TORTORA • FUNKE
• CASE
Microbiology
AN INTRODUCTION
EIGHTH EDITION
B.E Pruitt & Jane J. Stein
Chapter 13, part A
Viruses, Viroids, and Prions
Viruses
• Are very small
• Viruses contain DNA or RNA
• And a protein coat
• Capsid with Capsomeres
• Some are enclosed by an envelope
• Acellular
• Obligate intracellular parasites
• No ATP generating system
• No Ribosomes or means of Protein
Synthesis
• Some viruses have spikes
• Most viruses infect only specific
types of cells in one host
Viruses: only 1 nucleic acid
• Nucleic Acid
• DNA or RNA (But never both)
• ssDNA
• ds DNA
• ss RNA
• ds RNA
Viruses are small
Figure 13.1
Shapes of Viruses: Helical
Figure 13.4a, b
Shapes: Polyhedral
icosahedral
Figure 13.2a, b
Shapes: Complex Viruses
Figure 13.5a
Envelope
• Membrane may be
around capsid
• It may contain spikes
Viral Taxonomy
• 1. Nucleic Acid
• 2. Morphology
• 3. Strategy for replication
• 4. Symptoms
• Viral species: A group of viruses sharing the same
genetic information and ecological niche (host).
Common names are used for species
• Subspecies are designated by a number e.g.
HHVII
Growing Viruses
• Viruses must be
grown in living
cells.
• The are never
grown in culture
media
• Bacteriophages
form plaques on
a lawn of
bacteria.
Plaques on E. coli Ø
Growing Viruses
• Animal viruses
may be grown in
living animals or
in embryonated
eggs.
• Vaccine virus in
eggs
Figure 13.7
Viral cultures
• 1. Primary Cell Lines
• die out after a few generations
• 2. Diploid Cell Lines
• derived from human embryos
• maintained for up to 100 generations
• 3. Continuous Cell Lines
• Transformed Cells (Cancerous Cells)
• may be maintained indefinitely
• HeLa Cells
• Henrietta Lacks 1951 (Cervical Cancer)
Growing Viruses
• Animal and plants viruses may be grown in cell
culture.
• Continuous cell lines may be maintained
indefinitely.
Figure 13.8
Virus Identification
• Cytopathic effects CPE Negri bodies rabies
• Serological tests
• Detect antibodies against
viruses in a patient
• Use antibodies to identify viruses
in neutralization tests, viral
hemagglutination, and Western
blot
• Nucleic acids
• RFLPs
• PCR
Virus Identification
Cytopathic effects CPE
Figure 13.9
Multiplication of Bacteriophages (Lytic Cycle)
APBMR - Bacteriophage Lytic multiplication cycle
• Attachment
Phage attaches by tail fibers to
host cell
• Penetration
Phage lysozyme opens cell wall,
tail sheath contracts to force tail
core and DNA into cell
• Biosynthesis
Production of phage DNA
and proteins
• Maturation
Assembly of phage particles
• Release
Phage lysozyme breaks cell wall
Bacterial
cell wall
Bacterial
chromosome
Capsid
DNA
Capsid
Sheath
Tail fiber
1 Attachment:
Phage attaches
to host cell.
Base plate
Pin
Cell wall
Tail
Plasma membrane
2 Penetration:
Phage penetrates
host cell and
injects its DNA.
Sheath contracted
Tail core
3 Biosynthesis: DNA
is copied and
capsomeres are
produced
Figure 13.10.1
Tail
DNA
4 Maturation:
Viral components
are assembled into
virions.
Capsid
5 Release:
Host cell lyses and
new virions are
released.
Tail fibers
Figure 13.10.2
One-step Growth Curve
Burst size and Burst time during Eclipse Period
Eclipse period no
whole virions
Figure 13.11
Lytic cycle versus Lysogenic
• Lytic cycle
Phage causes lysis and death of
host cell
• Lysogenic cycle
Prophage DNA incorporated in
host DNA
The Lysogenic Cycle
Figure 13.12
Specialized Transduction
Prophage
gal gene
Bacterial DNA
1 Prophage exists in galactose-using host
(containing the gal gene).
Galactose-positive
donor cell
gal gene
2 Phage genome excises, carrying
with it the adjacent gal gene from
the host.
gal gene
3 Phage matures and cell lyses, releasing
phage carrying gal gene.
4 Phage infects a cell that cannot utilize
galactose (lacking gal gene).
Galactose-negative
recipient cell
5 Along with the prophage, the bacterial gal
gene becomes integrated into the new
host’s DNA.
6 Lysogenic cell can now metabolize
galactose.
Galactose-positive recombinant cell
Figure 13.13
Multiplication of Animal viruses
Same as bacteriophage except must uncoat
• Attachment
Viruses attaches to cell membrane
• Penetration
By endocytosis or fusion
• Uncoating
By viral or host enzymes
• Biosynthesis
Production of nucleic acid and proteins
• Maturation
Nucleic acid and capsid proteins
assemble
• Release
By budding (enveloped viruses) or
rupture