This document discusses various systems for classifying viruses. It begins by describing common properties used to classify viruses such as morphology, genome type, replication strategy, and host organisms. It then summarizes five major classification systems: Baltimore classification (based on mRNA synthesis strategy), Holmes classification (based on host organism), LHT classification (based on physical/chemical properties), Casjens and Kings classification (based on nucleic acid and structure), and ICTV classification (the current standard system maintained by the International Committee on Taxonomy of Viruses). The document also discusses classification based on nucleic acid type, number of genome strands, capsid structure, presence of an envelope, and host organism.

1. SRIPARAMAKALYANICOLLEGE
(REACCREDITEDWITHBGRADEWITH ACGPAOF2.71INTHEIICYCLEBYNAAC
AFFILIATEDTOMANONMANIAMSUNDARANARUNIVERSITY,TIRUNELVELI)
ALWARKURICHI627412TAMILNADU,INDIA
POSTGRADUATE& RESEARCHCENTRE -DEPARTMENT OFMICROBIOLOGY
(GOVERNMENTAIDED)
IISEM-CORE–VIROLOGY
UNIT–1
TAXONOMY,CLASSIFICATIONANDNOMENCLATUREOFVIRUS
Submitted to,
GUIDE: Dr.C.MARIAPPAN, Ph.D,
ASSISTANT PROFESSOR,
SRI PARAMAKALYANI COLLEGE,
ALWARKURICHI.
H. Ahamed Samsun Fahmitha
REG NO: 20211232516101
I M.SC.MICROBIOLOGY
Sri Paramakalyani College

2. Contents
◦ Classification of virus
◦ Classification of system
1. Baltimore classification
2. Holmes classification
3. LHT System of Virus Classification
4. Casjens and Kings classification of virus
5. ICTV classification
◦ Nomenclature
◦ Rules of Nomenclature

3. CLASSIFICATION OF VIRUS
◦ Viruses are classified on the basis of morphology, chemical composition, and mode
of replication.
◦ The viruses that infect humans are currently grouped into 21 families, reflecting
only a small part of the spectrum of the multitude of different viruses whose host
ranges extend from vertebrates to protozoa and from plants and fungi to bacteria.
The following properties have been used as a basis for the classification of viruses.
◦ Virion morphology, including size, shape, type of symmetry, presence or absence of
peplomers, and presence or absence of membranes.
◦ Virus genome properties, including type of nucleic acid (DNA or RNA), size of
genome in kilobases (kb) or kilobase pairs (kbp), strandedness (single or double),
whether linear or circular, sense (positive, negative, ambisense), segments (number,
size), nucleotide sequence, G + C content,).
◦ Genome

4. ◦ And presence of special features (repetitive elements, isomerization, 5′-terminal
cap, 5′-terminal covalently linked protein, 3′-terminal poly(A) tract.
◦ Genome organization and replication, including gene order, number and position of
open reading frames, a strategy of replication (patterns of transcription, translation),
and cellular sites (accumulation of proteins, virion assembly, virion release).
◦ Virus protein properties, including number, size, and functional activities of
structural and nonstructural proteins, amino acid sequence, modifications
(glycosylation, phosphorylation, myristylation), and special functional activities
(transcriptase, reverse transcriptase, neuraminidase, fusion activities).
◦ Antigenic properties.
◦ Physicochemical properties of the virion, including molecular mass, buoyant
density, pH stability, thermal stability, and susceptibility to physical and chemical
agents, especially ether and detergents.
◦ Biologic properties, including natural host range, mode of transmission, vector
relationships, pathogenicity, tissue tropisms, and pathology.

5. Virus classification involves naming and placing viruses into a taxonomic system.
Like the relatively consistent classification systems seen for cellular organisms, virus
classification is the subject of ongoing debate and proposals. This is largely due to the
pseudo-living nature of viruses, which are not yet definitively living or non-living. As
such, they do not fit neatly into the established biological classification system in
place for cellular organisms, such as plants and animals, for several reasons.
Virus classification is based mainly on phenotypic characteristics, including
morphology, nucleic acid type, mode of replication, host organisms, and the type of
disease they cause. A combination of two main schemes is currently in widespread
use for the classification of viruses. David Baltimore, a Nobel Prize-winning biologist,
devised the Baltimore classification system, which places viruses into one of seven
groups. These groups are designated by Roman numerals and separate viruses based
on their mode of replication, and genome type. Accompanying this broad method of
classification are specific naming conventions and further classification guidelines set
out by the International Committee on Taxonomy of Viruses.

