Introduction to viruses
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This document provides a comprehensive overview of viruses, detailing their definitions, characteristics, historical discoveries, and classification systems. It covers various sub-viral agents like satellites and viroids, describes bacteriophages, and outlines the properties and structural features of viruses. The text also discusses the Baltimore classification system categorizing viruses based on genetic content and replication strategies, along with taxonomy and nomenclature practices.
Introduction to viruses
- 1. INTRODUCTION TO VIRUSES Dr. Vinod Kumar Singh Department of Vet. Microbiology DUVASU, Mathura
- 2. 4/14/2021 2 Terminology 1. Virus (latin : slimy liquid or poison) is a broad general term for any aspect of the infectious agent and includes: • the infectious or inactivated virus particle • viral nucleic acid and protein in the infected cell 2. Virion is the physical particle in the extra-cellular phase which is able to spread to new host cells; complete intact virus particle
- 3. 4/14/2021 3 Sub-viral agents 3. Satellites • Contain nucleic acid • Depend on co-infection with a helper virus • May be encapsidated (satellite virus) • Mostly in plants, can be human e.g. hepatitis delta virus • If nucleic acid only = virusoid 4. Viroids • Unencapsidated, small circular ssRNA molecules that replicate autonomously • Only in plants, e.g. potato spindle tuber viroid • Depend on host cell polII for replication, no protein or mRNA 5. Prions • No nucleic acid • Infectious protein e.g. BSE
- 4. 4/14/2021 4 Bacteriophage • Bacteriophage is any one of viruses that infect bacteria • They do this by injecting genetic material, which they carry enclosed in an outer protein capsid. • The genetic material can be ssRNA, dsRNA, ssDNA, or dsDNA along with either circular or linear arrangement
- 5. 4/14/2021 5 Bacteriophage T4 20 Sided polyhedron Double-stranded Locate bacterium Anchor to bacterium Needle which injects DNA into bacteria
- 6. 4/14/2021 6 Limits of resolution Electron Microscope 0.003 m Light Microscope 0.2 m Unaided Human eye 40 m Microscopic protozoa & fungi 4 - 40 m Classes of organisms Bacteria 0.1 - 10 m Viruses 0.03 - 0.3 m 0.001 0.01 0.1 1 10 100 Size (m) Relative size of Microorganisms
- 7. 4/14/2021 7 Relative size of Microorganisms
- 8. • China 1000 B. C. – Prevention without knowledge of the agent, based on recognition that survivors of smallpox were subsequently protected against disease – Variolation: Inoculation of healthy individuals with dry material from smallpox pustules • Roman Empire was weakened by viral epidemics of measles and smallpox between 160 A.D. through 266 A.D • Smallpox helped destroy the Aztec Empire during the conquest of Mexico by Spain 4/14/2021 8 History
- 9. • 1718 - Lady Marry Wortley Montagu dicovered the Ottoman Empire concept of variolation and brought the idea to Britain • 1796 - E. Jenner performed his vaccination studies showing smallpox infection could be stopped by prior infection with cowpox • 1885 - L. Pasteur experimented with rabies vaccination, using the term virus to describe the agent – Although Pasteur did not discriminate between viruses and other infectious agents, he originated the terms virus and vaccination in honor of Jenner 4/14/2021 9 History
- 10. • 1879 - Adolf Mayer • named tobacco mosaic disease based on the dark and light spots on infected leaves • Experiment on transmission of disease by inoculating extracted juice from infected leaves • Unable to culture the agent • 1892 – Dimitri Ivanowski • repeated Mayer’s observation • additionally filter the sap of leaves using Chamberland filter • 1898 - Martinus Beijerinck • Showed that sap of infected tobacco plant retain its infectivity after filteration by chamberland porcelin filter • Extented these studies by showing that filtered sap diluted regain its strength after replication in living growing tissue of plant • Called this filterable agent as “Contagium vivum fluidum” -contagious living liquid 4/14/2021 10 History
- 11. 4/14/2021 11 • 1898 - Freidrich Loeffler and Paul Frosch: applied filteration criteria to a disease transmission in cattle which later k/a Foot-and-mouth disease • 1900 -Walter Reed demonstrated that yellow fever is spread by mosquitoes • 1908 - Karl Landsteiner and Erwin Popper proved that poliomyelitis is caused by a virus. Landsteiner and Popper were the first to prove that viruses could infect humans as well as animals • 1911 - Francis Peyton Rous: discovered 1st tumor causing virus History
- 12. 4/14/2021 12 • 1915 - Frederick Twort: detected existance of filterable agents that could kill bacteria • 1917 - Felix d’ Herelle: • also noted killing of bacteria by agent he called Bacteriophage • defined Plaque Assay for quantification of bacteriphage • 1931 - Woodruff and Goodpasture used ECE for virus culture • 1935 - Wendell Stanley established the chemical nature of viruses when he crystallized TMV and found it to be mostly protein History
- 13. 4/14/2021 13 • 1939 - Emory Ellis and Max Delbruck: one step growth curve of bacteriophage` • 1957 - Isaacs and Lindemann discover Interferon (INF) • 1979 - WHO declared smallpox globally eradicated History
