General Virology for DEN New PDF

GENERAL DESCRIPTION  Viruses is not living organism, as they lack two essential properties of life- they cannot replicate independently, and cannot survive long –term independently so it is obligate intracellular organism.  They are much smaller than bacteria human viral p...

GENERAL DESCRIPTION  Viruses is not living organism, as they lack two essential properties of life- they cannot replicate independently, and cannot survive long –term independently so it is obligate intracellular organism.  They are much smaller than bacteria human viral pathogens usually 20nm in diameter where the bacteria around 1000 nm in diameter). It infect animal and human viral infection it infect bacteria bacteriophage It has no ribosome's or any structure requirement for harvesting of energy They can only seen under electron  microscope Due to very small structure ranging from  10-300 nm They can not cultivated on artificial  culture media  Structure and composition: Structure Virus particle is called virion it is consisted of 1. Genome  (either DNA or RNA) never both.  The most common forms of viral genomes found in nature are ds DNA or ss RNA. the RNA can be positive strand which function as messenger RNA or negative strand RNA which function as template for production of mRNA. It is the infectious part of the virus which mediates viral virulence 2.Capsid :repeated protein units this protein shell enclosing the genome it gives the virus shape. for most virus families, found in either DNA or RNA Functions of viral capsid 1-it is responsible for viral morphology 2-nucleic acid protection 3-anigenic properties 4-mediates viral attachment to the host Viral symmetry: 1-complex symmetry e ,g brick shaped box virus 2. Helical capsid : (coiled shape) the helical capsid is constructed of multiple copies of single poly peptide type arranged in helix shape with the nucleic acid coiled between the turn of the helix ,each poly peptide unit called protomer 3. Icosahedral symmetry:  an icosahedron is a structure with 12 corners and 20 triangular faces(crystal like).  It is more complex than helical symmetry.  Consist of several different polypeptides (protein) group called capsomer. Icoshadral Structuer Helical Structure 3. envelope: is membrane surrounding the nucleocapsid  It is important structural feature to define viral family is the presence or absence of lipid- containing membrane surrounding the nucleocapsid.  A virus that is not enveloped called naked virus.  The envelope is made of inner structural protein layer ,outer lipid layer and projecting spike of glycoprotein.  Entry of the virus depend on prescience or absence of envelop. VIRAL REPLICATION A. Adsorption (attachment): Initial attachment of virus particles to a host cell involves an interaction between Specific molecular structure on the virion surface(viral attachment proteins) and molecule in the host cell membrane by cell receptor molecules. Some require more than one receptor as HIV virus. Naked viruses: attachment between capsid and cell receptors. Enveloped viruses: attachment between glycoprotein spikes and cell receptor. B. Penetration : Passage of the virion from the surface of the cell across the cell membrane and into the cytoplasm by 2 mechanisms: 1. receptor-mediated endocytosis: naked viruses and enveloped virus. 2. membrane fusion : enveloped viruses  :C-uncoating. the viral coat , exposing nucleic acid  by remove the capsid by cellular enzyme lysosyme in the cytoplasm D. Replication and transulation:  In DNA virus the DNA direct the cell to replicate in the nucleus use enzyme in the host or viral encoded enzyme-- protein synthesis  The larger the viral genome the fewer host cell enzyme are involved in replication ex poxvirus, parvovirus.  RNA viruses must provide its own polymerases  Formation of m RNA from RNA viruses occurs as the following  1-in double stranded RNA viruses one strand is first transcribed by viral RNA dependent RNA polymerase (RdRp) into messenger RNA  In s s RNA viruses there are 3 routes for m RNA formation  A-The strand of positive sense acts directly as a messenger RNA  B-the negative stranded virus which must be first transcribed by viral RdRp into positive sense strand acts as a mRNA  Retroviruses which contain-3 positive ss RNA by action of reverse transcriptase will produce complementary ss DNA which is converted to ds DNA Ds DNA enters the nucleus and is either integrated in host cell genome causing transformation or is transcribed by host cell polymerase into mRNA E. Synthesis of daughter viral nucleic acid using the parent viral nucleic acid as template. F. Assembly of intact viral within the cell. G. Release of the viral particles from the cell and cell membrane envelope the virus and form outer coat.  