1. MICROBIOLOGY AND IMMUNOLOGY (BIOL200/343), SUMMER, 2017
Lecture:5
MICROBIOLOGY AND IMMUNOLOGY (BIOL200/343), SUMMER, 2017
Dr.Q.M.I.Haq

2. VIRUS
Obligate
Intracellular
Parasite

3. Virion Structure Nucleic Acid Lipid Envelope Protein Capsid Virion
Spike
Projections
Protein
Capsid
Lipid Envelope
Virion
Associated
Polymerase
·         This overhead shows the standard features found in some (but not all) viruses.
·         A virus particle is essentially a piece of nucleic acid surrounded by a protein coat.
·         The protein coat (i.e. the capsid) is a delivery system for transferring the virus genome from one cell to another. The protein serves to:
Ø     Provide protection to the nucleic acid against the environment - e.g. nucleases etc.
Ø     Function in receptor recognition - targeting a virus to a susceptible host and cell type.
·         Surrounding this coat there may be a lipid envelope - this envelope is derived from one of the cell membranes and is not determined by the virus. There may be some modification to the lipid composition induced during virus maturation.
·         Inserted into the lipid envelope there are usually virus proteins which are present as spike projections - these are normally glycoproteins.
Due to restrictions on the coding size of many virus genomes the capsid of the virion is made up of repeating subunits, which coat the virus genomic nucleic acid. The redundancy also allows for the fact that if there is an inactivation of part of the capsid the virus does not completely lose its infectivity For example the poliovirus RNA (7kb) can specific at most 250,000 Daltons of protein altogether (some must be used for replication) but the poliovirus virion capsid weighs 6 x 106 Daltons.
Genomic Nucleic Acid
Viruses only possess a single type of genomic nucleic acid – either DNA or RNA but not both. This nucleic acid can be in a variety of physicla forms that can be used as a valuable classification feature.

4. Morphology of viruses

5. General properties of viruses
They are very small in size, from m and are only seen by electron microscope.
They contain one kind of nucleic acid (RNA or DNA) as their genome, enclosed in a simple protein shell known as a capsid.
Viral capsids (coats) are made of individual protein subunits, are called capsomeres.
They are obligate intracellular parasites.
They are unaffected by antibiotics.
They are sensitive to interferon.

6. Viruses are inert (nucleoprotein ) filterable Agents.
Viruses cannot make energy or proteins independent of a host cell.
Some viruses have particles that are surrounded by a membrane.
Viruses have a naked capsid or envelope with attached proteins.
Virus proteins protect the viral genome, identify the appropriate target cells, and get the genome into the target cells.
Some viruses contain proteins with enzymatic functions that are needed for genome Replication and enter host cells.

7. A virus contains only one type of nucleic acid, either DNA or RNA
A virus contains only one type of nucleic acid, either DNA or RNA. The virus containing DNA is called deoxy-virus or DNA virus.
The virus containing RNA is called RNA virus or ribovirus.
Rarely a virus contains both DNA and RNA. Such viruses are called RNA-DNA viruses.
Majority of animal and bacterial viruses are DNA viruses. Majority of plant viruses are RNA viruses.
However, a few plant viruses are known to possess DNA and a few animal viruses are know to possess RNA.

8. Beijerinck (1897) coined the Latin name “virus” meaning poison
Beijerinck (1897) coined the Latin name “virus” meaning poison. He studied filtered plant juices & found they caused healthy plants to become sick.
Smallpox
Edward Jenner (1796) developed a smallpox vaccine using milder cowpox viruses. Deadly viruses are said to be virulent. Smallpox has been eradicated in the world today.

