1. Virus
L. Dalia Kamal Eldien
MSc in Microbiology
Lecture NO: 6

3. Outlines At the end of this lecture, the student should know:-
Definition of virus
General properties of virus
Structure of virus
Classification of viruses
Transmission routes of virus
Virus growth curve
Steps of virus replication

4. Virus Viruses are small, obligate, intracellular particles.
All viruses seen only with the electron microscope, except the pox virus
Size of viruses range nm (1nm =0.001 µm )
The poxviruses are the largest viruses (250 × 350 nm), and the smallest human virus is the poliovirus, which is 25 nm in diameter
They must infect and take over a host cell in order to replicate.
Viruses have no organelles, no cytoplasm, and no cell nucleus or nucleoid

5. Electron microscope

6. Light microscope

7. Comparison of virus with bacteria & RBCs

8. General properties of viruses
They are very small in size.
They contain one kind of nucleic acid (RNA or DNA ) as their genome.
They are obligate intracellular parasites i.e. can only replicate inside living cells.
Attach to their host cell with a specific receptor
They cannot be grown on artificial culture media and are grown in tissue culture, embryonated eggs or living animals.

9. Structure of viruses All viruses consist of:-
Core is a mass of nucleic acid DNA or RNA
Surrounded by a protective protein coat called a capsid, the capsid consists of a number of identical small protein subunits called capsomeres.
C. May or may not have a phospholipid envelope, a membrane surrounding the nucleocapsid, derived from the plasma membrane of the host cell from which the virus is released

10. Structure of viruses

11. Capsomeres

12. Enveloped vs non enveloped (naked) virus

13. Question :
Define the virus?

14. Answer:-
An infectious, obligate intracellular parasite comprising genetic material (DNA or RNA) surrounded by a protein coat and/or an envelope derived from a host cell membrane

15. Structure of viruses
The nucleic acid together with the capsid form the nucleocapsid.
The symmetry, or pattern, of capsides is one of the features used to classify viruses.

16. Structure of viruses
Viruses may be classified into several different morphological types on the basis of their capsid symmetry as being:
Icosahedral
Helical
Complex

17. Icosahedral (Polyhedral Viruses)
The capsid of most polyhedral viruses is in the shape of an icosahedron.
A regular polyhedron with 20 triangular faces and 12 corners.
The capsomeres of each face form an equilateral triangle

18. Helical viruses
Helical viruses resemble long rods that may be rigid or flexible.
Capsids with helical symmetry consist of repeated units of a single polypeptide species that—in association with the viral nucleic acid—self-assemble into a helical cylinder

19. Complex virus
Complex, meaning the capsid symmetry is have a combination of helical and icosahedral symmetry
One example of a complex virus is a bacteriophage

20. Viruses Capsid symmetry

21. Identify the following viruses capsid symmetry?

22. Some important terms Term Definition Virion
The entire virus particle (infectious particle)
Capsid
Protein shell which surrounds and protects the genome
Nucleocapsid
Genome + capsid
Envelope
Phospholipid membrane which surrounds some viruses

23. Function of the capsid/envelope
Protect nucleic acid from the host’s acid and protein-digesting enzymes
Assist in binding and penetrating host cell
Stimulate the host’s immune system

25. Transmission routes of viruses
1- Direct contact, e.g. sexually transmitted viruses e.g. HIV 2-Ingesting viruses in food or water contaminated with faeces, e.g. Enteroviruses, Rotaviruses, and Hepatitis A 3-Inhaling viruses in airborne droplets, e.g. influenza virus 4- Mother infecting her child during pregnancy (vertical transmission)

26. Transmission routes of viruses
5-Blood transfusion, e.g. HIV 1& 2, Hepatitis B virus.
6-Bite of an infected, blood-sucking mosquito, sandfly, tick or midge. Arthropod-borne viruses, e.g. Dengue virus
7- Bite of an animal host, e.g. Rabies virus

