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33 Cards in this Set

  • Front
  • Back
Properties of viruses
20 nm in diameter

10nm smallest Foot + Mouth
HB 15 nm
< ribosome

250 lrgst in Pox grp
Properties of viruses
All have 2 basic parts

1. Central core of nucleic acid (DNA or RNA)

2. Coat (protein + lipid)

envelope (optioanal, host cellular material)
Properties of viruses

Only in living cells
aninmal, plant, bacteria, fungal

Not known to cross species

Viruses may invade more than one host
Properties of viruses

Infection of a host by a virus does not necessarily = disease

Infection may be asymptomatic

Disease may be mild or severe
Properties of virus

Viral replication
Viral replication does not necessarily destroy the host
Properties of viruses

Cytopathic Effect
The Cytopathic Effect is visible cell changes and/or destruction in laboratory cell cultures
Properties of viruses

Viruses exhibit a high degree of specificity for host cell type.

eg.: Influenza virus found on respiratory cells

Polio virus found on nerve nad gut cells

Hence, there is mostly a relationship between cell preference and disease locality

Campbell p. 321
Properties of viruses

Depending on the type and arrangement of nucleic acid, viruses reproduce in quite different ways to the binary fission of bacteria
Properties of viruses
Viruses have no metabolism per se
no replicative process
limits treatment

viruses are obligate intracellular parasites, they can reproduce only within a host cell. An isolated virus is unable to reproduce, except infect a host cell. Viruses lack the enzymes for metabolism, have no ribosomes. Isolated viruses are merely packaged sets of genes in transit from one host to another (Campbell p 321)
Properties of viruses

Unaffected by bacterial treatments

Anti-viral therapy requires interference w/ sp. viral replicative processes non-essential to the host
Structure of Viruses
3 types

(eg Human papilloma, adenovirus)

Helical (eg Influenza A, measles)

Complex (eg Pox)

Phages infect bacteria, polyhedral head (DNA) and tail apparatus (injection)

All viruses have a protein + coat and a nucleic acid core, some have an envelope

Physical and chemical effects

At room temperature and outside the body, many viruses are very labile, and only survive a few hours. (eg. Influenza, HIV, throat washings must be stored and transported cool)

Some viruses are much more resistant days, weeks months: eg. Enteroviruses
Viruses - Physical and chemical Effects


Heat: Pathogenic viruses generally inactivated by moderate heat 56 - 60 C)

Cold: Like bacteria, all viruses are resistent to extremes of cold- preserved at -70 and -199 C

Liquid nitrogen
Freeze drying is used for preserving viruses and vaccines
Viruses - Physical and chemical effects

Viable between pH 5-9

Viruses are destroyed by extremes of acidity and alkalinity

eg. do not pass thr the stomach well
Viruses - Physical and Chemical effects

Oxidizing agents:

Organic iodine derivatives
H2O2 hydrogen peroxide
Potassium permanganate
Soap/ detergent

Proprietary mixed disinfectants
Viruses - Physical + chemical effects

Antibiotics and other therapeutic agents
Interferon 1st line of defense

AB, (penicillin, streptomycin, tetracyclin, sulphonimides) have no effect, virus lacks metabolism

some agents are anti-viral and used for tx.

AB can prevent superinfection, immuno suppressed, used prin. in severe respiratory infection
Viral Replication

6 Stages
1. Electrostatic adsorption followed by chemical attachment at specific receptors

Us. best at 37 C and Mg + Ca ions help

Viruses must bind to cell surface to get into it. interfere w/ metal ions help
Viral replication

6 stages
2. Penetration

Penetration of virus (sometimes just viral nucleic acid)occurs by:

Pinocytosis (engulfment) commonest
Membrane fusion envelop virus
from pinocytosed lysosome

Enzymes possibly assist
Viral replication

6 stages
3. Uncoating (Eclipse phase)

No visible activity - no cell changes

Metabolic re-organization

Microscope will not see anything. virus switch off host cell genes to allow it to take over, make dau. virus particles.
Viral replication

6 stages
4. Formation of viral proteins and NA's

Occurs characteristically in the nucleus or cytoplasm depending on the virus type

Often visible as inclusion bodies

May cause alteration in cell morphology (cytopathic effect or CPE)

see changes, visible areas of material
Viral Replication

Newly formed nucleic acid and protein coats assemble to form daughter virions.

Envelope where present added last at cell membrane
Viral Replication

Capsid: Protein coat that encloses the viral genome

maybe rod shaped, (helical), polyhedral, or complex

capsids are built from a large no. of protein units called capsomeres

capsomere # of units are genetically detemined.

Eg adenovirus has 252 id protein mol. arranged into a polyhedral capsule w/ 20 triangular facets
Viral replication
Tx strategy

Continuous proteins -> capsomeres
enzymes arrange assembly
Stop proteases, cont. protein capsomeres, favored for attack w/ HIV virus
Viral Replication

Release of virions

Note: A virion is the whole infective virus particle w/ enzymes
Daughter virions may leave cell singly or in a single burst. (the latter normally destroying the cell)

Normally ONE virus invades and several hundred daughter virions are produced
DNA virus replication

How does it work?
Entry of viral DNA into host

Transcription of viral DNA into mRNA by host cell RNA polymerase

Translation of mRNA -> virus specific proteins
RNA virus replication

+ ve stranded
input strand ?
input strand = mRNA
RNA Virus Replication

-ve standed

input strand?
input strand not mRNA
Retrovirus replication

How does it work?
Reverse transcriptase makes a DNA copy of viral RNA

DNA copy is integrated into the host genome = provirus DNA

provirus DNA is trnascribed into both new genomic RNA and mRNA for translation into viral proteins
DNA Virus families
of medical significance
ds DNA
Poxviruses smallpox, cowpox, vaccinia
Herpes viruses
Adenoviruses respiratory
papovavirus papilloma

RNA Viruses of medical importance
Coronaviruses SARS
Rhabdoviruses rabies
Arena virus
Toga virus rubella

flavivirus yellow fever
Calicivirus human diarrhea
Picornavirus polio
Orthomyxo virus measles
Paramyxovirus influenza
Other Replcative Entities



Viroids: Small virus like ssRNA; some times protein free; mainly plant disease; eg. Hep D Delta hepatitis

Prions: Infectious proteins with no nucleic acids (eg. CJD, BSE, Kuru)

Virinos: Small nucleic fragments, complexed with protein, derived from host cell
Diagnostic Virology Summary

Ddemonstrate relevant virus by:
Isolation in animals No
fertile hen eggs
cell culture
Electron microscopy
Molecular techniques (PCR)
Diagnostic virology summary

Demonstration of significant AB to a relevant virus by:
Enzyme-immunoassay (ELISA)
Heme agglutination - inhinition
Complement fixation