Acellular Microorganisms: Viruses, Viroids, Virusoids & Prions PDF

Acellular microorganisms: Viruses, Viroids, Virusoids & Prions Prof. Hazem Sawalha Copyright © 2010 Pearson Education, Inc. Student Learning Outcomes Differentiate a virus from a bacterium. Explain the difference between enveloped and nonenveloped virus...

Acellular microorganisms: Viruses, Viroids, Virusoids & Prions Prof. Hazem Sawalha Copyright © 2010 Pearson Education, Inc. Student Learning Outcomes Differentiate a virus from a bacterium. Explain the difference between enveloped and nonenveloped viruses. Define viral species. Describe how bacteriophages and animal viruses are cultured. Compare and contrast the lytic and lysogenic cycles of bacteriophages. Define oncogene and transformed cell. Discuss the relationship between viruses and cancer. Explain latent viral infections and give an example. Discuss how a proteins can be infectious (prions). Copyright © 2010 Pearson Education, Inc. What is a Virus  General definition:  Obligatory intracellular parasite, active inside the host cell and inactive outside the host cell.  Scientific definition:  Entity that is composed of a nucleic acid, either DNA or RNA surrounded by a protein coat. Sometimes it is surrounded by an external envelope Copyright © 2010 Pearson Education, Inc. First observation  1864. Diseases on humans, animals and plants with unknown causal agents  1886. Mayor in Holand noticed tobacco leaves with mosaic symptoms (green area mixed with yellow one)  1898. Beijerinck (Dutch microbiologist) proved that crude sap of plants with mosaic symptoms can pass through bacterial proof membrane and still infectious.  Contagium vivum fluidum ‫السائل الحيوي المعدي‬  Virus poison  The first human disease associated with a filterable agent was yellow fever Copyright © 2010 Pearson Education, Inc.  Foundations of Virology Non-living agents that infect all life forms (phages vs. animal viruses) Viral cultivation differs from bacterial cultivation Advent of EM allowed for visualization of viruses  1,500 known viruses (estimates:  400,000 exist) Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc.  General Characteristics of Viruses  Virus = Latin for poison  Obligatory intracellular parasites  Filterable: can be filtered through bacterial proof membranes  Contain DNA or RNA  Contain a protein coat = capsid made up of building units called capsomeres. Various shapes  Some are enclosed by an envelope (naked vs. enveloped)  Some viruses have spikes (COH/protein)  Most viruses are tissue specific  Host range is determined by specific host attachment sites and cellular factors (the key and the lock) Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Virus Shapes and Sizes Fig 13.1 Viral Shapes 1- Helical viruses: Rod shape viruses with a hollow in the center (the site of nucleic acid) 2- Polyhedral viruses: Spherical viruses with many faces (triangles) and many corners. Ex. Icosahedral (20 faces & 12 corners). 3- Enveloped viruses: Viruses with external envelopes (proteins, carbohydrates & lipids). Some times they have spikes (glycoproteins). 4- Complex viruses: Viruses with complicated structure as they have several parts as head, fibers, tail, sheath….etc. Ex. Bacteriophages Copyright © 2010 Pearson Education, Inc. Polyhedral  (d) Enveloped viruses: have envelopes made of proteins, lipids and carbohydrates. Some have spikes made of glycoproteins Morphology of an enveloped helical virus Fig 13.3 Copyright © 2010 Pearson Education, Inc.  Host Range and Specificity Animal viruses, plant viruses, Bacteria viruses …etc. Virus / host cell interaction usually very specific (narrow host range) – due to? Tissue tropism Copyright © 2010 Pearson Education, Inc.  Tissue tropism is the cells and tissues of a host which support growth of a particular virus or bacteria. Some viruses have a broad tissue tropism and can infect many types of cells and tissues. Other viruses may infect primarily a single tissue. For example rabies virus affects primarily neuronal tissue. Copyright © 2010 Pearson Education, Inc.  Taxonomy of Viruses  No evidence for common viral ancestor.  