1. Chapter 6 An Introduction to the Viruses

2. 2 Introduction  Viruses are a unique group of biological entities known to infect every type of cell including bacteria, algae, fungi, protozoa, plants, and animals  Viruses are considered to be noncellular particles with a definite size, shape, and chemical composition  Viruses are best described as infectious particles and as either active or inactive  Viruses are a type of obligate intracellular parasites that cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release quantities of new viruses  Because of this characteristic, viruses are capable of causing serious damage and disease

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4. 4 General Structure of Viruses  Size range  viruses represent the smallest infectious agents  viruses are ultramicroscopic  most <0.2 μm; requires electron microscope  animal viruses range in size from 20nm (0.02µm) in diameter to 450nm (0.45µm) in length. Some cylindrical viruses are relatively long 800nm (0.8µm) in length but so narrow in diameter 15nm (0.015µm)  Virion – fully formed virus able to establish an infection

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7. 7 General Structure of Viruses  Capsid  all viruses have a capsid or shell - protein coat that surrounds and protects their nucleic acid in the central core  each capsid is constructed from identical subunits called capsomers made of protein  the capsid together with the nucleic acid are nucleoscapsid  some viruses have an external covering called envelope; those lacking an envelope are naked

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11. 11 General Structure of Viruses  Two structural types:  helical - continuous helix of capsomers forming a cylindrical nucleocapsid  icosahedral - 20-sided with 12 corners  vary in the number of capsomers  each capsomer may be made of one or several proteins  some are enveloped

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16. 16 General Structure of Viruses  Viral envelope  mostly animal viruses  acquired when the virus leaves the host cell  exposed proteins on the outside of the envelope, called spikes, essential for attachment of the virus to the host cell

17. 17 Functions of Capsid/Envelope  Protects the nucleic acid when the virion is outside the host cell  Helps to bind the virion to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell

18. 18 General Structure of Viruses  Complex viruses: atypical viruses  poxviruses lack a typical capsid and are covered by a dense layer of lipoproteins  some bacteriophages have a polyhedral nucleocapsid along with a helical tail and attachment fibers

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21. 21 Nucleic acids  Viral genome – either DNA or RNA but never both  Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses  Number of genes varies for each type of virus – few to hundreds

22. 22 Nucleic Acids  DNA viruses  usually double stranded (ds) but may be single stranded (ss)  circular or linear  RNA viruses  usually single stranded, may be double stranded, may be segmented into separate RNA pieces  ssRNA genomes ready for immediate translation are positive- sense RNA  ssRNA genomes that must be converted into proper form are negative-sense RNA

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24. 24 General Structure  Pre-formed enzymes may be present  polymerases – DNA or RNA  replicases – copy DNA  reverse transcriptase –synthesis of DNA from RNA (AIDS virus)

25. 25 How Viruses are Classified and Named  Main criteria presently used are structure, chemical composition, and genetic makeup  No taxa above Family (no kingdom, phylum, etc.)  Currently recognized: 3 orders, 63 families, and 263 genera of viruses  Family name ends in -viridae, i.e.Herpesviridae  Genus name ends in -virus, Simplexvirus  Herpes simplex virus I (HSV-I)

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28. 28 Modes of Viral Multiplication  General phases in animal virus multiplication cycle:  adsorption - binding of virus to specific molecule on host cell  penetration - genome enters host cell  uncoating – the viral nucleic acid is released from the capsid  synthesis – viral components are produced  assembly – new viral particles are constructed  release – assembled viruses are released by budding (exocytosis) or cell lysis

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30. 30 Adsorption and Host Range  Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane  Spectrum of cells a virus can infect – host range  hepatitis B – human liver cells  poliovirus – primate intestinal and nerve cells  rabies – various cells of many mammals

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32. 32 Penetration/Uncoating  Flexible cell membrane is penetrated by the whole virus or its nucleic acid by:  endocytosis – entire virus is engulfed and enclosed in a vacuole or vesicle  fusion – envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm

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34. 34 Replication and Protein Production  Varies depending on whether the virus is a DNA or RNA virus  DNA viruses generally are replicated and assembled in the nucleus  RNA viruses generally are replicated and assembled in the cytoplasm  positive-sense RNA contain the message for translation  negative-sense RNA must be converted into positive-sense message

