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Viruses, Viroids, and Prions Viruses, Viroids, and Prions Bui Thi Viet Ha, Ph D Department of Microbiology, HUS 1 Contents VIRUS STRUCTURE AND GROWTH General Properties of Viruses Nature of the Virion.
Viruses, Viroids, and Prions Bui Thi Viet Ha, Ph.D Department of Microbiology, HUS Contents VIRUS STRUCTURE AND GROWTH General Properties of Viruses Nature of the Virion The Virus Host Quantification of Viruses VIRAL REPLICATION General Features of Virus Replication Viral Attachment and Penetration Production of Viral Nucleic Acid and Protein VIRAL DIVERSITY SUBVIRAL ENTITIES Overview of Bacterial Viruses Defective Viruses Virulent Bacteriophages and T4 Viroids Temperate Bacteriophages, Lambda and P1 Prions Overview of Animal Viruses Retroviruses What are Viruses? Viruses are genetic elements that cannot replicate independently of a living cell, called the host cell Viruses are therefore obligate intracellular parasites that rely on entering a suitable living cell to carry out their replication cycle Viruses possess their own genetic information and are thus independent of the host cell’s genome What are Viruses? • Unlike plasmids, viruses have an extracellular form, the virus particle, that: – enables them to exist outside the host for long periods – facilitates transmission from one host cell to another • To multiply, viruses must enter a cell in which they can replicate, a process called infection • Viruses infect bacterial cells on agar results in cleared zones called plaques where the cells have been lysed What are Viruses? • Viruses are the most numerous microorganisms infect all types of cellular organisms – Therefore, they are interesting in their own right • However, scientists also study viruses for what they can tell us about the genetics and biochemistry of cellular processes and, for many viruses, the development of disease • Furthermore, viruses are also important tools in microbial genetics and genetic engineering Viruses differ from living cells • In summary, viruses differ from living cells in at least three ways: – (1) their simple, acellular organization; – (2) the presence of either DNA or RNA, but not both, in almost all virions (human cytomegalovirus has a DNA genome and four mRNAs); – (3) their inability to reproduce independent of cells and carry out cell division as procaryotes and eucaryotes • Although bacteria such as Chlamydia and Rickettsia are obligately intracellular parasites like viruses, they not meet the first two criteria Minireview A virus is an obligate intracellular parasite that cannot replicate without a suitable host cell The virus genome may be introduced into a new host cell by infection The virus redirects the host metabolism to support virus replication A virion is the extracellular form of a virus and contains either an RNA or a DNA genome inside a protein shell Viruses are classified by their nucleic acids and type of host Questions? – How does a virus differ from a plasmid? – How does a virion differ from a cell? – What is a bacteriophage? Viral History Brief history - Mayer discovered that the disease was contagious when he found he could transmit it from plant to plant by spraying sap extracted from diseased leaves onto healthy plants - He searched for a microbe in the infectious sap but found none Mayer conduded that the disease was caused by unusually small bacteria that could not be seen with the microscope - This hypothesis was tested a decade later by Dimitri Ivanowsky, a Russian who passed sap from infected tobacco leaves through a filter designed to remove bacteria Modes of HIV Transmission • HIV is transmitted by exposure to infected body fluids • Body Fluids – Blood – Semen – Vaginal Secretions – Breast Milk • How are these fluids transferred from one person to another? High Risk Sexual Contact – unprotected vaginal sex – unprotected oral sex – unprotected anal sex • Needles – Intravenous Drug Abuse (sharing dirty needles) – accidental needle sticks How are these fluids transferred from one person to another? • Blood to Blood Contact – open sores or wounds – Transfusions – Organ Transplants – Artificial Insemination • Mother to Child – placenta – as baby passes thru the birth canal – breast milk HIV and the Immune System • Cellular Immune System – cells phagocytize microbes • Humoral Immune System – antibodies to destroy or inactivate microbes Clinical Stages of an HIV Infection • Acute Infection – Initial infection of HIV (exposure to infected body fluids) – Viremia • Fever • Headaches • Weakness • Muscle and joint aches – May last for a couple of weeks – Normal CD4 cell count 1200mm3 Asymptomatic Disease • • • • • • CD4 cell count < 1000mm3 Virus is ‚latent‛ inside CD4 cells Median latency period - 10 yrs No signs or symptoms of illness (asymptomatic) HIV Positive - antibodies can be detected in your blood Seroconversion – to weeks Symptomatic Disease • CD4 cell count < 600mm3 • Viral Genome is ‚turned on‛, Symptoms begin to appear • What causes HIV Genome to be turned on? – – – – – – – Other infections stress shock to the system alcohol drug abuse nutrition exercise ? Symptomatic Disease • Symptoms – chronic fatigue – low-grade fever – night sweats – diarrhea – weight loss • Susceptible to Infections – bacterial pneumonia – meningitis – oral and vaginal yeast infections – tuberculosis Advanced Disease (AIDS) • CD4 cell count < 200mm3 • Severe Opportunistic Infections – Pneumocysitis carinii pneumonia (PCP) Fungi – Kaposi’s Sarcoma ( Cancer - Skin and Blood vessels) – Toxoplasmosis (Brain) Protozoan – Cryptosporidiosis (G.I Tract) Protozoan – Other Bacterial, Fungal and Viral Infections HIV Infection and Immune Response (Graph) Blood Test - ELISA • Enzyme Linked Immunosorbant Assay – tests for HIV Antibodies • If ELISA is positive, same sample is tested again • If ELISA is positive again, then a Western Blot Test is done – Western Blot - test for Viral antigens Treatment for HIV Infection • No Cure • AZT ( Azidothymidine) – Thymine analog – lacks a 3’ OH – Chain Terminator – Inhibits Reverse Transcriptase AIDS Cocktail (Combination Therapy) • AZT • 3TC ( 2’-deoxy-3’-thiacytidine) • Protease Inhibitor Vaccine for HIV ? • HIV mutates too rapidly – Reverse Transcriptase causes at least mutation each time it is used • million variants during Asymptomatic Disease • 100 million variants during Advanced Disease (AIDS) THANK YOU FOR YOUR ATTENTION THE END 141 ... Attachment and Penetration Production of Viral Nucleic Acid and Protein VIRAL DIVERSITY SUBVIRAL ENTITIES Overview of Bacterial Viruses Defective Viruses Virulent Bacteriophages and T4 Viroids Temperate...Contents VIRUS STRUCTURE AND GROWTH General Properties of Viruses Nature of the Virion The Virus Host Quantification of Viruses VIRAL REPLICATION General Features of Virus Replication Viral Attachment... animal viruses, plant viruses, viruses that infect other kinds of eukaryotic cells 30 Viral Taxonomy • Family names end in -viridae • Genus names end in -virus • Viral species: A group of viruses