Download Overview of Viruses - Food Science and Human Nutrition

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