Classification based on type of NA, strategy for replication, and morphology.  Family names end in –viridae  Genus and species names end in -virus.  Viral species: A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species.  Subspecies are designated by a number. Copyright © 2010 Pearson Education, Inc. Taxonomy of Viruses  Herpesviridae  Retroviridae  Herpesvirus  Lentivirus  Human herpes virus  Human HHV-1, HHV-2, HHV-3 immunodeficiency virus HIV-1, HIV-2 Copyright © 2010 Pearson Education, Inc.  Isolation, Cultivation, and Identification of Viruses Fig 13.6  Viruses must be grown in living cells Bacteriophages form plaques on a lawn of bacteria. So you have to mix the phage with bacteria and culture the mixture plaques will appear Animal viruses may be grown in animal hosts, embryonated eggs, or cell culture Fig 13.8 Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc.  Virus Identification  Cytopathic effects: refers to damage to host cells during virus invasion, organelle malformation, formation of inclusion bodies (virus particles + host protein)  Serological tests (ELISA: enzyme-linked immunosorbant assay)  Detect antibodies against viruses in a patient  Use antibodies to identify viruses (antibody-antigen reaction)  Nucleic acids  PCR: polymerase chain reaction  Novel methods such as Biophotonics:  a combination of biology and photonics techniques that deal with the interaction between biological items and photons Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc.  PCR Reagents  1X Buffer  10mM Tris-HCl, 50mM KCl  MgCl2  1mM - 4mM (1.5mM)  dNTPs  200μM  Primers  100nM-1μM, 200nm (or less) for real time analysis  DNA polymerase  Taq DNA polymerase is thermostable  1-4 Units (1 unit)  DNA  10pg-1μg (20ng) Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc.  More Cycles = More DNA Size Number of cycles Marker 0 10 15 20 25 30 Copyright © 2010 Pearson Education, Inc. Terms  Viral genome: part of nucleic acid which carries information needed for virus replication  Viron: mature well-developed virus particle Copyright © 2010 Pearson Education, Inc.  Viral Replication  Obligate intracellular parasites using host cell machinery  Very limited number of genes encode proteins for  Capsid formation  Viral nucleic acid replication  Movement of virus into and out of cell  Kill (lytic) or live in harmony within the host cell (lysogenic) – Outside the cell, viruses are inert  Lytic cycle: ends with cell lysis  Lysogenic cycle: the genome incorporates into the host genome and stays for a period of time Copyright © 2010 Pearson Education, Inc.  Bacteriophage: The Lytic Cycle 1. Attachment to cell surface receptors (chance encounter – no active movement) 2. Penetration – only genome enters. Lysozymes are produced by the tail and make a hole in the bacterial cell wall through which the tail core is driven and the genome is injected into cytoplasm 3. Biosynthesis – Production of phage DNA and proteins 4. Maturation – assembly to form intact phage 5. Release due to phage induced lysozyme production See Fig 13.11 Lytic Cycle of a T-Even Bacteriophage 1 2 3 Fig 13.11 Copyright © 2010 Pearson Education, Inc. Lytic Cycle of a T-Even Bacteriophage 4 Fig 13.11 Copyright © 2010 Pearson Education, Inc. Results of Multiplication of Bacteriophages  Lytic cycle  Lytic or virulent phage  Phage causes lysis and death of host cell  Lysogenic cycle  Lysogenic or temperate phage  Phage DNA incorporated in host DNA  Prophage  Phage conversion  Specialized transduction: is the process by which DNA is transferred from one bacterium to another by a virus ANIMATION Viral Replication: Virulent Bacteriophages ANIMATION Viral Replication: Temperate Bacteriophages Copyright © 2010 Pearson Education, Inc. Lytic and Lysogenic Cycles Fig 13.12 Copyright © 2010 Pearson Education, Inc. Some animal viruses exit the host cells via budding. It is a form of viral shedding by which enveloped viruses acquire their external envelope from the host cell membrane, which bulges (‫ )ينتفخ‬outwards and encloses the virion. HSV envelopment and release Compare Copyright © 2010 to Fig. Pearson 13.20 Education, Inc. Multiplication of DNA Viruses Foundation Fig 13.15 ANIMATION Viral Replication: Animal Viruses Multiplication of a Retrovirus Fig 13.19 The Biosynthesis of RNA Viruses  Virus multiplies in the host cell's cytoplasm using RNA-dependent RNA polymerase  ssRNA; + (sense) strand  Viral RNA serves as mRNA for protein synthesis  ssRNA; – (antisense) strand  Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis  dsRNA—double-stranded RNA Copyright © 2010 Pearson Education, Inc.  