6. Classification systems
◦ Baltimore classification
◦ Holmes classification
◦ LHT System of Virus Classification
◦ Casjens and Kings classification of virus
◦ ICTV classification

7. Baltimore classification
◦ The Baltimore Classification of viruses is based on the method of viral mRNA
synthesis..
◦ The Nobel Prize-winning biologistDavid Baltimore devised the Baltimore
classification system.
◦ The ICTV classification system is used in conjunction with the Baltimore
classification system in modern virus classification.
◦ As per Baltimore classification there are 7 classes of viruses

9. Holmes Classification
Holmes(1948) used Carolus Linnaeus system of binomial nomenclature classification
system to viruses into 3 groups under one order ,order virales.they are placed as
follows:
◦ Group I: phaginae(attacks bacteria)
◦ Group II: phytophaginae(attacks plants)
◦ Group III: zoophaginae(attacks animals)

10. LHT CLASSIFICATION
• The LHT System of Virus Classification is based on chemical and physical
characters like
• Nucleic acid (DNA or RNA),
• Symmetry (Helical or Icosahedral or Complex),
• Presence of envelope,
• Diameter of capsid,
• Number of capsomers.
• Andre Loff, Robert Horne, and Paul Tournier (1962)
• This classification was approved by the Provisional Committee on Nomenclature of
Virus (PNVC) of the International Association of Microbiological Societies (1962).

12. Casjens and Kings classification of virus
Casjens and Kings(1975) classified virus into 4 groups based on type of nucleic acid,
presence of envelope,symmetry and site of assembly[citation needed]. It is as follows:
◦ Single Stranded RNA Viruses
◦ Double Stranded RNA Viruses
◦ Single Stranded DNA Viruses
◦ Double Stranded DNA Viruses

13. INTERNATIONAL COMMITTEE ON TAXONOMY OF
VIRUSES (ICTV) CLASSIFICATION
◦ Established in 1966
◦ Only body takes the task of developing refining and maintainingthe Universal
viruse taxonomy
◦ Governed by the Virology Division of the International Union of Microbiological
Societies.

15. On the Basis of Genetic Material Present
◦
◦ Viruses are small, nonliving parasites, which cannot replicate outside of a host cell.
◦ A virus consists of genetic information — either DNA or RNA — coated by a protein.
◦ Accordingly, they are classified as DNA viruses and RNA viruses.
◦ The nucleic acid may be single or double stranded, circular or linear, segmented or
unsegmented.
DNA viruses
◦ As their name implies, DNA viruses use DNA as their genetic material.
◦ Some common examples of DNA viruses are parvovirus, papillomavirus, and
herpesvirus.
◦ DNA viruses can affect both humans and animals and can range from causing begin
symptoms to posing very serious health.

16. RNA viruses
◦ The virus that possesses RNA as genetic material are called RNA viruses.
◦ Rotavirus, polio virus, yellow fever virus, dengue virus, hepatitis C virus, measles
virus, rabies virus, influenza virus and Ebola virus are examples of RNA virus.
DNA-RNA viruses
◦ The RNA tumor viruses called Leukoviruses and Rous’s viruses unusually contain
both DNA and RNA as genetic material

18. On the basis of the presence of a number of
strands
Double-stranded DNA
◦ It is found in pox viruses, the bacteriophages T2, T4, T6, T3, T7 and Lamda, herpes
viruses, adenoviruses etc.
Single-stranded DNA
◦ It is found in bacteriophagesφ, X, 74 bacteriophages.
Double-stranded RNA
◦ It has been found within viral capsid in the reoviruses of animals and in the wound
tumour virus and rice dwarf viruses of plants.

19. Single-stranded RNA
◦ It is found in most of the RNA viruses eg: tobacco mosaic virus, influenza virus,
poliomyelitis, bacteriophage MS-2, Avian leukemia virus.

20. Classification of Virus on the Basis of Structure
1. Cubical virus: They are also known as icosahedral symmetry virus Eg. Reo virus,
Picorna virus.
2. Spiral virus: They are also known as helical symmetry virus Eg. Paramyxovirus,
orthomyxovirus.
3. Radial symmetry virus: eg. Bacteriophage.
4. Complex virus: eg. Pox virus.

22. On the Basis of the Type of Host
The virus can be classified on the basis of the type of host. They are:
1. Animal viruses
2. Plant viruses
3. Bacteriophage
1. Animal Viruses
◦ The viruses which infect and live inside the animal cell including man are called
animal viruses. Eg; influenza virus, rabies virus, mumps virus, poliovirus etc. Their
genetic material is RNA or DNA.
◦

23. 2. Plant Viruses
◦ The viruses that infect plants are called plant viruses. Their genetic material is RNA
which remains enclosed in the protein coat.
◦ Some plant viruses are tobacco mosaic virus, potato virus, beet yellow virus and
turnip yellow virus etc.
3. Bacteriophages
◦ Viruses which infect bacterial cells are known as bacteriophage or bacteria eaters.
They contain DNA as genetic material.
◦ There are many varieties of bacteriophages. Usually, each kind of bacteriophage
will attack only one species or only one strain of bacteria.

