Viral Structure and Classification M.E 2022 PDF

Intended Learning outcomes By the end of the session and the associated learning resources, the student should be able to: Know the characteristics of viruses. Recognise the structural components of the viral particle. Describe the classification systems of pathogenic viruses. List the viruses that i...

Intended Learning outcomes By the end of the session and the associated learning resources, the student should be able to: Know the characteristics of viruses. Recognise the structural components of the viral particle. Describe the classification systems of pathogenic viruses. List the viruses that infect human. Viruses The word is from the Latinv rī us referring to poison. Viral discovery Viral infections were known before the discovery of viruses. Smallpox Yellow fever Rabies Viral Discovery Jenner in 1796 use cowpox-infected material to vaccinate a boy against smallpox. Viral Discovery Louis Pasteur was unable to find a causative agent for rabies. He speculated that the causative agent too small to be detected by light microscope. Viruses: Discovery Charles Chamberland: developed “Chamberland filter” filtering particles that are smaller than bacteria (1884 ). Dmitry Ivanovsky: provided the first solid evidence for the existence of a non-bacterial infectious agent (1892). Martinus Beijerinck: (1898) first to adopt the name “VIRUS”. Viruses: Discovery The first images of viruses were obtained upon the invention of Electron Microscopy in 1930s by Ernst Ruska and Max Knoll. TMV Characteristics of viruses Very small: (ranging from 20nm to 300nm, nm = 10-9 meter). Obligate intracellular parasites: replicate only inside cells outside cells they are “inert”. The viral particle is composed of nucleic acid surrounded by protein coat. The nucleic acid is either DNA or RNA but not both. Lack organelles and machineries for “energy generation” and “protein synthesis”. Viruses: Shape Due to their small sizes viruses are only seen by “Electron microscope”. Viruses exhibits different shapes e.g. : Spherical. Rods. Bullet. Filamentous. Cuboids. Viruses: Structure Virion : The complete, infective form virus particle possessing all structures: Nucleic acid : either DNA or RNA. Capsid : The protein shell, or coat, that encloses the nucleic acid genome. Envelope : A lipid-containing membrane that surrounds the viral particle. not ( all virions have envelope ). Viruses: Envelope Lipid membrane that surrounds some viruses. It is acquired from host cell membrane through exit (egress) from the infected cell. Viral encoded “glycoprotein spikes” are exposed from the envelope surface. Glycoprotein spikes Viruses: Envelope The envelope is sensitive to heat, dryness as well as alcohol and ether. Non enveloped viruses are termed “Naked” are more environmentally stable than “enveloped viruses”. Glycoprotein spikes Viruses: Envelope The envelope glycoproteins spikes have certain functions: Attachment to specific host cell receptors (cell entry). Antigenic: site for antibody binding (can be used in serology). Some has enzymatic activities e.g. neuraminidase. influenza ( virus ). Some can adsorb RBC: Haemadsorption. influenza ( virus ) Glycoprotein spikes Viruses: Matrix protein Structural proteins layer linking the viral envelope with the virus core. found in many enveloped viruses. May assist in viral replication and release from the host cell Viruses: Capsid Protein coat surrounding the nucleic acid. Nucleic acid + Capsid= “Nucleocapsid” (Core). It is made up of structural subunits called “Capsomers”. Capsomers are composed of five or six protein molecules; “protomers”. Viruses: Capsid Viral capsid proteins can arrange into different geometric symmetries: i. Icosahedral symmetry. ii. Helical symmetry. iii. Complex symmetry. Viruses: Capsid Icosahedral symmetry. Icosahedron is a 3 dimensional geometric structure that has 20 triangular faces, 12 vertices. Viruses: Capsid Helical symmetry: The capsid is composed of multiple copies of a single kind of protein subunit arranged in a close-packed helix. Complex symmetry: no identifiable arrangement or shape e. g. poxviruses Viruses: Capsid proteins General functions: Protect viral nucleic acid. Attachment of host cell receptors (in naked viruses). Bind specifically to “neutralising antibodies”. Viruses: basic structure Naked + Icosahedral (e.g. Adenovirus) Enveloped + Icosahedral (e.g. Herpesviruses) Naked + helical. (no animal virus) plant virus (Tobacco Mosaic Virus TMV) Enveloped + helical (e.g. Influenza virus) Enveloped + complex. (poxviruses) Viruses: Nucleic acid Either DNA or RNA (not both). Size of the genome: Is measured by the number of bases (nucleotides) in the nucleic acid, and shown as thousands of bases (kilobases, kb). RNA viruses have smaller genomes and encode for fewer proteins than DNA viruses. Viruses: Nucleic acid DNA can be: Linear or circular. Single stranded or double stranded. Viruses: Nucleic acid RNA can be: Linear or circular. Single stranded or double stranded. Segmented or non segmented. RNA can be: Viruses: Nucleic acid Positive sense or negative sense: Positive sense RNA: the viral genome can be directly translated into viral encoded proteins (acting as mRNA). Negative sense RNA: the viral genome cannot be directly translated into viral encoded proteins (need to be converted into mRNA). Viral like structures Defective viruses: A virus particle that is functionally deficient in some aspect of replication. Viroids: are infectious small circular RNA molecules that lack protein shells. (Hepatitis delta agents) Prions: contain proteins without nucleic acid. Causing “Spongiform encephalitis”: e.g. Creutzfedt Jacob disease (CJD). Bovine spongiform encephalitis (BSE). Classification of viruses The process of designation and placing viruses into a taxonomic system. Classification of viruses The classification is based on: Phenotypic characteristics. Genotypic characteristics. Mode of replication. The nature of the host. The type of disease the cause. Classification systems Two important classification systems are used: International Committee on Taxonomy of Viruses (ICTV). Baltimore classification. ICTV classification Authorizes and organizes the taxonomic classification and the nomenclatures for viruses. Based on similarities between viruses in : Physical properties of viruses. Type of the nucleic acid. Type of the capsid. Presence or absence of the envelope. Other properties. Updated ICTV classification Classify viruses into: Realm Kingdom Phylum Class Order; end with suffix –virales :(not for all families) Families; end with suffix –viridae. Subfamilies; end with suffix –virinae. (not for all genera) Genus end with suffix -virus. Species. Example: human herpes viruses Family Sub-Family Sub-Family Genus Genus Genus Genus Genus Genus Baltimore classification Classify viruses into Seven groups according to: Type of the viral genome (DNA or RNA). The strategy for replication and synthesis of mRNA. Baltimore classification Group I: double stranded DNA viruses: Adenoviridae. (e.g. adenovirus) Herpesviridae. Poxviridae. Papovaviridae. ( e.g. Human Papilloma virus) I dsDNA mRNA Baltimore classification Group II: single stranded DNA viruses: Parvoviridae. (e.g human parvovirus B19) II ssDNA mRNA Baltimore classification Group III: double stranded RNA viruses: Reoviridae. (e.g. rotavirus) III dsRNA mRNA Baltimore classification Group IV: single stranded RNA viruses with positive sense: Enveloped: Non enveloped: Astroviridae. Coronaviridae. E.g. MERS Caliciviridae. Togaviridae. Picornaviridae. E.g. poliovirus E.g. rubella virus Hepeviridae. E.g HEV Flaviviridae. E.g. HCV IV ssRNA +ve mRNA Baltimore classification Group V: single stranded RNA viruses with negative sense: Non segmented RNA: Paramyxoviridae. E.g. measles virus Filoviridae. E.g. Ebolavirus Rhabdoviridae. E.g. Rabies Segmented RNA: Orthomyxoviridae. E.g. influenza Arenaviridae. Lassa virus Bunyaviridae. Rift Valley fever virus V ssRNA -ve ssRNA +ve mRNA Baltimore classification Group VI: single stranded RNA viruses replicate through DNA intermediate via process of “reverse transcription”: Retroviridae. HIV VI ssRNA R T dsDNA mRNA Baltimore classification Group VII: double stranded DNA viruses form RNA intermediate and then via “reverse transcription” form the DNA: Hepadnaviridae. dsDNA Hepatitis B virus R VII T ssRNA mRNA Further Reading Warren Levinson, Review of Medical Microbiology and Immunology, 16th Edition. Jawetz Melnick & Adelbergs, Medical Microbiology, 27th Edition. Greenwood Medical Microbiology, 18th Edition. Sherris Medical Microbiology, 7th Edition. https://asm.org/ https://microbiologyinfo.com/ https://www.medscape.org/infectiousdiseases https://www.microbiologybook.org/