- 14. 4/14/2021 14 Virus Properties 1. Ultramicroscopic 2. Filterable agents 3. Obligate intracellular parasite: cannot be grown on artificial media (but not all obligate intracellular agents are virus eg. Anaplasma, Ehrlichia, Rickettsia) 4. Do not possess cellular organelles viz. mitochondria, golgi, ER associated ribosome's) 5. Multiplication not by binary fission but resembles an assembly line in which different parts of virus come together from different sites in host cell to form new virus particle 6. Genome either DNA or RNA
- 15. 4/14/2021 15 Property Bacteria Mycoplasma Rickettsia/ Chlamydiae Viruses Fungi Size (> 300 nm diameter) + + + < 300 nm + Culture on Artificial media + + _ _ + Obligate intracellular _ _ + + _ DNA and RNA + + + DNA or RNA + Functional ribosomes + + + _ + Characteristics of Infectious Agents
- 16. 4/14/2021 16 • Composition varies in individual virus family • Simplest virus consist of a core of nucleic acid surrounded by a protein coat (Capsid) and +/- Envelope • Components of virus particle include: 1. Nucleic acid- DNA or RNA • single stranded / double stranded • Segmented / non segmented • linear / circular Composition of Virus
- 17. 4/14/2021 17 2. Proteins • Structural -capsid made of capsomeres • serve as antigens which elicit an immune response • Enzymes • differ from host cell • targets of antiviral therapy 3. Envelope • lipoprotein envelope containing viral and host cell components Composition of Virus
- 18. 4/14/2021 18 Structure of Virus
- 19. 4/14/2021 19 • Viruses varies in shape, size and symmetry 1. Shape: Spherical, Rod, Elongated/Filamentous, Bullet, Brick, cone, etc. 2. Size: usually ranges from 20 nm to 300 nm 3. Symmetry: refers to the way capsomere are arranged in the virus capsid • Virus particles exhibit 3 types of capsid symmetry 1. Icosahedral: isometric or cubic 2. Helical: tubular 3. Complex Structure of Virus
- 20. 4/14/2021 20 Icosahedral capsids 1 2 6 7 9 5 4 3 11 12 8 10 • 12 vertex • 20 faces • 60 sides • 3 axis of rotation
- 21. 4/14/2021 21 Icosahedral: Axis of symmetry 2-FOLD 5-FOLD 3-FOLD
- 22. 4/14/2021 22 Icosahedral: Axis of symmetry
- 23. 4/14/2021 23 Helical Symmetry Tobacco mosaic virus Amplitute : daimeter Pitch: distance covered by each complete turn
- 24. • Virus Classification I - the Baltimore classification – genetic content and replication strategy • Virus classification II - the Classical system – based on three principles 1. classifying virus itself not host 2. nature of nucleic acid 3. shared physical properties of the infectious agent (e.g capsid symmetry, dimensions, lipid envelope) • Virus classification III - the genomic system – based on DNA/RNA sequence – By the year 2014 there were 3180 viruses of plants, animals and bacteria - in 7 order, 104 families, 23 subfamilies and 505 genera 4/14/2021 24 Classification of virus
- 25. 4/14/2021 25 Baltimore classification of virus • David Baltimore (Nobel prize 1975) proposed a classification based on combination of • type of viral genome (genetic content) • information about genome synthesis (replication strategy) • focus on synthesis of mRNA • (+) strand can be directly translated • (-) strand cannot be translated • Seven distinct virus groups
- 26. 4/14/2021 26 Baltimore Classification of Virus • Baltimore classification: seven classes of viruses : 1. ds DNA viruses (herpes, papova, adeno, pox) 2. ss DNA viruses (parvo) 3. ds RNA viruses (reo, rota) 4. (+) sense ss RNA viruses ((picorna, toga, flavi, corona) 5. (-) sense ss RNA viruses (arena, bunya) 6. RNA reverse transcribing viruses (retro, lenti) 7. DNA reverse transcribing viruses (hepedna)
- 27. 4/14/2021 27 • Initially grouped based on disease and symptoms viz. • Hepatitis virus: viruses causing liver pathology • Mosaic viruses: viruses causing mottling in plants • 1966 – International Committee on Nomenclature of Viruses (ICNV) • International Committee on Taxonomy of Viruses (ICTV) Virus Taxonomy and Nomenclature
- 28. 4/14/2021 28 • Virus Classification: polythetic system • Order: highest taxa • Family: practical used highest taxa • Subfamily • Genus • Species • Viruses with similar structural, genomic and replication properties are grouped into families (suffix: viridae) e.g. Herpesviridae • Families subdivided into genera (suffix: virus) e.g. Herpes simplex virus, Cytomegalovirus, Varicella zoster virus • Subtypes based on nucleotide sequence and antigenic reactivities e.g. Herpes simplex virus type 1, Herpes simplex virus type 2 Virus Taxonomy
- 29. 4/14/2021 29 • Order: suffix: virales • Family: suffix: viridae • Subfamily: suffix: virinae • Genus: suffix: virus • Species : can contain more than one word • have no specific ending • for species having more than one word , 1st word plus any proper nouns are capitalised Virus Nomenclature • must be single word written in continuation without any space •1st alphabet should be capitalized • letter should be italics
- 30. 4/14/2021 30 • Eg. Newcastle disease virus • Order: Mononegavirales • Family: Paramyxoviridae • Subfamily: Paramyxovirinae • Genus: Avulavirus Virus Nomenclature