VPViro_L2_DK_Spring2024.pdf

Introduction to Microbiology: Virology Lecture 2 – Structure and classification of viruses Viral ecology and transmission Darryn Knobel BVSc MSc PhD [email protected] Structure and classification of viruses Structure and classification of viruses General virus structure Glycoproteins (viral pro...

Introduction to Microbiology: Virology Lecture 2 – Structure and classification of viruses Viral ecology and transmission Darryn Knobel BVSc MSc PhD [email protected] Structure and classification of viruses Structure and classification of viruses General virus structure Glycoproteins (viral proteins) Envelope (phospholipid bilayer from host cell) Nucleic acid (DNA or RNA) Nucleocapsid Capsid Structural unit (capsomer) comprising one or multiple protein subunits (protomers) Structure and classification of viruses Introduction Structure and classification of viruses General virus structure Structure and classification of viruses General virus structure Human immunodeficiency virus Bacteriophage Influenza virus Adenovirus Tobacco mosaic virus Ebola virus Structure and classification of viruses Classifying viruses Four characteristics are used in the taxonomic classification of all viruses: 1. 2. 3. 4. Nature of the nucleic acid in the virus particle (DNA or RNA) Symmetry of the capsid Presence or absence of an envelope Dimensions of the virion and capsid Nowadays the standard method for taxonomic classification is based on analysis of genome sequence similarities The above characteristics are still useful! Structure and classification of viruses Taxonomy International Committee on Taxonomy of Viruses (ICTV) (https://ictv.global/taxonomy) Kingdom Phylum Class Order (-virales): Mononegavirales Family (-viridae) Rhabdoviridae Genus (-virus) Lyssavirus Species Lyssavirus rabies Virus name: rabies virus (RABV) History of virology Classification of viruses Structure and classification of viruses Baltimore classification of genome types Defintions mRNA (ribosome ready) is always the (+) strand DNA of equivalent polarity is also the (+) strand RNA and DNA complements of (+) strands are negative (-) strands Structure and classification of viruses Baltimore classification of genome types II I VII VI IV III V Structure and classification of viruses Capsid symmetry Helical Icosahedral Complex Structure and classification of viruses Capsid symmetry § Capsomeres and nucleic acid are wound together and form a helical or spiral tube § In animal viruses: helical nucleocapsids are always enclosed within a lipoprotein envelope § In plant viruses: naked helical nucleocapsids are common HELICAL CAPSID SYMMETRY Structure and classification of viruses Capsid symmetry HELICAL CAPSID SYMMETRY Structure and classification of viruses Capsid symmetry Enveloped RNA viruses with (-)ssRNA and helical nucleocapsids Paramyxoviridae (canine distemper virus, rinderpest virus) Rhabdoviridae (rabies virus) Orthomyxoviridae (influenza virus) Filoviridae (ebola virus) HELICAL CAPSID SYMMETRY Structure and classification of viruses Capsid symmetry Icosahedron: 20 faces (each an equilateral triangle), 12 vertices and 30 edges Threefold (20), fivefold (12) and twofold (30) axes of symmetry Allows formation of a closed shell with smallest number (60) of identical subunits ICOSAHEDRAL CAPSID SYMMETRY Structure and classification of viruses Capsid symmetry § Protomeres aggregate to form capsomers which are either hexons or pentons Crystallographic structure of a simple icosahedral virus ICOSAHEDRAL CAPSID SYMMETRY Structure and classification of viruses Capsid symmetry § Virions often appear spherical, but have icosahedral symmetry (not necessarily icosahedron shape) ICOSAHEDRAL CAPSID SYMMETRY Structure and classification of viruses Capsid symmetry § Virions are composed of several parts, each with separate symmetries and shapes § Bacteriophage: icosahedral head and helical tail Bacteriophage COMPLEX CAPSID SYMMETRY Pox virus Structure and classification of viruses Virus envelope § An envelope is the outer membrane of a virus, an extra lipid bilayer surrounding the protein capsid § Derived from a host cell membrane (plasma membrane/nuclear membrane/Golgi/endoplasmic reticulum) ENVELOPED vs NON-ENVELOPED (naked) virus Structure and classification of viruses Classifying viruses Property Parameters Nucleic acid § DNA § RNA Capsid symmetry § Icosahedral § Helical § Complex Envelope § Enveloped § Non-enveloped Genome architecture § Linear, circular, segmented § Strandedness: single-stranded (ss), double stranded (ds), ds with regions of ss § Sense: positive (+), negative (-), ambisense (+/-) Classification criteria RNA Nucleic acid Symmetry of capsid DNA Icosahedral Naked or enveloped Naked ds ds (+) ss (+) ss (+) ss Genome 10-18 2 seg. cont. cont. cont. architecture seg. Baltimore IV III IV IV III class Birna Picorna Calici Arteri SedoFamily reo (+) ss (+) ss (+) ss (+) ss cont. cont. 2 copies cont. Flavi IV Toga VI IV (–) ss (–) ss (–) ss (–) ss (–) ss (–) ss (–) ss ss linear ss circular ds ds ds cont. cont. cont. 3 seg. 8 seg. cont. 2 seg. (+) or (–) (+) or (–) circular circular linear V V V V V V V Retro Coronoa Filo Borna Rhabdo Bunya Ortho- Para- Arena myxo myxo II Parvo II Circo I Papilloma Complex Enveloped Envel- Naked/Env. oped (cytoplasmic) (cytoplasmic) Naked Enveloped Enveloped IV Icosahedral Helical I I Poly- Adeno oma ds linear I Herpes ds linear I Asfar ds linear (x-linked) I Pox Virus ecology and transmission Virus ecology and transmission Viral infection cycle Shedding from host Entry into susceptible host Clearance from host Local or general spread in the host, with secondary virus replication Primary virus replication Virus ecology and transmission Routes of entry Virus ecology and transmission Routes of transmission Transplacental Sexual Vector-borne Virus ecology and transmission Routes of transmission Virus ecology and transmission Spread through the host Spread via the blood (Hematogenous spread) Spread via the nerves (Neural spread) Virus ecology and transmission Shedding from host Respiratory secretions (coronavirus; influenza virus) Saliva (rabies; EBV/mono/kissing disease) Feces (norovirus; rotavirus) Blood (hematophageous vectors; VHFs incl. RVFV) Urine (hanta- & arenaviruses) Semen (HIV; equine arteritis virus) Milk (HIV) Skin lesions (Mpox) Tears Virus ecology and transmission Vertical transmission Transmission of a pathogen from mother to offspring during the period before, during or immediately after birth Transplacental (e.g. Via milk (e.g. Direct contact during or after birth (e.g. Virus ecology and transmission Cross-species transmission ‘Multi-host’ pathogens can infect multiple host species Animal -> animal (e.g. malignant catarrhal fever) Animal -> human (anthropozoonoses) Human -> animal Zoonoses (zooanthroponoses/’reverse zoonoses’) Virus ecology and transmission Cross-species transmission: Reservoir hosts The host population in which a viral population is maintained Reservoir hosts are generally efficient at transmitting virus Viruses can still be highly virulent to and even kill reservoir hosts Virus ecology and transmission Cross-species transmission: Spillover hosts Individuals are susceptible to infection But the host population cannot maintain the viral population Spillover hosts are generally inefficient at transmitting virus Infections in spillover hosts are often ‘dead-end’ infections or only short chains of tranmission Virus ecology and transmission Cross-species transmission: 5 evolutionary stages Wolfe, N., Dunavan, C. & Diamond, J. Origins of major human infectious diseases. Nature 447, 279–283 (2007). https://doi.org/10.1038/nature05775