Types of cell culture: There are three types of cell lines:  Primary cell lines:, e.g. monkey kidney.  Diploid cell lines: Prepared from human embryo tissue, e.g. lung..  Continuous cell lines: Prepared from tumor cells with unlimited number of subcultures of passages, e.g. HeLa cells from concer cervix Embryonated eggs It contains a variety of membranes that.support multiplication of certain viruses   Laboratory animals. It was the first method used for  cultivation of viruses. It is still used in the following: Isolation of coxsackie viruses and togaviruses by inoculation in suckling mice. Rabies virus inoculation in mice..For research  Classification of Viruses  A- Classification by symptomatology It is the old classification based on  diseases they produce, i.e. tropism, e.g. neurotropic viruses, enteroviruses,…etc B- The Hierarchical virus classification: It  is a scheme classifying viruses into order, family, subfamily, genus, species,.type, subtype, and strain  C- The Baltimore Classification: It is based on virus genome replication strategy. The central idea is that all viruses must generate positive strand mRNAs from their genomes, in order to produce proteins and replicate themselves. The precise mechanisms whereby this is achieved differ for each virus family. DIFFERENCES BETWEEN LOCAL AND SYSTEMIC VIRAL INFECTIONS Local infections Systemic infections Specific disease Example Respiratory (common Measles cold e.g. rhinovirus) Site of pathology Portal of entry. At distant Incubation period Relatively short Relatively long Viraemia Absent Present Duration of immunity Usually short Usually lifelong Role of secretory lgA Usually important Usually not important antibody in immunity LABORATORY DIAGNOSIS OF VIRAL INFECTIONS  I. Direct detection of viruses and / or their components: The following procedures are used:  1. Light microscopy: The ordinary microscope can be used in examination of large viruses as poxviruses, or giant cells in herpes infection, or inclsion bodies, e.g. Negri bodies in nerve cells in rabies.  2. Fluorescent microscopy: By using direct immunofluorescent antibody technique (IF), e.g. diagnosis of rabies in brain smears.  3. Electron microscopy (EM): This method is used when large number of viruses is present in the sample. It also gives an idea about the size and shape of viruses. 4. Immunoelectron microscopy (IEM): 5. Immunoassays: For detection of the virus antigens by the use of either ELISA or RIA; e.g. hepatitis B antigens in blood. Polymerase chain reaction (PCR): It is a.6 recent method in which amplification of a short sequence of a target nucleic acid of the virus allows it to be easily detected by different methods, e.g..probes Nucleic acid hybridization.7  III. Serological diagnosis - It is an indirect method to detect antiviral antibodies. - Usually 2 serum samples (paired.serum) are taken. The first in the acute phase and the second 2-3 weeks later, to demonstrate a rising titre (4 fold increase or more is diagnostic). - IV. Skin tests can be used as an indication of cell-mediated immunity (CMI) in some viral infections, e.g. mumps. SELECTED ANTIVIRAL DRUGS AND THEIR MECHANISM OF ACTION AND CLINICAL USE Virus infections Mechanism of action Antiviral drugs Examples of viral diseases I. Fusion inhibitors that Fuzeon (enfuvirtide) HIV block virus entry 1.Amantadine Influenza A virus IL Inhibition of uncoating of viruses Influenza A virus viruses 2. Rimantadine (less toxic) Influenza virus A 1. Oseltamivir (Tamiflu) III. Neuraminidase Influenzavirus B inhibitors interfere with production and release of virus from cells of Influenza virus A 2. Zanamivir respiratory tract. B  1. HIV Azidothymidine VI.Nucleoside HIV analogues that 3. Lamivudine HIV& HBV inhibit DNA synthesis by inhibiting viral reverse transcriptase HIV VII. enzyme. Protease 1. Indinavir HIV inhibitors that 2. Ritonavir HIV inhibit cleavage of precursors HIV polypeptide VIII. Inhibition of vial protein 1. Methisazone Poxviruses (smallpox)