9. Are viruses alive? Yes No They are not made of cells or organelles
They cannot reproduce without a host
They do not metabolize energy
They do not perform cellular processes
Reproduce
They have DNA or RNA
They can adapt to surroundings
The have organization

10. Used for Virus Identification
RNA or DNA Virus
Do or do NOT have an envelope
Capsid shape
HOST they infect
Viral Taxonomy
Family names end in -viridae
Genus names end in -virus
Viral species: A group of viruses sharing the same genetic information and ecological niche (host).
Common names are used for species
Subspecies are designated by a number
Examples
Retroviridae
Lentivirus
Human Immunodeficiency Virus 1, HIV 2

11. Virus Transmission Transmission is fundamental property of viruses.
The spreading of viruses from infected hosts to healthy hosts by means of some
Agencies is called transmission of viruses.
The mode of transmission of viruses is different for plants and animals including Man.
1. Virus transmission for plants
Viruses are transmitted from disease plants to healthy plants in several ways:
Mechanical transmission, Transmission by Propagules, Insect Transmission, Transmission by Pollen and Fungi.
2. Virus transmission for animals and Man
Sexual contact, Droplet Infection, Animal Transmission, d.Transmission by Food and water, Vertical transmission, Transmission by direct contact.

12. Diagram of T-4 Bacteriophage
Bacteriophages
Viruses that attack bacteria are called bacteriophage or just phage
T-phages are a specific class of bacteriophages with icosahedral heads, double-stranded DNA, and tails.
The most commonly studied T-phages are T4 and T7
They infect E. coli , an intestinal bacteria
Diagram of T-4 Bacteriophage

13. Bacteriophage Replication
Lytic Cycle Review
Attachment Phage attaches by tail fibers to host cell
Penetration Phage lysozyme opens cell wall, tail sheath contracts to force tail core and DNA into cell
Biosynthesis Production of phage DNA and proteins
Maturation Assembly of phage particles
Release Phage lysozyme breaks cell wall
The Lytic Cycle

14. Lysogenic Cycle
Phage DNA injected into host cell.
Viral DNA joins host DNA forming a prophage/provirus.
When an activation signal occurs, the phage DNA starts replicating. Viral DNA (part of prophage) may stay inactive in host cell for long periods of time.
Replicated during each binary fission.
Over time, many cells form containing the prophages. (Viral Latency)
Once a prophage cell is activated, host cell enters the lytic cell.

15. Bacteriophage Replication

16. Viral Latency Latency in Eukaryotes
Some viruses have the ability to become dormant inside the cell
Called latent viruses
They may remain inactive for long periods of time (years)
Later, they activate to produce new viruses in response to some external signal
HIV and Herpes viruses are examples
Latency in Eukaryotes
Some eukaryotic viruses remain dormant for many years in the nervous system tissues.
Chickenpox (caused by the virus Varicella zoster) is a childhood infection.
It can reappear later in life as shingles, a painful itching rash limited to small areas of the body.
SHINGLES

17. Potato famine in Ireland.
Viroids
Small ( nt), circular RNA molecules without a protein coat or envelope.
Infect plants.
Potato famine in Ireland.
Prions
Prions are “infectious proteins”
They have no DNA or RNA.
The main protein involved in human and mammalian
prion diseases is called “PrP”.
Prions form insoluble deposits in the brain. Causes neurons to rapidly degeneration. Mad cow disease (bovine spongiform encephalitis: BSE) is an example.
These agents are associated with diseases such as Creutzfeldt-Jakob disease in humans and scrapie in sheep.
BSE

18. Nucleic acid replication Virus protein processing Virus maturation
Antiviral Targets
Attachment/Entry
Nucleic acid replication
Virus protein processing
Virus maturation
Attachment/Entry
Picornaviruses
Nucleic acid replication
Human immunoideficiency virus (AZT)
Herpes simplex virus (Acyclovir)
Virus protein processing
HIV (Protease inhbitors)
Virus maturation
Influenza A virus (Neuraminidase blockers)
Problems of antivruals
Dificuly in finding a virus specific site against which to direct the antivrial
As with the use of antiiotics – resistant mutant scan be readily generated that are resistant to antiirals – this is particuarly a problem with those against HIV where the drug has to be used for prolonged periods of time.