1. General Properties of Viruses
Dr. Yagoub Hmadt Allah
Ass. Prof. medical microbiology

2. General Properties of virus
It is very smallest infectious agent (20 –350nm)
Obligate intracellular parasites
Contain only one type of nucleic acid, either DNA or RNA
Do not possess cellular organization
Lacks enzymes necessary for protein & NA synthesis
Depends on host cell machinery for replication
Causes a large no. of human diseases ranging from minor ailments like common cold to terrifying diseases such as rabies, HIV etc.
1/1/2013
Dr .yagoub, Microbiology

3. General Properties
Morphology – size, structure, shape, chemical properties, resistance
Replication
Hemagglutination
Cultivation
Viral assay
Viral infections: virus-host interactions
1/1/2013
Dr .yagoub, Microbiology

4. Morphology - Size Much smaller than bacteria
“Filterable agents” – can pass through filters that can hold back bacteria
Vary widely in size:
Largest – poxvirus (300nm)
Smallest – parvovirus (20nm)
Virion – extracellular infectious virus particle
1/1/2013
Dr yagoub, Microbiology

5. 1/20134/24/20174/24/2017
Dr Ekta, Microbiology

6. Morphology - Structure of Virus
ELECTRON
MICROSCOPE
Morphology - Structure of Virus
Nucleic acid
Capsid
Envelope
Peplomer
Size in nanometers
1/1/2013
Dr yagoub, Microbiology

7. Morphology – Structure & Shape of a virus
Nucleic acid & capsid with or without envelope.
Capsid – the protein coat surrounding the nucleic acid core. It
protects nucleic acid from inactivation
helps to introduce viral genome into host cell
Capsomers - the repeating protein subunits that make up the capsid
Protomers – the polypeptide chains which make up the capsomers
1/1/2013
Dr yagoub Microbiology

8. 4. Mature Capsid 1. Protomers 2. Capsomers 3. Pro-Capsid 1/1/2013
Dr yagoub, Microbiology

9. Morphology – Structure & Shape of a virus: Capsid
Capsomers – symmetrically arranged to form an impenetrable shell (capsid) around the nucleic acid core.
This symmetry is of two types:
Icosahedral (cubical)
Helical
1/1/2013
Dr yagoub, Microbiology

10. Morphology – Structure & Shape of a virus: Capsid
Icosahedron – a polygon with 12 corners (vertices) & 20 sides (facets)
Side – equilateral triangle
Two types of capsomers form the capsid
Pentagonal capsomers form the vertices
Hexagonal capsomers form the sides.
Helical – the capsomers & nucleic acid are wound together to form a helical or spiral tube.
The overall shape of virus is quite variable, but mostly they are spherical.
1/1/2013
Dr yagoub, Microbiology

11. 1/1/2013
Dr yagoub, Microbiology

12. Morphology – Structure & Shape of a virus: Envelope
May or may not be present
Derived from the host cell membrane
Lipoprotein in nature – lipid is of host cell origin while protein is from virus.
Protein subunits seen as projecting spikes on the surface of envelope – called Peplomer.
A virus may have more than one type of peplomer e.g. influenza virus.
Confers chemical, antigenic & biological properties.
Susceptible to lipid solvents
1/1/2013
Dr yagoub, Microbiology

13. 1/1/2013
Dr yagoub, Microbiology

14. Resistance Very heat labile but stable at low temperatures
Inactivated within seconds at 56C.
Can be kept frozen at -70C for long term storage.
Inactivated by sunlight, UV rays & ionising radiations.
More resistant than bacteria to chemical disinfectants.
Most active antiviral agents (virucidal) – oxidising agents like hydrogen peroxide, potassium permangnate, hypochlorites
1/1/2013
Dr yagoub, Microbiology

15. Viral replication
Viruses have no metabolic activity of their own. Therefore, they depend on living cells for providing energy and synthetic machinery for synthesis of :
1- viral nucleic acid , 2- viral protein.
The virus genome provides the host cell with the genetic information needed for its replication.
4/24/2017
Dr y.h, Microbiology

16. The sequence of events for virus replication
Stages in virus replication begin when virions infect cells
Attachment/ Adsorption
Penetration
Uncoating
Biosynthesis
Maturation & Assembly
Release
1/1/2013
Dr yagoub, Microbiology

