Microbiology 5 - Replication of Viruses.pdf

Microbiology Replication of Viruses Microbiology| Replication of Viruses Contents : Replication of Viruses 3 Biosynthesis 16 Maturation, Assembly & Release 19 Culture of Viruses 26 Microbiology| Replication of Viruses Life cycle : Viruses multiply only in living cells. The host cell must provide the...

Microbiology Replication of Viruses Microbiology| Replication of Viruses Contents : Replication of Viruses 3 Biosynthesis 16 Maturation, Assembly & Release 19 Culture of Viruses 26 Microbiology| Replication of Viruses Life cycle : Viruses multiply only in living cells. The host cell must provide the energy and synthetic machinery and the lowmolecular-weight precursors for the synthesis of viral proteins and nucleic acids. This “growth cycle” involves specific attachment of virus, penetration and uncoating, nucleic acid transcription, protein synthesis, maturation and assembly of the virions and their subsequent release from the cell by budding or lysis. Microbiology| Replication of Viruses Initiation Phase : This phase include the following steps Attachment Penetration Uncoating Microbiology| Replication of Viruses Attachment/Adsorption : Viral attachment protein recognizes specific receptors on the cell surface (These may be protein e.g. picorna-virus, or carbohydrate e.g., orthomyxo-viruses and paramyxoviruses, or lipid components of the cell surface). Cells without the appropriate receptors are not susceptible to the virus. Microbiology| Replication of Viruses PENETRATION (Virus enters the cell) : Virions are either engulfed into vacuoles by “endocytosis” or the virus envelope fuses with the plasma membrane to facilitate entry Enveloped viruses Non-enveloped viruses injection Microbiology| Replication of Viruses Fusion : A. Entry by fusing with the plasma membrane. Some enveloped viruses fuse directly with the plasma membrane. Thus, the internal components of the virion are immediately delivered to the cytoplasm of the cell. Microbiology| Replication of Viruses HIV Microbiology| Replication of Viruses Enveloped viruses : There is fusion of the virion envelope with the plasma membrane of the cell. Those systems involve the interaction of a viral fusion protein with a second cellular receptor or “coreceptor” (e.g., chemokine receptors for human immunodeficiency virus). Microbiology| Replication of Viruses Endocytosis : B. Entry via endosomes at the cell surface Many enveloped viruses, such as COVID-19, also enter the cell through endocytosis. Microbiology| Replication of Viruses Entry via the endosome guarantees low pH and exposure to proteases which are needed to open the viral capsid and release the genetic material inside. Further, endosomes transport the virus through the cell and ensure that no trace of the virus is left on the surface, which could be a substrate for immune recognition. Microbiology| Replication of Viruses In some systems, the step of penetration accomplished by receptor-mediated endocytosis, with uptake of the ingested virus particles within endosomes. There are also examples of direct penetration of virus particles across the plasma membrane. Microbiology| Replication of Viruses Entry via Genetic Injection : A third and more specific example, is by simply attaching to the surface of the cell via receptors on the cell, and injecting only its genome into the cell, leaving the rest of the virus on the surface. This is restricted to viruses in which only the gene is required for infection of a cell (most positive-sense, single-stranded RNA viruses because they can be immediately translated) and further restricted to viruses that actually exhibit this behavior. The best studied example includes the bacteriophages; for example, when the tail fibers of the T2 phage land on a cell, its central sheath pierces the cell membrane and the phage injects DNA from the head capsid directly into the cell. Microbiology| Replication of Viruses Uncoating : is the physical separation of the viral nucleic acid from the outer structural components of the virion so that it can function. When the nucleic acid is uncoated, infectious virus particles cannot be recovered from the cell - this is the start of the ECLIPSE phase – which lasts until new infectious virions are made. Uncoating is usually achieved by cellular proteases “opening up” the capsid, which required low PH in the endosome. Biosynthesis Microbiology| Biosynthesis Expression of Viral Genomes and Synthesis of Viral Components : mRNAs must be transcribed from the viral nucleic acid for successful expression and duplication of genetic information. Then, viruses use cell components to translate the mRNA. all of the virus specified macromolecules are synthesized in a highly organized sequence. Viral protein is synthesized in the cytoplasm on polyribosomes composed of virusspecific mRNA and host cell ribosomes. Microbiology| Biosynthesis Many viral proteins undergo modifications (glycosylation, acylation, cleavages, etc.). Viral DNA is usually replicated in the nucleus. Viral genomic RNA is generally duplicated in the cell cytoplasm, although there are exceptions. Maturation, assembly & release Microbiology| Maturation, assembly & release Maturation: The stage of viral replication at which a virus particle becomes infectious; nucleic acids and capsids are assembled together. Assembly: The stage of replication during which all the structural components come together at one site in the cell and the basic structure of the virus particle is formed. Microbiology| Maturation, assembly & release Release : Disintegration: naked virus cause the host cell lysis. Budding: enveloped viruses. Budding viruses do not necessarily kill the cell. Thus, some budding viruses may be able to set up persistence. Enveloped viruses are not infectious until they have acquired their envelopes. Microbiology| Maturation, assembly & release Microbiology| Replication of Viruses Defective Virus: deficiency in some aspects of replication. e.g.,. hepatitis D virus Abortive Infection: When a virus infects a cell (or host), but cannot complete the full replication cycle (lacking some functional viral gene), i.e. a non-productive infection. Defective virus can cause abortive infection (fail to produce infectious progeny). Microbiology| Replication of Viruses Interference: Infection of a cell with two viruses often leads to an inhibition of multiplication of one of the viruses Causes of interference 1. One virus may inhibit the ability of the second to adsorb to the cell either by blocking of the receptors or by destroying it. 2. One virus may compete with the second for components of the replication Apparatus. 3. Interferon, IFN. Microbiology| Replication of Viruses Interferons (IFNs): are a group of signaling proteins made and released by host cells in response to the presence of several pathogens, such as viruses, bacteria, parasites, and also tumor cells. These proteins cause prevents replication of the second virus. Culture of Viruses Microbiology| Culture of Viruses Microbiology| Culture of Viruses One step growth curve to study viral replication : 1. Adsorption of virus (initial phase). 2. Eclipse phase: This lasts for 10-12 hours, and it corresponds to the period during which the input virus becomes uncoated. As a result, no infectious virus can detected during this time (any infectious virus detected is simply virus that is still stuck on the cell membrane). Microbiology| Culture of Viruses 3. Synthetic phase: This starts around 12 hours post-infection and corresponds to the time during which new virus particles are assembled. 4. Latent period: during this period, no extracellular infectious virus can be detected. After ~18 hours, extracellular virus is detected. Ultimately, production will reach a maximum plateau level. Microbiology| Culture of Viruses Microbiology| Culture of Viruses Virus latency (or viral latency) : is the ability of a pathogenic virus to lie dormant (latent) within a cell, denoted as the lysogenic part of the viral life cycle. A latent viral infection is a type of persistent viral infection which is distinguished from a chronic viral infection. Latency: is the phase in certain viruses’s life cycles in which, after initial infection, proliferation of virus particles ceases. However, the viral genome is not fully eradicated. Microbiology| Culture of Viruses The result of this is that the virus can reactivate and begin producing large amounts of viral progeny without the host being infected by new outside virus, denoted as the lytic part of the viral life cycle, and stays within the host indefinitely. Virus latency is not to be confused with clinical latency during the incubation period when a virus is not dormant. Microbiology| Culture of Viruses