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PowerPoint to accompany Foundations in Microbiology Fifth Edition Talaro Chapter 6 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. An Introduction to the Viruses Chapter 6 2 3 Size of viruses Smallest infectious agents- must use electron microscope 4 Purified poliovirus crystals Light microscope1200X Purified poliovirus crystals Electron micrograph 150,000X Shows individual viruses Naming viruses • No taxa above Family (no kingdom, phylum, etc) • 19 families of animal viruses • Family name ends in -viridae , Herpesviridae • Genus name ends in -virus, Simplexvirus • Herpes simplex virus I (HSV-I) • • • • Family – Herpesviridae Genus – Varicellovirus Common name – chickenpox virus Disease - chickenpox Standardized species names not widely accepted Family Characteristics include: type of capsid, nucleic acid, presence and type of envelope, overall viral size, and area of host cell in which the virus multiplies 7 Table 6.2a Capsids • All viruses have capsids- protein coats that enclose & protect their nucleic acid • Each capsid is constructed from identical subunits called capsomers made of protein • Capsomers spontaneously self-assemble into finished capsids of 2 types: – helical – icosahedral 10 Naked Enveloped Helical Nucleocapsid=capsid and nucleic acid Enveloped viruses are mostly animal viruses. When a virus is released from a cell it takes a bit of membrane with 11 them to form an envelope Assembly of helical nucleocapsids Fig. 6.5 Naked helical virus Enveloped helical virus Influenza virus Tobacco mosaic virus Naked Icosahedral Enveloped 14 Icosahedral • 20-sided with 12 corners • Vary in the number of capsomers • Each capsomer may be made of 1 or several proteins • Some are enveloped 15 Fig. 6.7 Adenovirus model Fig. 6.8 Naked icosahedral virus Rotavirus Enveloped icosahedral virus Herpes simplex virus, 300,000X Complex or Atypical Polyhedral Vaccinia virus, a poxvirus Bacteriophage Bacteriophage Poxvirus, large DNA virus Mumps virus herpesvirus adenovirus Fig. 6.10 Flexible-tailed bacteriophage rhabdovirus HIV virus Helical nucleocapsid See page 165, fig 6.10 Icosahedral nucleocapsid papillomavirus Icosahedral capsid 6 steps in phage replication in Bacteria 1. adsorption – binding of virus to specific molecule on host cell 2. penetration –genome enters host cell 3. replication – viral components produced 4. assembly - viral components assembled 5. maturation – completion of viral formation 6. release – viruses leave cell to infect other cells Fig 6.11 22 penetration 23 Bacteriophage assembly line 24 Fig. 6.14 A bacteria cell rupturing with bacteriaphage: spent phage lined up along cell wall • Not all bacteriophages lyse cells • Temperate phages insert their viral DNA into the host chromosome & viral replication stops there until some later time. • Lysogeny- bacterial chromosome carries phage DNA 26 • Induction- the prophage will be activat3ed and return to the lytic cycle • Phages can serve as transporters of bacterial genes from one bacteria to anotherTransduction • Can transfer genes for toxin production and drug resistance 27 Host range • Spectrum of cells (host) a virus can infect – cell has to have a specific structure (receptor) on its surface for viral attachment – cell has to contain all of the enzymes and materials needed to produce new virions • May be one species or many – HIV (only humans) vs rabies (many animals) • May be one tissue or many within a host – Hepatitis (human liver) vs polio (primates’ intestinal & nerve cells) – Rabies infects nerve cells of most mammals 28 Animal virus replication 1. 2. 3. 4. 5. adsorption penetration/uncoating of genome duplication/synthesis assembly release 29 RNA replication in cytoplasm and DNA replicated in the nucleus 30 adsorption Enveloped coronavirus- the configuration of the spike has a complementary fit for cell receptors Adenovirus-naked capsid that adheres to its host cell by surface molecules on its capsid into the receptors 31 penetration Endocytosis and uncoating Fusion of cell membrane with the viral envelope 32 Release by budding 33 Parainfluenza virus budding and picking up envelope and spikes HIV virus leave the host T cell by budding off its surface Differences between phage and animal virus replication 1. Animal virus replication is more complex than phage replication because host cells are more complex. 2. Animal viruses cannot inject their DNA must be engulfed or fuse with the membrane. 3. Viral persistence-lysogeny for phage, latency for animal viruses 35 Table 6.3 Cytopathic effects (CPEs)- virusinduced damage to cells 1. 2. 3. 4. 5. 6. 7. changes in size & shape cytoplasmic inclusion bodies appear nuclear inclusion bodies cells fuse to form multinucleated cells-syncytia cell lysis alter DNA transform cells into cancerous cells-oncogenic, capacity to divide for an indefinite period Oncovirus-DNA virus, papilloma virus, genital warts associated with cervical cancer 37 Cytopathic changes in cells 38 How do we grow viruses? Obligate intracellular parasites require appropriate cells to replicate. Growing animal viruses 1. In vivo- live animals 2. bird embryos – chicken, duck; intact, selfsupporting unit, sterile, self-nourished 3. In vitro- cell culture-makes it possibloe to propagate most viruses (sterile conditions, special media with correct nutrients) pg 176 41 Fig. 6.22 Technician fertilizing chicken eggs with influenza virus, first step in preparing vaccines Monolayer of cells Plaques show signs of infection 43 Fig. 6.23a Petri dish of E. coli bacteria show macroscopic plaques Other noncellular infectious agents 1. prions - misfolded proteins, contain no nucleic acid – cause spongiform encephalopathies – holes in the brain – common in animals • • • scrapie in sheep & goats bovine spongiform encephalopathies (BSE), aka mad cow disease humans – Creutzfeldt-Jakob Disease 2. viroids - short pieces of RNA, no protein coat – only been identified in plants, so far 45 Diagnosis of viral diseases • 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) 46 • • • • Cytopathic changes Immunofluorescence techniques Electron microscope Screen samples for presence of indicator molecules (antigens) with polymerace chain reaction (PCR) • Certain infections require cell culture, embryos, or animals for definitive diagnosis 47 Diagnosis Fig. 6.24a Diagnosis