Microbiology
Viruses
I. Viruses: Living or Non-living?
Five characteristics of living things (review!):
1. Made up of one or more cells
2. Reproduce
3. Grow and develop (and die)
4. Obtain and use energy (metabolism)
5. Respond to environment
II. History
Virus = poison in Latin
1892: Dmitri Ivanovsky describes a non-bacterial
pathogen infecting tobacco plants.
II. History
1931: Electron microscope allows for first imaging
II. History
1983: Pasteur Institute isolates HIV retrovirus
II. History  TIMELINE
• Viruses are an important natural means of
transferring genes between different species,
which increases genetic diversity and drives
evolution.
• It is thought that viruses played a central role
in early evolution, before the diversification of
bacteria, archaea and eukaryotes, at the time
of the last universal common ancestor of life
on Earth.
III. Viral Structure  VIRION
Helical
Polyhedral
Spherical
Complex
IV. Viral Classification
1962: Linnaean
hierarchical system
Order (-virales)
Family (-viridae)
Subfamily (-virinae)
Genus (-virus)
Species (-virus)
2013: Baltimore
Classification system
V. Viral Reproduction
Able to multiply only within the living cells of a host
LYTIC AND LYSOGENIC CYCLE ANIMATION
Graphic Organizer Activity
1. groups of two to three
2. open bag and distribute steps of your cycle
3. as a team, organize steps (check!)
4. join group with other cycle and organize all steps
V. Viral Reproduction
LYTIC CYCLE
1. Attachment:
- to host cell
2. Penetration:
- enter host (virus or just genetic material)
- endocytosis or pore
3. Replication and protein synthesis:
- direct host to make viral genetic material and protein
4. Assembly:
- of viral nucleic acids and proteins
5. Release:
- of new viral particles (virions) through lysis or exocytosis
V. Viral Reproduction
LYSOGENIC CYCLE
- Steps 1 & 2
2a. Viral DNA inserted into DNA of host cell (prophage)
- may remain in host DNA for many generations
- activates to direct synthesis of new virions
(Lytic Cycle steps 3-5)
- Factors that activate the virus:
- change in temperature
- availability of nutrients
V. Viral Reproduction
• Bacteriophage: virus that infects and
replicates within a bacterium
• Viral Specificity: can only attack particular cells
of a particular organism
• Retroviruses: group of RNA viruses that insert
a DNA copy of their genome into the host cell
in order to replicate (e.g. HIV).
– HOW IS THIS USEFUL?
VI. Basic Lines of Defense
1. Primary
VI. Basic Lines of Defense
2. Secondary: phagocytic white blood cells engulf viruses
VI. Basic Line of Defense
3. Tertiary: white blood cells called lymphocytes produce antibodies
VI. Basic Lines of Defense
Antiviral Drugs
• nucleoside analogues
(fake DNA building-blocks)
• viruses mistakenly incorporate them into their genomes
• the life-cycle of the virus is halted because the newly
synthesized DNA is inactive
VII. Reducing Spread
1. Cover your nose and mouth when you sneeze
2. Keep your hands clean
3. Do not share cutlery or beverages
4. Stay home if you are sick
5. Get the appropriate antivirus immunization
*Stay healthy: get plenty of sleep, be physically active,
manage your stress, drink plenty of fluids, and eat nutritious
food. The healthier you are, the better your immune system
will be at defending your body against a virus.
VIII. Current Virus Definition
• An infective agent that typically consists of a
nucleic acid molecule in a protein coat, is too
small to be seen by light microscopy, and is able
to multiply only within the living cells of a host.
VIII. Current Virus Definition
Evidence for living:
- carry genetic material
- reproduce  multiply
- evolve by natural selection
VIII. Current Virus Definition
Evidence for non-living:
Five characteristics of life?
- not cellular  only made up of genetic material and
protein coat
- do not grow or develop
- do not do anything in nutrient agar (grow nor die)
- if undisturbed, will never age or die
- do not obtain or use energy  host cell is does this for
the virus
- completely at the mercy of their environment
 do not respond to it!
VIII. Current Virus Definition
Evidence for non-living:
- Five characteristics of life?
- Uses of HIV Retrovirus
* When a virus encounters a cell, a series of chemical
reactions occur that lead to the production of new viruses.
* These steps are completely passive: predefined by the
nature of the molecules that comprise the virus particle.
SO… WHAT DO YOU THINK?
Evaluate the evidence above
and state your choice