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Virology. What are viruses?. Viruses are very small particles which infect humans, animals, plants and even bacteria. Obligate intracellular parasites. Viruses range in size from 20-400nm. Are viruses living organisms?.
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Viruses are very small particles which infect humans, animals, plants and even bacteria. • Obligate intracellular parasites. • Viruses range in size from 20-400nm. Are viruses living organisms?
NO! They can only replicate in living cells , employ host cell’s replicative and metabolic pathways. • Outside cells they are essentially inert macromolecules (Protein & NA). Obligate intracellular parasites
Core of genetic material(nucleic acid): Single or double strands DNA or RNA, linear or circular, one piece or segmented. Protein coat or capsid: • Composed of a large number of subunits (capsomeres). • Protects viral genes from inactivation by adverse environmental factors, help attachment to specific receptors on host cells. • Core + capsid = nucleocapsid
Envelope: • Lipid or lipoprotein bilayer, not present in all virus species, derived at least partially from host cell membrane , contain viral glycoprotein • Structural • Functional Enzymes: • Found only in very few number of viruses.
Virion • Complete virus particle : nucleic acid + protein coat, which may be surrounded by an envelope. • It is the form in which the virus moves between cells or hosts.
Nucleic acid Capsid Envelope protein Membrane protein Viral envelope** Nucleocapsid Viral Structure - Overview Spike protein Fig 1. Schematic overview of the structure of animal viruses ** does not exist in all viruses
Continue • The nucleocapsid which may have • Icosahedral:(Cubic) Has 20 faces, each an equilateral triangle (e.g.adenoviruses). • Helical: Protein binds around DNA/RNA in a helical fashion (e.g. Influenza virus). • Complex: no specific symmetry (e.g. Bacteriophage , Poxviruses)
The host cell act as a factory providing substrates, energy, and machinery for synthesis of coat proteins , nucleic acid genomes. Recognition and attachment to the target cell: It depends on legends “ Surface protein ” of the virus and receptors in the host cell.
HIVbinds to CD4 receptors in cells of immune system. • EBV binds to CD21 receptors on B cells. • Poliovirus attach only to cells in CNS and GIT. • Attachment step initiateirreversible structural changes in the virion. • Attachment step is temperature dependent.
Penetration or engulfment: • Non enveloped “Naked” viruses penetrate by • Receptor mediated endocytosis. • Translocation of the virion across the host cell membrane. • Enveloped viruses penetrate cells through • Fusion of viral envelope with host cell membrane. • May or may not involve receptor mediated endocytosis.
Uncoating: • Enveloped viruses are usually uncoated upon fusion to the cell membrane. • The virus then delivered to the replication site. • DNA viruses replicates in the nucleus except poxvirus. • RNA viruses replicates in the cytoplasm except retroviruses (HIV).
Expression of viral genomes and synthesis of viral components: • It is the most important step in virus replication. • It depends on the formation of functional mRNA capable of binding to the ribosome and being translated into proteins. • DNA viruses that replicates in the nucleus utilize the cell’s DNA dependent RNA polymerase.
While poxvirus which replicates in the cytoplasm must encode for such enzyme. • RNA viruses must encodes for enzymes which transcript the RNA. • Protein synthesis takes place in two stages: Early stage: synthesis of proteins that inhibit the cell metabolism and enzymes (polymerases) necessary for nucleic acid replication.
Late stage: synthesis of protein capsids. • Replication differs according to the type of the nucleic acid core. Assembly: • Association of cores and coats. • Usually starts as soon as necessary pieces are synthesized. • It looks like a box of capsomeres enclosing the genome.
Release: • Naked viruses are released after lyses of the cell while enveloped viruses are released usually by budding.
Time course of replication • The time interval after penetration and before assembly is called eclipse cycle. • During the eclipse cycle infective virion couldn’t be isolated.
Structure: DNA or RNA (most satisfactory for microbiologists). • Morphology:icosahedral, helical or complex, also enveloped or non, and others, picorna virus (small), togavirus (cloak), coronavirus (crown), rhabdovirus (rod). • Disease:hepatitis virus, encephalitis virus, influenza virus (most satisfactory for medicine). • Tissue (tropism): adenovirus (glands), enterovirus (intestine) or myxovirus (mucous).
