viral growth
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Viral Growth
Dr Paul Brownpaul.brown@uwimona.edu.jm
BC10M: Introductory Biochemistry
Lecture 7
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Characteristics of Viruses
Non-living agents that infect all lifeforms (Bacteriophages, plant viruses, andanimal viruses)
One virus particle = virion (size: 10
500 nm)
Virus architecture Nucleic acid (DNA or RNA not both!) Protein coat = capsid ofvarious shapes
(isometric, helical, complex) Naked virions vs. enveloped viruses
Genome: ss-/ds-RNA, ss-/dsDNA; linearor circular
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Methods of Study
Much more expensive anddifficult to study animalviruses than bacteriophages
Cultivation in host cellsLiving animalEmbryonated chicken eggsCell or tissue culture (= in
vitro)
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Methods of Study cont:
QuantitationPlaque assay (useful for infective and lyticviruses)
Virion counting with EM
Quantal assay(ID
50or LD
50)
Haemagglutination(e.g.influenza virus)
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Culturing viruses requires
an appropriate host cell.
In this example a bacteriophage
is grown using bacterial cells
as host cells. Animal or plant
viruses can be grown in tissue
culture.
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Plaques in a lawn of
bacterial cells caused
by viruses lysing
bacterial cells.
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Replication Cycle -
OverviewObligate intracellularparasites using host cellmachinery
Very limited number of genes
encode proteins for Capsid formation Viral nucleic acid replication Movement of virus into and out of
cell
Kill or live in harmony withinthe host cell Outside thecell, viruses are inert
Size comparison
(see also Table 13.1)
Fig 13.
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Bacteriophage (Phage)
Obligate intracellular parasites that multiplyinside bacteria by making use of some or allof the host biosynthetic machinery
SignificanceModels for animal cell viruses Gene transfer in bacteria (transduction)Medical applications
Identification of bacteria - phage typing Treatment and prophylaxis???
Examples: T4 and Lambda ()
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Composition and Structure
CompositionNucleic acid
Genome sizeModified bases
ProteinProtectionInfection
Structure (T4)
SizeHead or capsid
Tail
Tail
Tail Fibers
Base Plate
Head/Capsid
Contractile
Sheath
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Infection of Host Cells
Irreversible attachment
Adsorption
LPS for T4
Sheath Contraction
Nucleic acid
injection
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Lytic Bacteriophage
Lytic or virulent phage: Phage that can onlymultiply within bacteria and kill the cell by
lysis. (e.g., T4)
O f
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One-step growth curve of a lytic
bacteriophage
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Lytic Phage Multiplication
Cycle
Eclipse Early genes
Phage DNA
synthesis Late genes
Intracellularaccumulation
Lysis and Release
Total
Phage
Extracellular
Phage
Eclipse
Intracellular
accumulation
phase
Time after Infection
Numbe
rofInfec
tiou
sParticles
Lysis
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Time course of events occurring during one-
step growth curve of bacteriophage T4.
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Lysogenic Bacteriophage
Lysogenic or temperate phage:Phage that can either multiply via thelytic cycle or enter a quiescent statein the bacterial cell. (e.g.,)Expression of most phage genes
repressed
ProphageLysogen
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Lysogenic Phage
DNA integrates into host chromosome Phage DNA = Prophage Infected bacterial cell = lysogenic cell or lysogen
Prophage state can be indefinite
Lysogenic conversion confers new propertiesonto host cells (e.g.: toxin production ofS. pyogenes scarlet fever)
Phage induction converts lysogenic to lyticstate
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Replication cycle in a lysogenic bacteriophage
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Events Leading to Lysogeny
Circularization of the phage chromosome Cohesive ends
Ligase
Closed Circle
Cohesive Ends
Linear Double Stranded Opened Circle
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Events Leading to Lysogeny
Site-specificrecombination
Phage coded
enzyme
Repression of the
phage genome Repressor protein Specific
Immunity tosuperinfection gal bio
gal bio
gal
bio
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Termination of Lysogeny
InductionAdverse conditions
Role of proteases
recA proteinDestruction ofrepressor
Gene expression
ExcisionLytic growth
gal
bio
gal bio
gal bio
gal bio
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Lytic vs Lysogenic Cycle?
Role of repressor
Role ofcro gene product
Role of proteases
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Significance of Lysogeny
Model for animal virus transformation
Lysogenic or phage conversion
Definition: A change in the phenotype ofa bacterial cell as a consequence oflysogenyModification ofSalmonella O antigen
Toxin production by Corynebacteriumdiphtheriae
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Host Range of Phages
Phage host cell interaction usually very specific
Limiting factors for host range
1. Phage has to bind to bacterial surfacereceptors Bacterial surface receptors mutate resistant cell
Lysogenic conversion changes surface receptors andprotects host
1. Restriction modification system of hostcell
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Medical Applications of Phage
I strongly believe phage could become an
effective antibacterial tool - Carl Merril, Chief of theLaboratory of Biochemical Genetics, National Institute of Mental
Health, NIH.
It might be another string on the bow, such thatwhen (conventional antibiotics) fail, heres
something that has a chance of working. But its
not going to be a panacea - Joshua Lederberg, SacklerFoundation Scholar at The Rockefeller University
Reassessment of Medicinal Phage Spurs Companies to Study Therapeutic Uses
American Society for Microbiology News 64:620-623, 1998
M di l A li ti f Ph
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Medical Applications of Phage
Exponential Biotherapies (Rockville, MD) Vancomycin resistant Enterococcus faecium and
Streptococcus pneumoniae
Phage Therapeutics (Bothell, WA) Staphylococcus aureus and Staphylococcus epidermidis
Intralytix, Inc. (Baltimore, MD) Salmonella in meat and poultry
Biopharm Ltd. (Tblisi, Georgia) Infections associated with burns
University of Idaho Escherichia coliO157:H7 in cattle
Reassessment of Medicinal Phage Spurs Companies to Study
Therapeutic Uses. American Society for Microbiology News 64:620-623,
1998.
Phages eyed as agents to protect against harmful E. coli. American
Society for Microbiology News 65:666-667, 1999.
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