GENERAL PROPERTIES OF VIRUSES

A virus is an obligate intracellular parasite containing genetic material surrounded by protein
Virus particles can only be observed by an electron microscope
Most viruses range in sizes from 20 – 250 nanometers

1. INTRODUCTION TO VI R OLOGY

2. Introduction to Virology A virus is an obligate intracellular parasite containing genetic material surrounded by protein Virus particles can only be observed by an electron microscope Most viruses range in sizes from 20 – 250 nanometers 2

3. Viral Properties Viruses are obligate intracellular parasites Viruses are non-living entities Viruses cannot make energy or proteins independent of a host cell ( Depends on host cell for replication ) Viral genome are either RNA or DNA but not both. Viruses have a naked capsid or envelope with attached proteins Do not possess cellular organization Viruses do not have the genetic capability to multiply by division . They are NOT cultiviable on ordinary media.

4. Cellular Acellular Prokaryotes Eukaryotes Viruses Bacteria and archaea Fungi,algae,protozoa , helminths Viruses and bacteriophages Types of Microbes (b) VirusTypes Bacterial virus AIDS virus Envelope Capsid Nucleic acid (a) Cell Types Prokaryotic Chromosome Ribosomes Flagellum Cellwall Cell membrane Eukaryotic Flagellum Mitochondria Ribosomes Cell membrane Nucleus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. 5 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)

6. Viruses are Ultramicroscopic Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997 6

7. Shape of Viruses Spherical Rod-shaped Brick-shaped Tadpole- shaped Bullet-shaped Filament-shaped

8. Shapes of Viruses Spherical

9. Shapes of Viruses Rod-shaped

10. Shapes of Viruses Brick-shaped .                                                                             

11. Shapes of Viruses Tadpole-shaped

12. Shapes of Viruses Bullet-shaped

13. Shapes of Viruses Filament-shaped

14. 14 VIRAL STRUCTURE – SOME TERMINOLOGY virus particle = virion protein which coats the genome = capsid capsid usually symmetrical capsid + genome = nucleocapsid may have an envelope

15. V irion The complete infectious unit of virus particle 15

16. Viral Structure - Overview Fig 1. Schematic overview of the structure of viruses ** does not exist in all viruses Nucleic acid N u cl e oc a ps i d Capsid Viral envelope** Envelope protein Membrane protein Spike protein

17. Basic virus structure Capsid protein Nucleocapsid Naked capsid virus DNA RNA or = + Nucleocapsid Lipid membrane, glycoproteins Enveloped virus +

18. Med i c White, DO and Fenner, FJ. al Virology, 4 th Ed. 1994 Virus particle = virion

19. Viral genome Viral genome comprises viral nucleic acid genome (DNA or RNA). Controls viral heredity and variation and responsible for the infectivity. √ DNA: either double stranded ( ds ) or single stranded ( ss ); : either linear or circular √ RNA: either double stranded ( ds ) or single stranded ( ss ); : either linear or circular : ds RNA viruses: always linear : ss RNA viruses: either segmented or non-segmented : ss RNA viruses: + sence (mRNA) or – sence (strand complementary to mRNA)

20. HERPESVIRIDAE HE P A D N A VIRI D AE ENVELOPED PAPILLOMAVIRIDAE POLYOMAVIRIDAE (formerly grouped together as the PAPOVAVIRIDAE) CIRCULAR ADENOVIRIDAE LINEAR NON-ENVELOPED DOUBLE STRANDED PARVOVIRIDAE SINGLE STRANDED NON-ENVELOPED P O X V I R I D AE COMPLEX ENVE L OP ED DNA VIRUSES All families shown are icosahedral except for poxviruses Dr.T.V M .R o a d o ifi M ed D from Volk et al., Essentials of Medical Microbiology, 4th Ed. 19 4 9 8 1

21. FL A VIV I R I D AE TOGAVIRIDAE RETROVIRIDAE ICOSAHEDRAL CORONAVIRIDAE HELICAL E N VE L OP E D ICOSAHEDRAL PI C O R N A VIRI D AE CALICIVIRIDAE ASTROVIRIDAE NONENVELOPED SINGLE STRANDED positive sense O R THO M Y X O VIRI D AE PARAMYXOVIRIDAE RHABDOVIRIDAE FILOVIRIDAE BUNYAVIRIDAE ARENAVIRIDAE SINGLE STRANDED negative sense R E O VIRI D AE DOUBLE STRANDED RNA VIRUSES ENVELOPED HELICAL ICOSAHEDRAL NONENVELOPED Dr.T.V M .R o a d o ifi M ed D from Volk et al., Essentials of Medical Microbiology, 4th Ed. 19

22. Viral Capsid 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. Protomers 2. Capsomers 3. Pro- Capsid 4. Mature Capsid

23. Symmetry of Nucleocapsid Cubic ( I cosahedral ) a polygon with 12 corners (vertices) & 20 sides (facets), each an equilateral triangle. Eg . adenovirus Helical Protein binds around DNA/RNA in a helical fashion eg . Coronavirus Complex Is neither cubic nor helical eg. poxvirus ICOSAHEDRAL SYMMETRY HELICAL SYMMETRY COMPLEX SYMMETRY POXVIRUS FAMILY

24. Envelope A lipid-containing membrane that surrounds capsid . It is acquired during viral maturation by a budding process through a cellular membrane. Viruses can be divided into 2 kinds: enveloped virus and naked virus. Enveloped icosahedral virus Enveloped helical virus

25. Properties of Non-enveloped (naked) viruses Not damaged by drying, acid, detergent, and heat Released by lysis of host cells Can sustain in dry environment Can infect the GI tract and survive the acid and bile Can spread easily via hands, dust, fomites , etc Can stay dry and still retain infectivity Neutralizing mucosal and systemic antibodies are needed to control the establishment of infection Examples: Poliovirus, Hepatitis A, Hepatitis E etc.

26. Properties of Enveloped viruses Labile in dry Damaged by drying, acid, detergent, bile salts and heat Pick up new cell membrane during multiplication Insert new virus-specific proteins after assembly Examples: Influenza virus, HIV, Measles virus, Rubella virus, HSV, etc.

27. Enveloped Virus Non- Enveloped Virus Cubic Helical

28. 5 BASIC TYPES OF VIRAL STRUCTURE HELIC A L ENVELOPED HELICAL ENVELOPED ICOSAHEDRAL ICOSAHEDRAL nucl eoc a psi d icosahedral nucleocapsid helical nucleocapsid lipid bilayer C O MPLEX nucleocapsid lipid bilayer glycoprotein spikes = peplomers Adapted from Schaechter et al., Mechanisms of Mic D r r o .T b .V i . a Ra l o D M is D ease

29. ADSORPTION PENETRATION UNCOATING AND ECLIPSE SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN ASSEMBLY (maturation) RELEASE BASIC STEPS OF VIRAL REPLICATION

30. VIRAL REPLICATION Stages in virus replication begin when virion infects host cells Virus attachment and entry Uncoating of virion Migration of genome nucleic acid to nucleus Synthesis of viral nucleic acid and protein. Virion assembly Release of new virus particles

31. Chemical Properties - Resistance  Temperature:    Enveloped viruses are more heat-labile Inactivated at 56 C-30’ (exception- HBV) and few sec at 100 C. Some may be preserved by lyophilization    p H :   . stable between pH 5 and 9 Enteroviruses – resistant to acidic conditions

32.  Radiation: UV and ionising radiation inactivates viruses  Photodynamic Inactivation  Vital dyes- Toluidene blue  Disinfectants  Most viruses are destroyed by oxidising agents such as chlorine, iodine and hydrogen peroxide. However, majority of viruses are resistant to phenol; chlorination does not always inactivate enteroviruses or hepatitis viruses, particularly if present with organic or faecal material.

33. Transmission of Viruses Respiratory transmission Influenza A virus Faecal-oral transmission Enterovirus Blood-borne transmission Hepatitis B virus Sexual Transmission HIV Animal or insect vectors Rabies virus

34. Viruses enter the body of the host in a variety of ways, for example...

35. The commonest forms of transmission are via... INHALED DROPLETS in sneezing of coughing for example the COMMON COLD or INFLUENZA VIRUSES.

36. or b y ... drinking water or eating raw food, for example, HEPATITIS A and POLIOVIRUS.

37. The commonest forms of transmission are also via... sexual intercourse for example HIV and HEPATITIS B and...

38. also... vertical transmission - from mother to baby for example HIV, HEPATITIS B and RUBELLA...

39. also... bites of vector arthropods such as mosquitoes for example YELLOW FEVER , and DENGUE.

40. Most viral infections... do not lead to such serious complications and the host...

41. get well after a period of sickness to be immune for the rest of their lives. Examples are MEASLES INFECTION, RUBELLA or German measles , MUMPS and many others...