Virology - 01 Intro to Virology

VIROLOGY PARTS OF VIRION INTRODUCTION TO VIROLOGY OUTLINE • Introduction to Virology o Virus o Virion o Groups of Virus based on Arrangement o Measuring the Size of the Virus • Classification of Virus o Basis of Classification of Virus o Criteria for Virus Classification o Steps of Infection • Prions o Prion o Types • • • • • INTRODUCTION TO VIROLOGY VIRUS • • • • Intracellular organisms unable to reproduce itself outside the host cell Haploid except for retrovirus Energy less; do not multiply by binary fission Size ranges from 20-200 nm • GROUPS OF VIRUS BASED ON ARRANGEMENT STRUCTURE • • • • • • The entire infectious unit is the virion; internal core is the genome containing genetic material either RNA or DNA; Capsid protein coat surrounds the genome Subunits of capsid are called capsomeres (icosahedral, helical or complex) Some are with envelope; those without envelope are classified as naked. Capsid - Protein shell or coat that encloses the nucleic acid genome that is built of structural units of a virion Nucleic Acid - DNA (deoxyribonucleic acid) or RNA (ribonucleus acid) Nucleocapsid - Nucleic acid + Capsid (Enclosed) Capsomeres - Morphological units seen on the surface of particles and represent clusters of structure units or polypeptide Envelope - A lipid containing membrane that surrounds some viral particles o It is acquired during viral maturation by a budding process through a cell membrane o Envelope viruses has a pleomorphic appearance inside the envelope Structural Units - Protomers are the smallest functional unit equivalent to building units of the capsid. Basic protein that acts as building blocks of the coat • Lysis is required for icosahedral pattern and not for helical pattern Cubic symmetry o observed with animal viruses is the icosahedral pattern, the most efficient arrangement for subunits in a closed shell Helical symmetry o protein subunits are bound in a periodic way to the viral nucleic acid, winding into a helix MODE OF TRANSMISSION • • • • • • • Droplet infection Airborne Ingestion Direct contact Insect borne/arthropod borne Direct inoculation Transplacental • Complex Symmetry RESISTANCE OF VIRUSES • • • • • Resistant to antimicrobials, antibacterials More resistant to antibiotics than bacteria but sodium hypochlorite, formaldehyde and dilute HCL are efficient Can survive long periods at -70degC when freeze dried. Killed with moist heat but are somewhat resistant to dry heat UV and XRAY irradiation inactivates viruses Chloroform and ether will inactivate viruses that have lipid envelope • VIRION • • infectious unit complete viral particle MEASURING THE SIZE OF VIRUS • • • • Direct observation in the electron microscopy Filtration through membranes of graded porosity Sedimentation in the ultracentrifuge (Svedberg Units) Comparativeness measurements TYPES CLASSIFICATION OF VIRUSES BASIS OF CLASSIFICATION OF VIRUS • • • • • • • Virion morphology (size, shape, type of symmetry, presence or absence of membranes) Physicochemical properties of virion (molecular mass, buoyant density, pH stability, thermal stability and susceptibility to physical and chemical agents ex. Ether and detergents) Virus genome properties (type of nucleic acid, size, strandedness(single or double) , nucleotide sequence) Virus protein properties Genome organization and replication Antigenic properties Biologic properties (host range, MOT, vector relationships, pathogenicity and tissue pathology) • • • • • CRITERIA FOR VIRUS CLASSIFICATION • • Genome type/Nuclear material: RNA or DNA Number of strand single or double stranded o Capsid morphology (Helical, icosahedral, or complex) o Envelope (Presence or absence) o Site of Virion Assembly ( nucleus or cytoplasm ) o Baltimore Classification/Scheme - mostly used to distinguish different types of RNA viruses ▪ +sense or –sense • • STEPS OF INFECTION • • • • • Absorption or attachment o Specific for cell receptors Penetration o Direct penetration of the membrane o Envelope-membrane fusion o Endocytosis Uncoating o Release the genome from capsid RNA in cytoplasm DNA in the nucleus PRIONS PRION • • • • Is a proteinaceous infectious material Structure is composed of insoluble beta pleated sheet Affects NS such as fibrils and vacuole (spongy) Examples o Transmissible spongiform encephalopathies (TSE) ▪ Normal cellular protein (PrPc) misfolded when a portion of its α-helical coil structure refolds to a βsheet, producing a mutant (PrPsc) ▪ May be secreted in the urine o Animal infection only ▪ Scrapie • Sheep ▪ Chronic wasting disease • Mule, deer, and elk ▪ Transmissible mink encephalopathy • Mink ▪ Human infection- Prion diseases are a family of progressive neurodegenerative disorders that affect both humans and animals • Sporadic Genetic Variant Iatrogenic o Bovine spongiform encephalopathy (BSE) ▪ Cattle ▪ Development of variant Creutzfeldt-Jakob disease (vCJD) from infected meat of cows with BSE o Kuru ▪ Aborigines in New Guinea o Creutzfeldt-Jakob disease (CJD) o Fatal insomnia ▪ All human as natural host Symptoms o Dementia, memory loss, behavioral problems o Cerebellar ataxia and shivering-like tremor o Complete loss of motor and speech function o Death in 1 month to 1 year Transmission o Neural tissue o Although there might be cases of other tissue transmission Other problems o Survive formalin fixation o Very resistant to sterilization ▪ Recommend exposure to 20,000 ppm chlorine or 1 M sodium hydroxide to inactivate prions