35. 35 Release  Assembled viruses leave host cell in one of two ways:  budding – exocytosis; nucleocapsid binds to membrane which pinches off and sheds the viruses gradually; cell is not immediately destroyed  lysis – nonenveloped and complex viruses released when cell dies and ruptures  Number of viruses released is variable  3,000-4,000 released by poxvirus  >100,000 released by poliovirus

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37. 37 Damage to Host Cell  Cytopathic effects - virus-induced damage to cells  Changes in size & shape  Cytoplasmic inclusion bodies  Nuclear inclusion bodies  Cells fuse to form multinucleated cells.  Cell lysis  Alter DNA  Transform cells into cancerous cells

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40. 40 Persistent Infections  Persistent infections - cell harbors the virus and is not immediately lysed  Can last weeks or host’s lifetime; several can periodically reactivate – chronic latent state  measles virus – may remain hidden in brain cells for many years  herpes simplex virus – cold sores and genital herpes  herpes zoster virus – chickenpox and shingles

41. 41  Some animal viruses enter host cell and permanently alter its genetic material resulting in cancer – transformation of the cell  Transformed cells have increased rate of growth, alterations in chromosomes, and capacity to divide for indefinite time periods resulting in tumors  Mammalian viruses capable of initiating tumors are called oncoviruses  Papillomavirus – cervical cancer  Epstein-Barr virus – Burkitt’s lymphoma

42. 42 Multiplication Cycle in Bacteriophages  Bacteriophages – bacterial viruses (phages)  Most widely studied are those that infect Escherichia coli – complex structure, DNA  Multiplication goes through similar stages as animal viruses  Only the nucleic acid enters the cytoplasm - uncoating is not necessary  Release is a result of cell lysis induced by viral enzymes and accumulation of viruses - lytic cycle

43. 43 6 Steps in Phage Replication  Adsorption – binding of virus to specific molecule on host cell  Penetration –genome enters host cell  Replication – viral components produced  Assembly - viral components assembled  Maturation – completion of viral formation  Release – viruses leave cell to infect other cells

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47. 47 Lysogeny: The Silent Virus Infection  Not all phages complete the lytic cycle  Some DNA phages, called temperate phages, undergo adsorption and penetration but don’t replicate  The viral genome inserts into bacterial genome and becomes an inactive prophage - the cell is not lysed  Prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny  Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis

48. 48 Lysogeny  Lysogeny results in the spread of the virus without killing the host cell  Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology – lysogenic conversion  Corynebacterium diphtheriae  Vibrio cholerae  Clostridium botulinum

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50. 50 Techniques in Cultivating and Identifying Animal Viruses  The primary purposes of viral cultivation are:  to isolate and identify viruses in clinical specimens  to prepare viruses for vaccines  to do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells

51. 51 Techniques in Cultivating and Identifying Animal Viruses  Obligate intracellular parasites that require appropriate cells to replicate  Methods used:  cell (tissue) cultures – cultured cells grow in sheets that support viral replication and permit observation for cytopathic effect  bird embryos – incubating egg is an ideal system; virus is injected through the shell  live animal inoculation – occasionally used when necessary

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54. 54 Prions and Other Infectious Particles  Prions - misfolded proteins, contain no nucleic acid  cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases  common in animals:  scrapie in sheep & goats  bovine spongiform encephalopathies (BSE), aka mad cow disease  wasting disease  humans – Creutzfeldt-Jakob Syndrome (CJS)  Extremely resistant to usual sterilization techniques

55. 55 Other noncellular infectious agents  Satellite viruses – dependent on other viruses for replication  adeno-associated virus – replicate only in cells infected with adenovirus  delta agent – naked strand of RNA expressed only in the presence of hepatitis B virus  Viroids - short pieces of RNA, no protein coat; only been identified in plants, so far

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57. 57 Detection and Treatment of Animal Viral Infections  More difficult than other agents  Consider overall clinical picture  Take appropriate sample  infect cell culture- look for characteristic cytopathic effects  screen for parts of the virus  screen for immune response to virus (antibodies)  Antiviral drugs can cause serious side effects