Viruses grow only in living cells which may be: I) Tissue culture: • Animal or human tissue culture pieces treated with trypsin to separate the cell. • They are grown in presence of growth medium containing serum. • A monolayer or sheet of cells is formed on the flat surface of the container (glass or plastic bottle or tube).
There are three types of tissue cultures: • a) Primary cell lines: fragments of tissue e.g. monkey kidney. They can only divide for 4-6 subcultures, then degenerate. • b) human diploid cell lines: Usually fibroblasts from human embryo tissue. They grow rapidly and can be subcultured 50 times. • c) Continuous cell lines: are derived from tumor cells and can be subcultured indefinitely e.g. Hella cells derived from the carcinoma of the cervix.
II) Chick or duck embryo: • The virus is allowed to grow in one of the following cavities within the fertilized egg. a) Embryo e.g. yellow fever virus. b) Allantoic or Amniotic cavity: e.g. influenza virus. c)Chorioallantoic membrane: e.g. pox and herpes viruses.
III) Intact animal: • The white suckling mouse is widely used for encephalitis viruses, calves are used for pox virus. • Cultivation in animals has the disadvantage of easy transmission of infection. • Rarely used now.
Cytopathogenic or cytopathic effect (CPE): • Characteristic changes due to permanent or temporary cell damage as cell death (poliovirus), cluster formation (adenovirus), giant cell formation (measles and mumps viruses) or cell transformation (tumor viruses). • Inclusion bodies: • Observed by light microscope. • They are aggregates or just sites of replication of the virus.
e.g. • Cytoplasmic Negri bodies in brain cells of rabies infected animal, cytoplasmic Guarnieri's bodies of poxvirus. • Nuclear bodies of adenovirus. • Nuclear and cytoplasmic bodies in case of cytomegalovirus.
Haemadsorption: • Viruses which contain haemagglutinin spikes e.g. influenza virus are able to form clumps of RBCs if added to the tissue culture. • Interference: • Host cells infected with a virus may acquire resistance to infection by a second virus, of the same type or another one (it is not common to all viruses e.g. not found between mumps, measles and rubella).
Some viruses do not produce CPE, however, their growth can be proved by their ability to interfere with another CPE producing virus e.g. rubella and ECHO (Enteric Cytopathic Human Orphan) virus. • Fluorescent antibody staining. • Detection of viral antigens by serology. • Acid production: • Normal cells produce acids, thus: • Normal tissues + Phenol red Yellow (acid color). • Infected tissues + Phenol red Red (alkaline color).
Local infections: • Skin: e.g. warts caused by papillomavirus. • Respiratory tract infections affecting the mucous membranes e.g. influenza and common cold. • Alimentary tract e.g. more than 60% of diarrhea in infants is caused by rotavirus. • Local infections are characterized by short incubation period and short lasting immunity.
Systemic infection: • The virus passes through lymphatics,blood, tissues or nerves, reaching the target tissue (tropism), e.g. hepatitis, measles, poliomyelitis. • Administration of neutralizing antibodies before the viraemic phase could prevent the development of the disease. • Systemic infections are characterized by long incubation period and long lasting immunity.
Persistent infection: • Due to escape of the virus from the host defense mechanism. It is either • Latent infection: the virus remain hidden most of the time with intermittent reactivation and development of the disease e.g. herpes simplex and adeno infections. The virus is not detected during latency. • Chronic infection: the virus usually has long incubation period and is always shedding i.e. can be detected all the time, during symptoms and in absence of symptoms, e.g. hepatitis B.
Slow infection: are viruses with long incubation period and slow multiplication e.g. subacutesclerosingpanencephalitis caused by a variant of measles virus. • Congenital or teratogenic infection: many viruses penetrate the placenta during pregnancy causing congenital defect in the embryo, e.g. rubella, cytomegalo, hepatitis viruses and HIV.
Oncogenicity (tumor causing): resulting in transformation of infected cells. • Viral genes become integrated into host cell chromosome e.g. of DNA viruses (herpes simplex type 2 virus, Epestein Barr virus, hepatitis B&C viruses and papovirus) e.g. of RNA viruses is the retrovirus group {human T lymphotropic viruses 1&2 (HTLV 1&2)} c.f. HIV formerly known as HTLV 3 which is non oncogenic. • With RNA tumor viruses, viral RNA acts as a template for synthesis of viral DNA- through the action of reverse transcriptase. The DNA copy of the viral RNA is integrated into the host cell chromosome.