Figure 13.17a Pathways of multiplication used by various RNA-containing viruses. Attachment Capsid Nucleus RNA Host cell Cytoplasm Entry Maturation and uncoating and release Translation and synthesis RNA replication by viral RNA– of viral proteins dependent RNA polymerase – strand is transcribed Uncoating releases KEY from + viral genome. viral RNA and proteins. Capsid Viral Viral + or sense protein genome protein strand of (RNA) viral genome – or antisense strand of viral genome + strand ss = single-stranded ssRNA; mRNA is transcribed + or sense strand; ds = double-stranded from the – strand. Picornaviridae Copyright © 2010 Pearson Education, Inc.  Figure 13.17b Pathways of multiplication used by various RNA-containing viruses. Attachment Capsid Nucleus RNA Cytoplasm Host cell Entry Maturation and uncoating and release Translation and synthesis RNA replication by viral RNA– of viral proteins dependent RNA polymerase The + strand (mRNA) must first be Uncoating releases Capsid transcribed from the – viral genome viral RNA and proteins. KEY protein before proteins can be synthesized. Viral Viral genome protein + or sense (RNA) strand of viral genome – or antisense strand of viral genome – strands are ssRNA; – or ss = single-stranded incorporated Additional – strands are antisense strand; ds = double-stranded into capsid transcribed from mRNA. Rhabdoviridae Copyright © 2010 Pearson Education, Inc.  Figure 13.17c Pathways of multiplication used by various RNA-containing viruses. Attachment Capsid Nucleus RNA Cytoplasm Host cell Entry Maturation and uncoating and release Translation and synthesis RNA replication by viral RNA– of viral proteins dependent RNA polymerase RNA polymerase initiates production of mRNA is produced inside the Uncoating releases – strands. The mRNA and – strands form the capsid and released into the viral RNA and proteins. dsRNA that is incorporated as new viral genome. cytoplasm of the host. KEY Viral Viral genome protein + or sense (RNA) strand of viral genome – or antisense strand of viral genome ss = single-stranded Capsid proteins and RNA- dsRNA; + or sense ds = double-stranded dependent RNA polymerase strand with – or antisense strand; Reoviridae Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Cancer Cancer uncontrolled mitotic divisions Benign tumor is a mass of cells that lacks the ability to invade neighboring tissue or metastasize malignant tumor is not self-limited in its growth, is capable of invading into adjacent tissues, and may be capable of spreading to distant tissues Oncology means a branch of science that deals with tumors and cancers. The word “onco” means bulk, mass, or tumor while “-logy” means study. Three important characteristics of cancer cells: 1. Rapid cell division 2. Loss of anchoring junctions (Cells within tissues and organs must be anchored to one another and attached to components of the extracellular matrix ) and loss of contact inhibition (meaning that when they come in contact with another cell, they do NOT stop growing. ) metastasis 3. Dedifferentiation of cells: the reversion of the cells of differentiated tissue to a less specialized form Copyright © 2010 Pearson Education, Inc. Relationship between Viruses and cancer  First Evidence: Ellerman and Bang 1908 (Denmark)  Chicken leukemia transferred to healthy chicken by cell free filtrate  2nd Evidence: Rous 1936  Adenocarcinomas (cancer of glandular epithelial tissue of mouse) is transferred from mother to offspring through milk Copyright © 2010 Pearson Education, Inc. The viral cause of cancer can often go unrecognized for several reasons:  Most of the particles of some viruses infect cells but do not induce cancer.  