17. 1- Attachment: virus and cell are brought into contact by random collision, but attachment occurs if the cell membrane contains specific receptors for the virus, e.g HIV binds to CD4 receptors on helper T cells.
2- penetration and uncoating: non envoloped virions are taken into animal cells by endocytosis where they are uncoated by lysosomal enzymes.
Enveloped viruses penetrate the membrane by fusion between the virus envelope and the cell membrane releasing the nucleocapsid into the cell; uncoating may occur at the cell surface. Uncoating renders viral nucleic acid accessible for transcription and replication.
4/24/2017
Dr yagoub.h, Microbiology

18. Pathways for Viral Entry into Host Cell
Surface Fusion
Receptor-Mediated Endocytosis
Fusion in Endosome
Lysis of Endosome
1/1/2013
Dr yagoub, Microbiology

19. Viral Replication
3- Eclipse phase : it is the period after penetration during which no infectious virus componant can be detected inside the host cell. During this phase the cell rediracted toward synthesizing early proteins (enzymes) which are essential for viral replication. {from the stage of penetration till the appearance of mature daughter virions, the virions cannot be detected inside the host cell}.
1/1/2013
Dr yagoub, Microbiology

20. 4- Intracellular viral synthesis: it includes synthesis of both viral nucleic acid and proteins. The viral nucleic acid (genome) replicates by using strand of the parental nucleic acid as a template for the production of progeny DNA or RNA molecules. The essential step in protein synthesis is transcription of mRNA from viral nucleic acid.
4/24/2017
Dr .Yagoub.H, Microbiology

21. -ss DNA RNA polymerase mRNA
The mRNA is translated to viral proteins (capsid and enzymes) using host cell ribosomes. The mRNA transcription varies depending on the nucleic acid type:
In double stranded (ds) DNA viruses, mRNA is transcribed from DNA by DNA dependent RNA polymerase ( transcriptase).
+ss DNA
ds DNA-
-ss DNA RNA polymerase mRNA
4/24/2017

22. b. In single stranded RNA viruses of positive polarity (+ sense) the ssRNA itself acts as mRNA for translation into proteins. + ssRNA = mRNA c. The ssRNA viruses of negative polarity ( - sense) must be transcribed by RNA dependent RNA polymerase – which is present in the virus into complementary (+ sense) m RNA . – ssRNA RNA polymerase + ssRNA = mRNA
4/24/2017
Y.H, Microbiology

23. d. The ssRNA of retroviruses (+ sense) is transcribed by a unique virion associated reverse transcriptase into complementary ssDNA, which is converted into dsDNA, which becomes integrated into the cellular genome causing malignant transformation of cells in vivo and in vitro. It may be transcribed into mRNA Integrated
+ssRNA Reverse transcriptase ssDNA dsDNA
Transcribed to mRNA
4/24/2017
Dr y.h Microbiology

24. e. Assembly of viral nucleic acid and protein coats to form mature virus particles occurs in the cytoplasm ( e.g. poliovirus) or in the nucleus e.g herpes viruses. f. Release: mature virus particles will accumulate in the cell in enonmous number and are liberated by rupturing the cell i.e cytolysis. The viruses may release by a slow process of leaking or budding through the cell membrane. Enveloped viruses will acquired lipoprotein envelope during budding.
4/24/2017
Microbiology

25. Classification of medical important viruses:
The classification of viruses depends on their structure, antigenic composition and other properties. Viral famillies and genera have been designated, though differentiation into species is still incomplete.
Viruses are classified into two major divisions depending on the type of nucleic acid.
4/24/2017
Dr y.h, Microbiology

26. 1- deoxyriboviruses, which contain DNA.
2- riboviruses, which contain RNA.
Both of these are further subdivided mainly on the basis of size and shape of the virion, symmetry of the nucleocapsid and strandness of the nucleic acid.
4/24/2017
Dr yagoub hamadta Allah Microbiology

27. Major families of viruses are briefly are: A- DNA viruses:
parvoviridae:
Size : 18 – 26 nm
Symmetry : icosahedral
Envelope : absent
DNA : single stranded
Example : parvovirus which cause gastroenteritis and haemolytic disease
4/24/2017
Dr .yagoub hamadt allah, Microbiology

28. Symmetry : icosahedral Envelope : absent DNA : double stranded
2- papovaviridae:
Size : 40 – 55 nm
Symmetry : icosahedral
Envelope : absent
DNA : double stranded
Example : i.papilloma virus which causes cutaneous, genital and laryngeal warts.
ii. Polyomavirus which cause neurological diseases
4/24/2017
Dr yagoub hamadt allah, Microbiology

29. Symmetry : icosahedral Envelope : absent DNA : double stranded
3- Adenoviridae:
Size : 70 – 90 nm
Symmetry : icosahedral
Envelope : absent
DNA : double stranded
Example : adenovirus.some can cause respiratory disease and conjunctivitis.
4/24/2017
Dr yagoub hamadtallah, Microbiology

30. Symmetry : icosahedral Envelope : present DNA : double stranded
4- herpesviridae:
Size : 100 – 200 nm
Symmetry : icosahedral
Envelope : present
DNA : double stranded
Example : i. herpes simplex virus
ii. Varicella /zoster viruses.
4/24/2017
Dr yagoub hamadtallah, Microbiology

31. Size : 300 – 450 nmX170 -260nm(brick shaped) Symmetry : unknown
5- poxviridae:
Size : 300 – 450 nmX nm(brick shaped)
Symmetry : unknown
Envelope : present
DNA : double stranded
Example : poxviruses –Variola, cowpox, monkey pox…..
4/24/2017
Dr yagoub hamadt allah, Microbiology

32. DNA : partially double stranded Example : hepatitis – B virus
6- hepadnairidae:
Size : 42 nm
Symmetry : unknown
Envelope : present
DNA : partially double stranded
Example : hepatitis – B virus
4/24/2017
Dr yagoub hamadt allah, Microbiology

33. Riboviruses(RNA viruses):
RNA viruses included: 1- Picornaviridae. 2- Reoviridae (double stranded). 3- Orthomyxoviridae. 4- Paramyxoviridae. 5- Rhabdoviridae. 6- Bunyaviridae.
4/24/2017
Dr y.h, Microbiology

34. 7- Coronaviridae. 8- Togaviridae. 9 Arenaviridae. 10- Retroviridae.
4/24/2017
Dr Ekta, Microbiology

35. Abnormal Replicative Cycles
Incomplete viruses - A proportion of daughter virions are not infective, due to defective assembly.
Defective viruses – genetically defective, unable to give rise to fully formed progeny.
Abortive infection – defect in the type of cell (non permissive cell), not in the parental viruses.
1/1/2013
Dr yagoub, Microbiology

36. Viral Hemagglutination
Originally seen with the Influenza virus by Hirst in 1941.
A convenient method of detection & assay of Influenza virus.
Due to the presence of Hemagglutinin spikes on the surface.
Reversal of hemagglutination – Elution
Due to the presence of Neuraminidase enzyme, Receptor Destroying Enzyme (RDE)
Destruction of receptor – reversal of hemagglutination – release of virus from the red cell surface
Found only in Myxoviuses.
1/1/2013
Dr yagoub, Microbiology

37. Virus Culture Embryonated Egg Cell Lines/ Tissue cultures
Chorioallantioc membrane (CAM)
Allantoic cavity
Amniotic cavity
Yolk Sac
Cell Lines/ Tissue cultures
Primary
Diploid/ Secondary
Continuous
Animal inoculation
Suckling mice
1/1/2013
Dr yagoub, Microbiology

38. Embryonated Hen’s Egg
Chorioallantoic membrane (CAM) – visible lesions called pocks. Each infectious virus particle forms one pock. e.g. Variola, Vaccinia virus
Allantoic cavity – Influenza virus (vaccine production) & paramyxoviruses
Amniotic cavity – primary isolation of Influenza virus
Yolk sac – Chlmyadia, Rickettsiae & some viruses
1/1/2013
Dr yagoub, Microbiology

39. Embryonated Hen’s Egg
1/1/2013
Dr yagoub, Microbiology

40. Viral Assay Viral content of a specimen: Total no. of
Virus particles – EM, HA
Infectious virions only
Assay of Infectivity: two types
Quantitative assays – actual no. of infectious particle in an inoculum
Quantal assays – indicate the presence or absence of infectious viruses, carried out in animals, eggs or tissue cultures
1/1/2013
Dr yagoub, Microbiology

41. Viral Assay Assay of Infectivity: Quantitative assays
Plaque assay in monolayer cell cultures
Pock assay on CAM
*Each plaque/ pock represents one
infectious virus.
Plaques are clear zones that develop on lawns of host cells.
The virus plaque is analogous to the bacterial colony.
1/1/2013
Dr yagoub, Microbiology