topic 8; modified virus: Comprehensive Virus Structure, Classification, and History Overview

Contagium vivum fluidum

Early term used for infectious, filterable agents; used historically for plant diseases; emphasized infectious agents smaller than bacteria

Virus (term origin)

Adopted widely in the 1930s; derived from the Latin word virus meaning poison

Electron microscope

Instrument whose invention made it possible to see viruses for the first time

Adolf Mayer (1886)

First described tobacco mosaic disease and demonstrated transfer of disease between plants

Dmitri Ivanovsky (1892)

Showed infected sap remained infectious after filtration through Chamberland filters; suggested infectious agent smaller than bacteria

Martinus Beijerinck (1898)

Confirmed filterability; demonstrated serial transmission with no loss of potency; defined viruses as a new infectious agent

Loeffler and Frosch (1898)

Discovered first filterable agent from animals; identified Foot and Mouth Disease virus

Virus size

Approximately 20 to 300 nanometers

Obligate intracellular parasite

Requires living host cells to multiply; has few or no metabolic enzymes; hijacks host metabolic machinery

Viral genetic material

Contains only one nucleic acid type: DNA or RNA, never both

Capsid

Protein coat that surrounds viral nucleic acid

Envelope

Lipid bilayer with proteins and carbohydrates present in some viruses

Viral replication

Occurs only inside living host cells using host cell machinery

Viruses vs bacteria (cellular nature)

Viruses are acellular; bacteria are unicellular prokaryotes

Plasma membrane comparison

Viruses lack plasma membranes; bacteria possess plasma membranes

Ribosomes

Viruses lack ribosomes; bacteria contain ribosomes for protein synthesis

Nucleic acids comparison

Viruses contain either DNA or RNA; bacteria contain both DNA and RNA

Binary fission

Viruses do not undergo binary fission; bacteria reproduce by binary fission

ATP metabolism

Viruses cannot generate ATP; bacteria can produce ATP

Dependency

Viruses require host cells; bacteria can live independently

Filterability

Viruses pass through bacteriological filters; bacteria generally do not

Antibiotic sensitivity

Viruses are not sensitive to antibiotics

Interferon sensitivity

Viruses are sensitive to interferons; bacteria are not

Host range

Spectrum of host cells a virus can infect

Virus host types

Viruses infect invertebrates, vertebrates, plants, protists, fungi, and bacteria

Specificity

Most viruses infect specific cell types and usually only one host species

Bacteriophage

Virus that infects bacteria

Host range determinant

Presence of specific receptors on host cell surface

Virion

Complete, fully developed viral particle with nucleic acid and coat

Capsomeres

Protein subunits that make up the capsid; may be one or multiple protein types

Nucleocapsid

Viral genome packaged within the capsid

Capsid symmetry

Icosahedral, helical, or complex

Envelope acquisition

Envelope obtained by budding through host cellular membrane, usually plasma membrane

Envelope glycoprotein functions

Bind host receptors; mediate membrane fusion; participate in uncoating; destroy host receptors

Envelope immunologic importance

Envelope epitopes are important for protective immune responses

Virus classification basis

Type of nucleic acid and replication strategy

Family naming rule

Family names end in -viridae

Genus naming rule

Genus names end in -virus

Baltimore classification

Classification based on nucleic acid type and replication strategy

Adenoviridae

Nonenveloped; 70-90 nm; genus Mastadenovirus; icosahedral capsid; linear dsDNA; respiratory infections and tumors

Papovaviridae

Nonenveloped; 55 nm; Papillomavirus and Polyomavirus; icosahedral; circular dsDNA; cause warts and cervical/anal cancer

Poxviridae

Enveloped; largest animal viruses; 300×200 nm; complex symmetry; linear dsDNA; brick-shaped; smallpox, molluscum contagiosum, cowpox

Herpesviridae

Enveloped; 120-200 nm; icosahedral; linear dsDNA; fever blisters, chickenpox, shingles, mononucleosis; cancers; Aujeszky, ILT, Marek disease

Asfarviridae

Enveloped; 175-215 nm; icosahedral; linear dsDNA; causes African Swine Fever

Iridoviridae

Infect fishes, amphibians, and insects

Parvoviridae

Nonenveloped; 18-25 nm; icosahedral; linear ssDNA; Parvovirus B19 causes fifth disease and anemia

Circoviridae

Nonenveloped; smallest viruses; 17-22 nm; circular ssDNA; Porcine circovirus; Type 2 causes multisystemic disease

Reoviridae

Nonenveloped; 60-80 nm; icosahedral; segmented dsRNA; mild respiratory infections; arthropod-borne; Colorado tick fever

Birnaviridae

Nonenveloped; 60 nm; icosahedral; two segments of dsRNA

Picornaviridae

Nonenveloped; 30 nm; icosahedral; positive ssRNA; polio, coxsackie, echovirus, hand-foot-mouth, Hep A; rhinoviruses cause colds

Caliciviridae

Nonenveloped; 27-40 nm; icosahedral; positive ssRNA; gastroenteritis; hepatitis E

Astroviridae

Nonenveloped; 28-30 nm; icosahedral; positive ssRNA

Togaviridae

Enveloped; 70 nm; icosahedral; positive ssRNA; arboviruses; EEE, WEE, chikungunya; rubella

Flaviviridae

Enveloped; 40-60 nm; icosahedral; positive ssRNA; yellow fever, dengue, Zika, West Nile; Hepatitis C

Coronaviridae

Enveloped; 120-160 nm; helical; positive ssRNA; common cold; SARS; MERS

Arteriviridae

Enveloped; 40-60 nm; icosahedral; positive ssRNA

Rhabdoviridae

Enveloped; bullet-shaped; helical; negative ssRNA; rabies and animal diseases

Filoviridae

Enveloped; 80-14000 nm; helical; Ebola and Marburg viruses

Paramyxoviridae

Enveloped; 150-300 nm; helical; negative ssRNA; parainfluenza, mumps, Newcastle disease

Bornaviridae

Enveloped; 90 nm; icosahedral; negative ssRNA

Bunyaviridae

Enveloped; 80-120 nm; helical; segmented negative or ambisense ssRNA; hantavirus hemorrhagic fever and pulmonary syndrome

Retroviridae

Enveloped; 80-100 nm; diploid positive ssRNA; reverse transcriptase; HIV; leukemia and tumors

Hepadnaviridae

Enveloped; 42 nm; partial dsDNA; reverse transcriptase; hepatitis B; liver tumors

Deltaviridae

Satellite RNA virus; 32 nm; hepatitis D virus; requires hepatitis B coinfection