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Chapter 17
Name: ___________________
Microbiology Notes Package
Viruses:
History
 Viruses were first discovered about 100 years ago.
 Tobacco plants were getting diseased. The leaves of diseased plants were crushed and the juices were spread on
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the surface of healthy leaves.
The juice caused the healthy leaves to become diseased which led researchers to the conclusion that something in
the juices was causing the disease.
They filtered the juices so that not even cells could pass through, but the juices still caused disease.
Researchers concluded that there must be some particles that are responsible for the disease and named them
viruses.
It is likely that viruses originated after cells and may have even started as pieces of a cell’s genetic material that
evolved over time.
In 1955, two researchers showed that the RNA and protein from a tobacco mosaic virus would assemble by
themselves when put together.
Living or Non-living?
 Viruses are not cells. They are a noncellular particle made up of genetic material and protein that can invade living
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cells.
All living things share 5 characteristics:
1. Living things are made up of cells
2. Living things reproduce
3. Living things grow and develop
4. Living things obtain and use energy
5. Living things respond to their environment
Against: Viruses are not made of cells, Viruses do not grow or develop, Viruses do not obtain or use energy,
Viruses do not respond to their environment.
For: Viruses do reproduce, but not independently. Viruses require a host to reproduce.
Since viruses require living host cells in order to survive, they are known as parasites.
The disease causing abilities of viruses makes them one type of pathogen.
Structure of Viruses
 A typical virus is composed of a core of nucleic acid (genetic material can be DNA or
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RNA) surrounded by a protein coat called a capsid.
Some viruses (like the bacteriophage seen in the picture) are composed of a head and
tail region. The head includes the capsid and genetic material, and the tail includes the
sheath and tail fibres.
Viruses come in a large variety of sizes and shapes. Some are complex like the
bacteriophage and others are simple like helices and cube shaped. Viruses can range in
size from 20 – 400 nanometers.
Infection
 Viruses infect a host in order to reproduce. However, the virus is only activated when it comes into contact with
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the right host cell – by chance.
The cells they reproduce inside are known as host cells.
Viruses only infect the right kind of host cell. The T4 bacteriophage has molecules on its tail fibers that attach to
the surface of the bacterium.
There are two main methods of infection: The lytic cycle and the lysogenic cycle.
The Lytic Cycle
 Once the virus attaches to the right host cell, the genetic material is injected into the host cell.
 The host cell recognizes the viral genetic material as its own and starts producing proteins just like it does using its
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own DNA.
The proteins made from the viral genetic material start attacking the host cell.
The host cell then makes thousands of copies of the genetic material and viral proteins until the host cell is full of
viral genetic material and viral proteins.
New virus particles are put together and the cell lyses (bursts) and the new viruses go and infect more cells.
The Lysogenic Cycle
 The virus attaches to the right host cell
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and injects its genetic material into the
cell.
The genetic material incorporates into
the host cell genome and can stay there
for generations. This incorporated
genetic material is known as a
prophage.
The presence of the prophage may
inhibit further infection from other
viruses and may also have some
positive effects on the host cell.
The prophage will remove itself after a
while and continue to produce new
virus particles.
Once the cell fills with new viral particles, the cell lyses and the new viruses are released.
The Immune Response to Viruses
 Primary line of defence
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Any part of the human body that passively or non-specifically prevents infection
Examples: the skin acts as a barrier against viral infection. Sweat and tears are a way for the body to flush
away pathogens.
o The human nose has ciliated mucous membranes to capture pathogens as they are breathed in.
Secondary line of defence (also known as the Inflammatory response)
o Includes any non-specific mechanisms that the body has against infection.
o This includes certain molecules in our bodies such as macrophages which are white blood cells that
engulf anything foreign that enters our bodies.
o Upon inflammation, the body begins to produce more white blood cells and also increases the body’s
temperature (fever)
o The increase in white blood cells means more phagocytes (also white blood cells that fight infection) to
engulf pathogens. The increase in body temperature also may stop certain pathogenic processes as these
processes only occur at a narrow temperature range, thus stopping the growth of certain microorganisms.
o Another secondary line of defence that is specific to viral infection is the production of proteins known as
interferons. This protein is produced by virally infected cells and helps surrounding cells resist viral
infection.
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Tertiary line of defence
o The third line of defence is specific to the pathogen responsible for infection.
o This line of defence is triggered by the presence of specific substances known as antigens. Antigens can
be surface proteins, lipids, or carbohydrates found on viruses, bacteria, or other pathogens.
o The key to this line of defence are a specific white blood cell known as lymphocytes. B-lymphocytes (a
type of lymphocyte known also as a B-cell) produce special proteins called antibodies that bind to the
antigens of the pathogen.
o Antibodies are shaped like a ‘Y’ and contain two identical binding sites that are specific to a particular
antigen. The purpose of these antibodies is to link viruses together into
a large mass known as agglutination. This makes the entire complex
much too large to enter any cell and also makes it easier for phagocytes
to engulf the pathogens.
o B-cells contain genes that are constantly rearranging so that the types of
antibodies being produced vary. In this way there is most likely an
antibody for any type of antigen.
o Upon B-cell activation, the B-cells that produce the proper antibody
grow and divide rapidly. These specialised B-cells are also known as
plasma cells. This entire process is assisted by certain lymphocytes that
have matured in the thymus gland known as T-cells. The T-cells aid in
the B-cells production of antibodies and are therefore sometimes called
helper T-cells.
Immunity
o Once the body has been exposed to a certain pathogenic antigen, a large group of B-cells and T-cells are
capable of producing an extremely large amount of specific antibodies in an extremely short amount of
time. This production of antibodies that are specific to the particular antigen is so fast that the disease
never gets a chance to develop.
o Active immunity – The injection of a weaker or mild
form of a pathogen is known as vaccination. The
presence of the pathogen triggers an immune response
which starts producing antibodies specific to the
pathogen. Ex. Influenza vaccination
o Passive immunity – antibodies to a specific pathogen can
be injected into the body and will passively provide
immunity against that pathogen as long as the antibodies
are in circulation within the body.
Ex. Travel vaccines.
Viral Diseases
 Some viral-caused human diseases include: small-pox, polio,
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measles, AIDS, mumps, influenza, yellow fever, rabies, and the
common cold.
Diseases mostly caused by viruses infecting cells much like the
bacteriophage does the bacteria. Living cells in our bodies are used for viral reproduction which ends up
destroying the cells which causes symptoms.
Despite popular belief, most viral infections are not curable; most are only preventable.
AIDS:
o Viruses infect helper T-cells which stops them from aiding the B-cells in
antibody production.
o Weakens the immune system so that future pathogens have very little
resistance when they infect the host.
Influenza:
o Binds to epithelial cells (nose, lungs, etc.) and injects genetic material into the
cell.
o Spreads through coughing, sneezing, or talking. Also through direct contact with hands and then touching
the mouth or nose.
o The recent H1N1 flu virus contained genes from four different known viruses which made the antigen
expression different than that of the regular flu virus.
Precautions
 The best way to control viral disease is through prevention.
 Avoid contact with infected people. Wash your hands regularly with soap and water. Avoid touching your mouth
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or nose with your hands frequently.
Get lots of sleep
Eat healthy
Get vaccinated
Exercise