24. Virus Classification by Capsid Structure
Naked icosahedral:
Hepatitis A virus, polioviruses
Enveloped icosahedral:
Epstein-Barr virus, herpes simplex virus, rubella virus, yellow fever virus, HIV-1
Enveloped helical:
Influenza viruses, mumps virus, measles virus, rabies virus
Naked helical:
Tobacco mosaic virus
Complex with many proteins:
some have combinations of icosahedral and helical capsid structures. Herpesviruses,
smallpox virus, hepatitis B virus, T4 bacteriophage.

25. On the Basis of Presence of Envelope
◦ The envelope is a lipid-containing membrane that surrounds some virus particles. It is acquired
during viral maturation by a budding process through a cellular membrane
◦ Virus encoded glycoproteins are exposed on the surface of the envelope. These projections are
called peplomers.
◦Enveloped Virus
◦ DNA viruses: Herpesviruses, Poxviruses, Hepadnaviruses
◦ RNA viruses: Flavivirus, Toga virus, Coronavirus, Hepatitis D, Orthomyxovirus, Paramyxovirus,
Rhabdovirus, Bunyavirus, Filovirus, Retroviruses
◦
Non-Enveloped Virus
DNA viruses- parvovirus, adenovirus and papovavirus.
RNA viruses- Picornavirus, Hepatitis A virus and Hepatitis E virus.

26. Nomenclature
Principles of Nomenclature
1. The essential principles of virus nomenclature are:
(i) to aim for stability;
(ii) to avoid or reject the use of names which might cause error or confusion;
(iii) to avoid the unnecessary creation of names.
2. Nomenclature of virus taxa is independent of other biological nomenclatures. Virus
taxon nomenclature is recognized as an exception in the proposed International Code
of Bionomenclature (BioCode).
3. The primary purpose of naming a taxon is to supply a means of referring to the
taxon, rather than to indicate the characters or history of the taxon.

27. Rules of Classification and Nomenclature
General Rules
The universal scheme
◦ Virus classification and taxon nomenclature shall be international and shall be
universally applied to all classifiable members of the virosphere.
◦ The universal virus classification system shall employ the hierarchical levels of
realm, subrealm, kingdom, subkingdom, phylum, subphylum, class, subclass, order,
suborder, family, subfamily, genus, subgenus and species.

28. Rules of Nomenclature
This nomenclature method remain unchanged since the report of Fenner [1976]. They
are as follows:
◦ Rule 1 – The code of bacterial nomenclature shall not be applied to viruses.
◦ Rule 2 – Nomenclature shall be international.
◦ Rule 3 – Nomenclature shall be universally applied to all viruses.
◦ Rule 4 – An effort will be made towards a latinized nomenclature.
◦ Rule 5 – Existing latinized names shall be re-tained whenever feasible.
◦ Rule 6 – The law of priority shall not be observed.
◦ Rule 7 - Sigla may be accepted as names of viruses or virus groups, provided that
they are meaningful to workers in the field and are recommended by international
virus study groups.
◦ Rule 8 - No person’s name shall be used.
◦ Rule 9 - Names should have international meaning

29. Rules of Nomenclature
◦ Rule 10 – The rules of orthography of names and epithets are listed in Chapter 3,
Section 6 of the proposed international code of nomenclature of viruses [Appendix D;
Minutes of 1966 (Moscow) meeting].
◦ Rule 11 – For pragmatic purposes the species is considered to be a collection of
viruses with like characters.
◦ Rule 12 – Numbers, letters, or combinations there of may be accepted in
constructing the names of species.
◦ Rule 13 – These symbols may be preceded by an agreed abbreviation of the latinized
name of a selected host genus, or, if necessary, by the full name.Rule 14 – The genus
is a group of species shar-ing certain common characters.
◦ Rule 15 – The ending of the name of a viral genus is ‘. .. Virus’.
◦ Rule 16 – A family is a group of genera with common characters, and the ending of
the name of a viral family is ‘…viridae’.

30. Reference
◦ https://www.google.com/search?q=nomenclature+of+viruses&source=lmns&bih=
672&biw=360&client=ms-android-
oppo&prmd=inv&hl=en&sa=X&ved=2ahUKEwjtzJmio7L3AhWZgGMGHVS_CJIQ_
AUoAHoECAAQAw#ip=1
◦ https://bhagwattekale.blogspot.com/2022/01/classification-of-viruses.html?m=1
◦ https://biol.wwu.edu//young/321/stuff/collection/baltimore_classification.html
◦ https://www.bionity.com/en/encyclopedia/Virus_classification.html#Holmes_clas
sification
◦ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7149719/
◦ https://talk.ictvonline.org/information/w/ictv-information/383/ictv-code