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Overview of Viruses Dr. Angela M. Shaw Assistant Professor of Food Science and Human Nutrition Food Safety Extension and Outreach Specialist Viruses • Dmitri Iwanowski - 1892 – Experiments with tobacco plants – Found infectious agents smaller than bacteria • Martinus Beijerinck - 1898 – Named the Viruses Viruses • Most common cause of FBI in United States • Non-living – Must have a host to reproduce – Do not consume nutrients – Do not excrete waste products – Protein coat with DNA/RNA Viruses • • • • • Acellular organisms Protein coat surrounds genetic information Can infect any cellular organism Organism specific Smallest microbial contaminant – Submicroscopic – 15 to 400nm Baltimore Classification Viral Classification • Based on Four Factors 1. Host range 2. Size 3. Structure 4. Life cycle Host Range • Spectrum of organisms a particular virus attacks – Prokaryotic – Eukaryotic – Dependent upon receptors Size • • • • Much smaller than bacteria (25-300 nm) No cytoplasm, ribosomes, or nucleus Small genomes <100 genes Structure 1. 2. 3. 4. Nucleic acid core Protein coat: capsid Envelope Viruses are either enveloped or naked 1. Viral Nucleic Acid Core • 5 different forms: 1. dsDNA: traditional gene expression 2. ssDNA: converted to dsDNA 3. dsRNA: no need for transcription 4. ssRNA (+ or -) +sense: mRNA molecule -sense: converted to +sense dsDNA viruses & ssDNA viruses • dsDNA viruses – Viral dsDNA is transcribed into mRNA – mRNA is translated into viral proteins – Proteins are used to assemble new virus particles • ssDNA viruses – Must first convert the ssDNA into dsDNA – Then undergo synthesis like dsDNA viruses RNA viral synthesis • Very different from that of DNA viruses • Require an additional enzyme: – RNA-dependent RNA polymerase – Made by host cell – Makes complementary copy of ssRNA template • RNA-dependent RNA polymerase: – uses +ssRNA as a template to make -ssRNA – -ssRNA is used as a template to make more +ssRNA • packed into new virions +ssRNA • RNA-dependent RNA polymerase: – Replicates the viral genome – Not used for gene expression • Uses host cell enzymes – Host does not have RNA-dependent RNA polymerase • Makes it from viral encoded RNA -ssRNA • RNA-dependent RNA polymerase: – replicate the genome – gene expression • First used to transcribe the -ssRNA into +ssRNA for use in gene expression – New +ssRNA serves as mRNA to make viral proteins – Also used as a template dsRNA • RNA-dependent RNA polymerase: – Used in the same way as with -ssRNA viruses – Genome is used as a template to make +ssRNA – +ssRNA is used as mRNA to make viral proteins dsRNA, continue • +ssRNA also used as a template to make -ssRNA which combines with plus strands to make dsRNA for new virions 2. Viral Capsids • Surrounds the nucleic acid of a virus – Capsomeres – Helical – cylindrical or rod-shaped – Polyhedral – made of triangles – Complex – helical tail portion with polyhedral head • Can possess other structures 3. Viral Envelopes • Made of phospholipids and proteins • Formed from host cell’s CM – Phospholipid portion (host) – Proteins (virus) • Sensitive to non-polar solvents Viral Life Cycle • To replicate, a virus must infect a host cell • 5 processes occur during the viral life cycle: 1. Adsorption 2. Penetration & Uncoating 3. Synthesis 4. Maturation 5. Release Life Cycle 1. Adsorption – Virion attaches to a host cell 2. Penetration and Uncoating: – Some open the envelope and capsid outside the host – Others uncoat inside the host cell Life Cycle, cont. 3. Synthesis – Viral components are synthesized 4. Maturation – components assembled into virions 5. Release – host cell lyses, new virions are released Bacteriophages • Viruses which infect bacteria • First discovered in 1915 • Luria-Delbruck experiment – 1940’s: enumerate bacteriophages and examine replication Replication pathways • Two pathways a bacteriophage can enter: – Lytic pathway – more virions – Lysogenic pathway – dormant state • Two types of Phages – Virulent phages – Temperate phages Virulent phage: Life Cycle 1. 2. 3. 4. 5. Adsorption to host cell Penetration Maturation Release All of the above occurs rapidly Temperate phage: Life Cycle • Adsorption • Injection of nucleic acid • Lytic cycle or lysogenic cycle – Lytic: same as for virulent phage – Lysogenic: quiescent state • Prophage • Repressor • Viral genome is replicated along with host cell genome Animal viruses • First cultured in embryonated chicken eggs • Still used today for the production of vaccines • Cell cultures are used today Replication of animal viruses • Replicate the same way as bacteriophages: 1. Adsorption 2. Penetration & Uncoating 3. Viral synthesis 4. Maturation 5. Release Animal Viruses Replication 1. Adsorption: viruses are attracted to receptors – Very specific 2. Penetration: 3 ways – Envelope fusion – Phagocytosis – Nucleic acid injection Animal Viruses Replication, continue 2. Uncoating: all virions must be uncoated for gene expression to occur – Can happen before or after virus enters the cell 3. Viral synthesis: virions are synthesized Animal Viruses Replication, continue 4. Maturation – Unknown exactly how animal viruses mature – It is believed that host cells assist with capsid formation around the nucleic acid Animal Viruses Replication, continue 5. Release – Varies according to whether or not there is an envelope – Enveloped viruses: • acquire their envelope as they leave – Non-enveloped viruses: • assemble in the host cell cytoplasm Retroviruses • Genome is replicated very differently • Cellular organisms (and dsDNA viruses): DNA is a template to make RNA • RNA viruses use RNA • Retroviruses (+ss): use a reverse process – RNA is a template for making DNA – Requires reverse transcriptase Virus • Complete virus particle or virion consist of one or more molecules of DNA or RNA enclosed in a coat of protein and in layers that contain carbohydrates, lipids and additional proteins • Cause wide range of disease in plants, animals, and humans Virus • Difficult to concentrate and purify viruses in food • Most food borne viruses infect the cells lining the intestinal tract and spread through feces • Human hygiene and water sources are two main causes of foodborne virus illness Virus • Low Infective Doses – Fairly communicable through foods and direct contact • Many virus resistant to: – Sanitizers – Freezing – Heat • Resistance varies greatly Viruses • Tropism: Each group of viruses has its own specific range of host and cell preferences • Two Phases: Extracellular phase and Intracellular phase – Extracellular phase: cannot reproduce independent of living cells – Intracellular phase: replicating nucleic acids that induce host metabolism to synthesize virion compounds Viral Prevention • Hand hygiene – Soap and water for 20 seconds reduces Norovirus by ~1 logs • Exclusion and Isolation to infected persons – Minimize contact with the person or their fluids • Environmental Disinfection – High contact surfaces (i.e. door handles, tools, machinery) Additional Questions Dr. Angela Shaw Iowa State University Assistant Professor/ Extension and Outreach 2577 Food Science Building, Ames, IA 50011 Email: [email protected] Phone: 515-294-0868