General Virology Lecture 7 PDF

Level (1) - Semester (1) MICROBIOLOGY D R Z I A D M A LECTURE (7) H A N GENERAL VIROLOGY A -  In Latin virus means poison.  Viruses are defined as: sub- microscopic, obligate intracellular parasites only active inside viable dividing host cells....

Level (1) - Semester (1) MICROBIOLOGY D R Z I A D M A LECTURE (7) H A N GENERAL VIROLOGY A -  In Latin virus means poison.  Viruses are defined as: sub- microscopic, obligate intracellular parasites only active inside viable dividing host cells.  Virus particles (virions) do not grow or undergo division.  Virions are produced from the assembly of pre-formed viral protein components.  Any symmetry may be enveloped or non-enveloped. -  The outer membrane surrounding some viruses.  It contains lipoproteins derived from host cell and glycoproteins of viral origin.  Viruses attach to the target cell by the envelope glycoproteins in most cases.  Capsid: The protein shell that encloses the nucleic acid.  Capsomers: clusters of structure units making the capsid.  Nucleocapsid: the capsid together with its enclosed nucleic acid.  The nucleic acid comprising the virus genome may be RNA or DNA.  Viral nucleic acid may be: ① Single-stranded or double-stranded, ② Linear or circular ③ Segmented or non-segmented.  Viruses are classified according to: their nucleic acid properties. -  Viruses have reactive sites on their surface which interact with specific receptors on suitable host cells.  This is a passive reaction.  The specificity of the reaction between viral protein and host receptor defines the host species and cell tropism.  Viral penetration is an active process, i.e. the cell must be metabolically active for this to occur.  Three mechanisms may be involved: ① Translocation of the entire virion across the cell membrane ② Endocytosis of the virus into intracellular vacuoles; eventually into the cytoplasm. ③ Fusion of the viral envelope with the cell membrane. -  Essential facts: ① Mammalian cells can only make DNA and RNA in the nuclear compartment i.e. viruses replicating in the cytoplasm must make their own polymerases. ② Viruses with an RNA genome have to make or bring their own polymerases.  Messenger RNA is transcribed from viral DNA (Positive sense RNA viruses) and are translated by the host cell ribosomes.  “Early” proteins are usually the DNA or RNA polymerases and “Late” proteins are structural, e.g. capsid or envelope proteins.  Nucleic acid replication occurs by viral or cellular polymerases to produces new viral genomes.  During this step, transcriptional errors may result in new mutants with new clinical importance. -  Assembly of viral proteins making complete viral particles may take place in the nucleus; in the cytoplasm; or at the cell surface, while budding.  Accumulation of virions at sites of assembly may form inclusion bodies that are useful in viral diagnosis.  This may occur by budding from the cell surface, as in many enveloped viruses.  Some viruses use exocytosis to exit the cell. Virus particles enclosed within vesicles are released to the outside of the cell when the transport vesicle fuses with the cell membrane.  Lysis of the infected cell can also result in the release of intact infectious virions. -  Resulting in host cell death and production of new viruses.  No cell damage and no progeny virus are produced due to factors in the virus or nature and resistance of the cell.  The host cell may be damaged but not killed, and progeny viruses are released for some time. Infected cells my change to tumor cells.  Some viruses e.g.: Herpes viruses can cause latent infection. The viral genome is present inside a host cell with no effect on cell nor production of progeny virus.  Such latent viruses can be reactivated due to change in immune status of the host, leading to a productive infection " cell damage and new virus production".  -  There are three basic types of tests for diagnosis of viral infections: ① Viral genome (RNA or DNA) detection by PCR, ② Viral antigen detection, and ③ Detection of antibodies to the virus.  PCR or antigen detection tests: are used to assess acute infection, whereas antibody tests provide evidence of prior infection.  Viral antigen detection is done in samples obtained from the sites of possible virus presence.  Antigen detection can be done using different methods like: ① Immunofluorescence, ② ELISA and ③ Most common now by Rapid Tests.  Serum samples are obtained and tested for presence of specific antiviral antibodies using: ① Immunofluorescence, ② ELISA and ③ Most common now by rapid tests.  - ① Vaccination:  Prevention of viral infection depended for long time on viral vaccines.  Antiviral drugs are developing within last years after using molecular biology techniques. COVID-19 Vaccine ① Live attenuated Virus Vaccines. ③ Sub-unit Vaccines. ② Inactivated Vaccines. ④ Genetically engineered vaccines. ② Public health measures. ③ Infection prevention. ④ Infection control.  - ① Antiviral drugs: to cure or control virus infections. ② Antiviral agents: are specific, and each drug has limited indications.  Antiviral drugs are available for viral diseases.  Examples: Acyclovir Zidovudine (AZT), Lamivudine (3TC), Protease inhibitors, Tamiflu, Sovaldi and daclatasvir. Remdesivir and Favipravir.  - Clinical Case  Grandfather Ali suffered from low grade fever, sneezing, running nose, and generalized body aches. His friend Mosaad advised him to go to the pharmacy and take the flu treatment composed from 2 capsules of antibiotics and one ampoule of vitamin C. Gedo Ali said flu is caused by a virus not a bacterium. Viruses are causing many human diseases and are different from bacteria. I will take an antiviral drug then I will receive the seasonal vaccine for influenza. ① What are main differences between bacteria and viruses? ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. ② Influenza is a virus infection? Do you know types of influenza and sources of infection? ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. ③ Mention some antiviral agents and the disease treated by them. ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. ④ Why do we have different vaccine for influenza each year? ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. ……………………………………………………………………………..………….. Assignment Can we do culture and isolation of viruses from pathological samples?  - Summary ① Many human infections are caused by viruses. ② Viruses are very small in size, and their genome may be DNA or RNA and some viruses have envelopes. ③ Viruses can replicate only inside active cells and replication cycle has 6 steps: adhesion, uptake, uncoating, genomic and protein synthesis, assembly and release. ④ Virus infected cells may die (lyse) or change to permeant infected cells. ⑤ Diagnosis of viral infection depends mainly on detection of virus genome (by PCR), detection of viral antigens or specific antibodies formed against the virus. ⑥ Prevention of viral diseases has been successfully done by vaccination. ⑦ Antiviral drugs are available but very specific.