Cancer might not develop until long after viral infection  Cancers do not seem to be contagious, as viral diseases usually are. Copyright © 2010 Pearson Education, Inc. Transformation of normal cell into cancer cells  Anything that can alter the genetic material of a eukaryotic cell has the potential to change a normal cell into cancer cell.  These cancer-causing alterations to cellular DNA affect parts of the genome called oncogenes.  Oncogenes first identified in cancer causing viruses (part of the genome)  Bishop and Varmus (1989) proved that cancer inducing genes are derived from animal cells and carried by viruses (Ex. SRC gene in avian sarcoma viruses is derived from chicken genes)  Viruses capable of inducing tumors in animals are called oncogenic viruses Copyright © 2010 Pearson Education, Inc. What is transformation?  Tumor cells undergo transformation; that is, they acquire properties that are distinct from the properties of uninfected cells or from infected cells that do not form tumors.  Characteristics of tumor cells transformed by viruses: contains a virus-specific antigen on the cell surface called tumor-specific transplantation antigen (TSTA) contains an antigen in the nucleus called T- antigen. Copyright © 2010 Pearson Education, Inc. Viruses and Cancer (summary)  The genetic material of oncogenic viruses becomes integrated into the host cell’s DNA (provirus).  Activated oncogenes transform normal cells into cancerous cells  Transformed cells have increased growth, loss of contact inhibition, tumor-specific transplant antigens, and T antigens  Oncogenic Viruses are responsible for 10 % of human cancers Copyright © 2010 Pearson Education, Inc. Types of Oncogenic Viruses  DNA Oncogenic Viruses:  Papillomaviruses causes uterine (cervical) cancer  RNA Oncogenic Viruses:  Retroviruses cause human T-cell leukemia (HTLV- 1, HTLV-2) Copyright © 2010 Pearson Education, Inc.  Oncogenic DNA Viruses and RNA Viruses Papilloma virus (HPV) Hepatitis C virus cervical cancer (HCV) liver cancer Epstein-Barr virus human T-cell (EBV) Burkitt’s leukemia virus lymphoma (HTLV-1) HV8 Kaposi’s sarcoma Hepatitis B virus (HBV) liver cancer Latent, Persistent and Acute Viral Infections Latent: Virus remains in asymptomatic host cell for long Fig 13.21 periods Persistent: Disease processes occurs over a long period; generally is fatal An acute disease: is a disease with a rapid onset and/or a short course Copyright © 2010 Pearson Education, Inc. Prions  Small proteinaceous infectious particles (very resistant to inactivation) without nucleic acid.  Inherited and transmissible by ingestion, transplant, and surgical instruments  Causes spongiform encephalopathies:  Sheep scrapie, (degenerative disease of nervous system)  Creutzfeldt-Jakob disease (is a degenerative neurological disorder that is incurable and invariably fatal)  Gerstmann-Sträussler-Scheinker syndrome (fatal neurodegenerative disease that affects patients from 20 to 60 years in age)  Fatal familial insomnia (sleeplessness, is a sleep disorder in which there is an inability to fall asleep)  Mad cow disease (is a fatal neurodegenerative disease (encephalopathy) in cattle that causes a spongy degeneration in the brain and spinal cord)  PrPC: Normal cellular prion protein, on cell surface. Involved in cell death.  PrPSc: Scrapie protein; accumulates in brain cells, forming plaques. Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Spongiform Encephalopaties Caused by altered protein:  Mutation in normal PrPc gene, or  contact with the abnormal PrPSc protein ANIMATION Prion: Overview ANIMATION Prion: Characteristics ANIMATION Prion: Diseases Viroids and virusoids  Viroids are plant pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary, circular, single-stranded RNA.  Ex. Potato spindle tubers  Virusoids : viroids enclosed in a protein coat, cause disease only when the cell is infected by a virus  Example: Hepatitis D (satellite RNA ) requires coinfection by Hepatitis B virus. Copyright © 2010 Pearson Education, Inc.