Classification of Medically Important VirusesDr Zakuan Zainy DerisM.D (USM), M.Path (Microbiology), FADUSMLecturer/Clinical Microbiologist

ObjectivesKnow the defination of virus, virusoid and prion Know the example of medically important virusesKnow the different classification of the virusKnow how the virus classify

VirusesSmallest infectious agents (20nm-300nm)Consist of one kind of nucleic acid (RNA or DNA)Surrounded by one or more proteinsSome have outer membrane envelope -lipidDo not have ribosomes or enzymesObligate intracellular parasitesInert in extracellular environmentReplicate only in living cells(parasites at genetic level)

Virusoid Nucleic acids that need the helper viruses virus-like particles. E.g.: Hepatitis D delta agentViroids molecules of naked, cyclical, small RNAs and restricted to plantsPrions Abnormal proteins that can spread from cell to cells and changes the normal cellular protiens disrupts the normal cell functions. E.g.: CJD, BSE, Kuru

Term used in virologyCapsid The protein shellEnvelope A lipid-containing membrane that surrounds some virus particleNucleocapsid The protien-nucleic acid complex representing the package from the viral genomeVirion The complete virus particleDefective virus a virus particle that is functionally deficiency in some aspect of replication

Example of VirusGlycoprotienCapsidRNA

HistoryThe first classification of viruses as a group distinct from other microorganisms was based on their capacity to pass through filters of a small pore size (filterable agents). Initial subclassifications were based primarily on pathologic properties such as specific organ tropism (e.g., hepatitis viruses) or common epidemiologic features such as transmission by arthropod vectors (e.g., arboviruses).

ClassificationCurrent classification systems are based on The type and structure of the viral nucleic acid and the strategy used in its replication, The type of symmetry of the virus capsid (helical versus icosahedral), and The presence or absence of a lipid envelope

The Nucleic Acid

Viral RNA

CapsidIcosahedral Capsid

HELICAL CAPSID

Viral envelopeELECTRON MICROSCOPY OF HERPES SIMPLEX VIRUS (HSV)

Other properties has bee used for classificationVirion morphology size, shapePhysiologycal properties of the virion molecular mass, pH stability, thermal stability and susceptible to physical and chemical agentsOther viral genome properties size of NA, strandedness, linear or circular, segmentViral protein properties number, size, function, structure, amino acid sequenceAntigenic properties

Virus ToxonomyA system to separate the virus to major group Families (suffix -viridae) based on virion morphology, genome structure and strategies of replicationGenera (suffix -virus) based on physicochemical and serologic differences

Classification of Viruses RNA-Containing Viruses

RNA-Containing Viruses (Cont)

DNA-Containing Viruses