27. Classification of viruses
Viruses are divided into related groups, or families, and, sometimes into subfamilies based on:
Morphology (Icosahedral, Helical & Complex)
Presence or absence of envelope
Type and structure of the viral nucleic acid
Host organisms
Type of disease they cause

28. Nucleic acid
The type of nucleic acid (genome) found in the virus particle is perhaps the most fundamental and straightforward of viral properties.
It may be RNA or DNA, either of which may be single stranded (ss) or double stranded (ds).
The most common forms of viral genomes found in nature are ssRNA and dsDNA.
However, both dsRNA and ssDNA genomes are found in viruses of medical significance
The nucleic acid can be either linear or circular

29. Nucleic acid

30. Nucleic acid The types of virus according to nucleic acid: ss RNA
ds RNA
ss DNA
ds DNA

31. Nucleic acid Functions of the nucleic acid :
It is the infectious part of the virus
It carries the genetic information for virus replication .

32. Host organisms
As a group, viruses can infect almost any cellular organism, include:-
Bacterial cells (known as bacteriophages, or phages)
Parasites
Fungi
Plants
Animals
Human

33. Viral replication The one-step growth curve
The growth curve shows that when one virion infects a cell, it can replicate to produce hundreds of virions within that cell.
Note that the time required for the growth cycle varies; it is minutes for some bacterial viruses and hours for some human viruses.
The one-step growth curve begins with the eclipse period, which is followed by a period of exponential growth

34. Viral growth curve Eclipse period :
Following initial attachment of a virus to the host cell, the ability of that virus to infect other cells disappears.
This is the eclipse period, and it represents the time elapsed from initial entry and disassembly of the parental virus to the assembly of the first progeny virion.
The eclipse period for most human viruses falls within a range of 1 to 20 hours.

35. Viral growth curve Exponential growth:
The number of progeny virus produced within the infected cell increases exponentially for a period of time, then reaches a plateau, after which no additional increase in virus yield occurs.

36. Viral growth curve

37. Steps of viral replication
Adsorption: the initial attachment of a virus particle to a host cell.
Penetration: is the passage of the virion from the surface of the cell across the cell membrane and into the cytoplasm.
“Uncoating” is the stepwise process of disassembly of the virion that enables the expression of the viral genes that carry out replication.
Release of progeny viruses

38. Steps of viral replication

39. Discus with your colleagues

40. Infection of cells by viruses
Because viruses possess neither a cellular structure nor organelles, they are dependent on their host cells for energy and replication.
Outside of living cells, viruses are metabolically inert.
The information required for a virus to replicate is contained in its nucleic acid.

41. Infection of cells by viruses
Following infection, a virus ‘takes over’ the synthesizing activities of its host cell, directing the cell to transcribe and, or, translate its genetic information to produce the protein and nucleic acid components required to make new virions.
Mature virions are released from host cells either by rupture of the cell membrane, or by a process called budding in which enveloped virions are extruded from the cell membrane.

42. Viral Infections
Initiating an viral Infection require three points must be met to ensure successful infection in an individual host:
Sufficient virus must be available to initiate infection
The cells at the site of infection must be susceptible for the virus.
The local host antiviral defense systems must be absent or at least initially ineffective

43. Effects of virus on living cells
There are four main effects of virus infection on the cell:
(1) Death : cell can be directly killed by the virus
(2) Cell fusion: Infection of cells with certain viruses causes the cells to fuse, producing giant, multinucleate cells.
(3) Transformation: Some viruses transform normal cells into malignant cells.
(4) Cytopathic effect (CPE): morphologic and functional change.

44. Question for revision Define the virus Identify the virus structure
What are the different mode of transmission of virus to man?
Classify the virus according to nucleic acid type

45. Home work
Differentiate between bacteria and virus

46. Sources
Lippincott's Illustrated Reviews: Microbiology, 3rd Edition (2012)- Richard A. Harvey- Bruce D. Fisher- Richard A. Harvey- (Chapter 23,1&3 )