Overview of Viruses. Dr. Angela M. Shaw Assistant Professor of Food Science and Human Nutrition Food Safety Extension and Outreach Specialist

Transcription

1 Overview of Viruses Dr. Angela M. Shaw Assistant Professor of Food Science and Human Nutrition Food Safety Extension and Outreach Specialist

2 Viruses Dmitri Iwanowski Experiments with tobacco plants Found infectious agents smaller than bacteria Martinus Beijerinck Named the Viruses

3 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

4 Viruses Acellular organisms Protein coat surrounds genetic information Can infect any cellular organism Organism specific Smallest microbial contaminant Submicroscopic 15 to 400nm

5 Baltimore Classification

6 Viral Classification Based on Four Factors 1. Host range 2. Size 3. Structure 4. Life cycle

7 Host Range Spectrum of organisms a particular virus attacks Prokaryotic Eukaryotic Dependent upon receptors

8 Size Much smaller than bacteria ( nm) No cytoplasm, ribosomes, or nucleus Small genomes <100 genes

9 Structure 1. Nucleic acid core 2. Protein coat: capsid 3. Envelope 4. Viruses are either enveloped or naked

10 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

11 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

12 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

13 +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

14 -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

15 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

16 dsrna, continue +ssrna also used as a template to make -ssrna which combines with plus strands to make dsrna for new virions

17 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

18 3. Viral Envelopes Made of phospholipids and proteins Formed from host cell s CM Phospholipid portion (host) Proteins (virus) Sensitive to non-polar solvents

19 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

20 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

21 Life Cycle, cont. 3. Synthesis Viral components are synthesized 4. Maturation components assembled into virions 5. Release host cell lyses, new virions are released

22 Bacteriophages Viruses which infect bacteria First discovered in 1915 Luria-Delbruck experiment 1940 s: enumerate bacteriophages and examine replication

23 Replication pathways Two pathways a bacteriophage can enter: Lytic pathway more virions Lysogenic pathway dormant state Two types of Phages Virulent phages Temperate phages

24 Virulent phage: Life Cycle 1. Adsorption to host cell 2. Penetration 3. Maturation 4. Release 5. All of the above occurs rapidly

25 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

26 Animal viruses First cultured in embryonated chicken eggs Still used today for the production of vaccines Cell cultures are used today

27 Replication of animal viruses Replicate the same way as bacteriophages: 1. Adsorption 2. Penetration & Uncoating 3. Viral synthesis 4. Maturation 5. Release

28 Animal Viruses Replication 1. Adsorption: viruses are attracted to receptors Very specific 2. Penetration: 3 ways Envelope fusion Phagocytosis Nucleic acid injection

29 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

30 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

31 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

32 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

33 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

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35 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

36 Virus Low Infective Doses Fairly communicable through foods and direct contact Many virus resistant to: Sanitizers Freezing Heat Resistance varies greatly

37 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

38 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)

39 Additional Questions Dr. Angela Shaw Iowa State University Assistant Professor/ Extension and Outreach 2577 Food Science Building, Ames, IA Phone: