kept out in the sun especially after rainy season.
2. Chemical Treatment: Wood, leather, textiles, paper and even grains are
chemically treated to stop the growth of microbes. Insecticides and pesticides
are used for this purpose. You may have seen naphthalene balls used in
cupboards; especially with woollen clothes is an old method of preserving
food and clothes. Usually, all clothes, shoes and books are kept out in the sun
especially after rainy season.
3. Preservatives: These are added to prevent microbial growth and enable food to
be stored for a longer time.
Eg: Sodium Benzoate and sodium meta-bisulphate are added to jams and
squashes
Vinegar and oil are used in pickling.
Salt or lime is added to fish. Salt works by drawing water out of the cells of
microbes, which kills them.
4. Pasteurisation: Milk is heated to 63ºC for 30 minutes or 72ºC for 15 seconds
and then cooled rapidly to 10ºC. This kills a wide variety of microbes, which
cause souring of milk. It is then packed in airtight containers such as bottles of
polyethylene bags.
5. Refrigeration: Low temperatures reduce microbe activity but do not stop it
altogether. It is suitable for short-term storage only. Meat, fish, vegetables and
fruits stay longer in a refrigerator.
6. Freezing: A great variety of food is marketed in the frozen form and large
quantities are shipped across the world in this state. Freezing does not kill
microbes they simply stop growing. Spore-bearing organisms are particularly
resistant to freezing. As frozen food is removed from the freezer, it begins to
thaw and bacterial growth commences immediately. So it has to be eaten as
soon as possible.
7. Dehydration: If water, which is essential for growth of microbes is removed
from food, the food is said to be dehydrated. It lasts for a long time but should
be kept in airtight containers. Fish or fruits can be preserved in this way.
8. Sterilisation: Food is boiled or heated to a high temperature to kill the
microbes. That is why well-cooked food keeps for a longer time. Food is
sterilised by heating it with steam to 150ºC. It is then stored in airtight
containers such as cans or bottles.
9. Radiation: Radioactive rays can also kill bacteria and other microbes. This
provides a very modern way of preserving food.