Lecture 3_ Ch 29.1-29.4 General Properties of Viruses.pdf

Jawetz, Melnick, & Adelberg’s Medical Microbiology, Twenty-Eighth Edition Slides by Dr. Jose Sapien Chapter 29: General Properties of Viruses 29.1 (Introduction) – 29.4 (Class...

Jawetz, Melnick, & Adelberg’s Medical Microbiology, Twenty-Eighth Edition Slides by Dr. Jose Sapien Chapter 29: General Properties of Viruses 29.1 (Introduction) – 29.4 (Classification) Jawetz, Melnick, & Adelberg’s Medical Microbiology, Twenty-Eighth Edition Introduction Viruses are the smallest infectious agents (ranging from about 20 to 300 nm in diameter). The nucleic acid is encased in a protein shell, which may be surrounded by a lipid-containing membrane. How do we call these two structures? The National Center for Biological Information (NCBI) database contains more than 8000 complete viral genomes as February 2019. (https://www.ncbi.nlm.nih.gov/labs/virus/vssi/#/ ) The entire structure of the virus (the genome, the capsid, and – where present – the envelope) makes up the virion or virus particle = infectious unit. © 2021 John Wiley & Sons, Inc. © McGraw-Hill Education, 2019  The Enigma of Viral Origin Proposed Theories 1. Escape from Cellular Control:  Cellular Origins: Some viruses might have evolved from cellular genetic elements that gained the ability to replicate independently.  Gene-like Behavior: These viral elements could be seen as "escaped genes" capable of autonomous existence. 2. Degenerated Cellular Organisms:  Reduced Complexity: Certain viruses, particularly larger ones like poxviruses, might represent simplified forms of once more complex intracellular organisms. Viruses do not: Encode ribosomal proteins Contain genes for energy metabolism This suggests viruses did not evolve from free-living organisms. © 2021 John Wiley & Sons, Inc. © McGraw-Hill Education, 2019  Identify these next concepts: Nucleocapsid Capsid Protomeres and Capsomeres Defective virus Envelope Structural units Subunit Virion Defective viruses require the coinfection of a helper virus for their replication; thus, they are parasitic on viruses. © McGraw-Hill Education, 2019 Terms and definitions in virology © McGraw-Hill Education, 2019 Classification Basis of Classification 1. Virion morphology: - Size (large or small) - Shape - Type of symmetry (Icosahedral, helical or complex) - Presence or absence of capsomeres/ protomers - Presence or absence of membranes. © 2021 John Wiley & Sons, Inc. © McGraw-Hill Education, 2019  Helical Nucleocapsids (1):  Consist of capsid proteins arranged in a helical pattern.  These proteins wrap around a helical filament of nucleic acid. Icosahedral Nucleocapsids (2):  Polyhedron having 20 equilateral triangular faces, 12 vertices, and 30 edges.  Common shape in many "spherical" viruses.  Characterized by the specific number and arrangement of capsomeres.  They could be polyhedral or spherical with fivefold, threefold, or twofold axes of rotational symmetry (1) (2) Medical Microbiology. 4th edition. Baron S, editor. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Copyright © 1996, The University of Texas Medical Branch at Galveston © McGraw-Hill Education, 2019  Classification 2. Virus genome properties: - Type of nucleic acid (DNA or RNA) - Size of the genome (Size of nucleic acid) - Strandedness (single or double) - Linear or circular - Sense (positive, negative, ambisense) - Segments (number, size) - Nucleotide sequence © McGraw-Hill Education, 2019  DNA Viruses: Nuclear replication Shared processes with host cell RNA Viruses: Cytoplasmic replication (typically) Viral-encoded RNA polymerase Independent of host cell replication machinery (carry or encode their own RNA-dependent RNA polymerase (RdRp), an enzyme that synthesizes RNA from an RNA template), but not independent ribosomes to express genes. Zhuang, X., et al. (2022) © 2021 John Wiley & Sons, Inc. © McGraw-Hill Education, 2019 Classification 3. Genome organization and replication: - Gene order, number and position of open reading frames - Strategy of replication (patterns of transcription, translation) - Cellular sites (accumulation of proteins, virion assembly, virion release). © McGraw-Hill Education, 2019 Classification 4. Virus protein properties: - Number - Size - Amino acid sequence - Modifications (glycosylation, phosphorylation, myristoylation), and functional activities of structural and nonstructural proteins (transcriptase, reverse transcriptase, neuraminidase, fusion activities). © McGraw-Hill Education, 2019 Classification 5. Antigenic properties, particularly reactions to various antisera. © McGraw-Hill Education, 2019 Classification 6. Physicochemical properties of the virion - Molecular mass - Buoyant density - Ph stability - Thermal stability - Susceptibility to physical and chemical agents, especially solubilizing agents and detergents. © McGraw-Hill Education, 2019 Classification 7. Biologic properties: Natural host range (bacteriophage) Mode of transmission (airborne viruses) Vector relationships (arthropod-borne viruses) Pathogenicity (Highly, moderate and mild) Tissue tropisms (Neurotropic, dermatotropic, hepatotropic) Pathology (oncogenic, hemorrhagic) © McGraw-Hill Education, 2019 The International Committee on Taxonomy of Viruses (ICTV) It was created at an international conference on microbiology in Moscow in 1966. Selected viral features that can be used for classification:  The nature of the viral genome  The presence of an envelope  The morphology of the virus capsid © 2021 John Wiley & Sons, Inc. © McGraw-Hill Education, 2019 The Baltimore scheme of virus classification In 1971, David Baltimore suggested a scheme for virus classification based on the way in which a virus produces messenger RNA (mRNA) during infection. Type of genome → mRNA → translation →protein Viruses with RNA genomes whose genome is the same sense as mRNA = positive‐sense (+ sense) RNA viruses Viruses whose genome is the opposite (complementary) sense of mRNA = negative‐sense (− sense) RNA viruses. Viruses with double‐stranded genomes obviously have both senses of the nucleic acid. © 2021 John Wiley & Sons, Inc. © McGraw-Hill Education, 2019 Universal System of Virus Taxonomy In the early days, they were named based on pathogenic properties (abbreviations from a few or initial letters) Viruses are classified into families (major groups) based on: Morphology (Virion structure) Genome type Replication strategy Family names end in -viridae. Genus (Subgroups) names carry the suffix -virus. There are 21 families and genera of medical importance. © McGraw-Hill Education, 2019 Universal System of Virus Taxonomy Virion structure Genome type Replication strategy Pathogenic characteristics Examples: Parvoviridae  Parvoviruses (from Latin parvus meaning small) are very small viruses. Adenoviridae  Adenoviruses (from Latin adenos meaning gland) are medium-sized. Hepadnaviridae  Hepadnaviruses (from Latin hepa meaning liver) are small. © McGraw-Hill Education, 2019 © McGraw-Hill Education, 2019 © McGraw-Hill Education, 2019  Additional source Gelderblom HR. Structure and Classification of Viruses. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 41. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8174/ © McGraw-Hill Education, 2019