Transcript Foundations in Microbiology

PowerPoint to accompany
Foundations
in
Microbiology
Fifth Edition
Talaro
Chapter
6
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
An Introduction to the Viruses
Chapter 6
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3
Size of viruses
Smallest infectious agents- must use electron microscope
4
Purified poliovirus crystals
Light microscope1200X
Purified poliovirus crystals
Electron micrograph 150,000X
Shows individual viruses
Naming viruses
• No taxa above Family (no kingdom,
phylum, etc)
• 19 families of animal viruses
• Family name ends in -viridae ,
Herpesviridae
• Genus name ends in -virus, Simplexvirus
• Herpes simplex virus I (HSV-I)
•
•
•
•
Family – Herpesviridae
Genus – Varicellovirus
Common name – chickenpox virus
Disease - chickenpox
Standardized species names not widely accepted
Family Characteristics include: type of capsid, nucleic acid,
presence and type of envelope, overall viral size, and area of host
cell in which the virus multiplies
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Table 6.2a
Capsids
• All viruses have capsids- protein coats that
enclose & protect their nucleic acid
• Each capsid is constructed from identical
subunits called capsomers made of protein
• Capsomers spontaneously self-assemble
into finished capsids of 2 types:
– helical
– icosahedral
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Naked
Enveloped
Helical
Nucleocapsid=capsid and nucleic acid
Enveloped viruses are mostly animal viruses.
When a virus is released from a cell it takes a bit of membrane with
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them to form an envelope
Assembly of helical nucleocapsids
Fig. 6.5
Naked helical virus
Enveloped helical virus
Influenza virus
Tobacco mosaic virus
Naked
Icosahedral
Enveloped
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Icosahedral
• 20-sided with 12 corners
• Vary in the number of
capsomers
• Each capsomer may be
made of 1 or several
proteins
• Some are enveloped
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Fig. 6.7
Adenovirus model
Fig. 6.8
Naked icosahedral virus
Rotavirus
Enveloped icosahedral virus
Herpes simplex virus, 300,000X
Complex
or Atypical
Polyhedral
Vaccinia virus,
a poxvirus
Bacteriophage
Bacteriophage
Poxvirus, large DNA virus
Mumps virus
herpesvirus
adenovirus
Fig. 6.10
Flexible-tailed bacteriophage
rhabdovirus
HIV virus
Helical nucleocapsid
See page 165, fig 6.10
Icosahedral
nucleocapsid
papillomavirus
Icosahedral capsid
6 steps in phage replication in Bacteria
1. adsorption – binding of virus to specific
molecule on host cell
2. penetration –genome enters host cell
3. replication – viral components produced
4. assembly - viral components assembled
5. maturation – completion of viral formation
6. release – viruses leave cell to infect other cells
Fig 6.11 22
penetration
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Bacteriophage assembly line
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Fig. 6.14
A bacteria cell rupturing with bacteriaphage: spent phage lined up
along cell wall
• Not all bacteriophages lyse cells
• Temperate phages insert their viral DNA
into the host chromosome & viral
replication stops there until some later time.
• Lysogeny- bacterial chromosome carries
phage DNA
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• Induction- the prophage will be activat3ed
and return to the lytic cycle
• Phages can serve as transporters of bacterial
genes from one bacteria to anotherTransduction
• Can transfer genes for toxin production and
drug resistance
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Host range
• Spectrum of cells (host) a virus can infect
– cell has to have a specific structure (receptor) on its surface for
viral attachment
– cell has to contain all of the enzymes and materials needed to
produce new virions
• May be one species or many
– HIV (only humans) vs rabies (many animals)
• May be one tissue or many within a host
– Hepatitis (human liver) vs polio (primates’ intestinal & nerve
cells)
– Rabies infects nerve cells of most mammals
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Animal virus replication
1.
2.
3.
4.
5.
adsorption
penetration/uncoating of genome
duplication/synthesis
assembly
release
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RNA replication in
cytoplasm and DNA
replicated in the
nucleus
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adsorption
Enveloped coronavirus- the
configuration of the spike has a
complementary fit for cell receptors
Adenovirus-naked capsid that
adheres to its host cell by surface
molecules on its capsid into the
receptors
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penetration
Endocytosis and uncoating
Fusion of cell membrane with the viral envelope
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Release by budding
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Parainfluenza virus budding and picking up envelope and spikes
HIV virus leave the host T
cell by budding off its
surface
Differences between phage and
animal virus replication
1. Animal virus replication is more complex than
phage replication because host cells are more
complex.
2. Animal viruses cannot inject their DNA must be
engulfed or fuse with the membrane.
3. Viral persistence-lysogeny for phage, latency for
animal viruses
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Table 6.3
Cytopathic effects (CPEs)- virusinduced damage to cells
1.
2.
3.
4.
5.
6.
7.
changes in size & shape
cytoplasmic inclusion bodies appear
nuclear inclusion bodies
cells fuse to form multinucleated cells-syncytia
cell lysis
alter DNA
transform cells into cancerous cells-oncogenic,
capacity to divide for an indefinite period
Oncovirus-DNA virus, papilloma virus, genital warts
associated with cervical cancer
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Cytopathic changes in cells
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How do we grow viruses?
Obligate intracellular parasites
require appropriate cells to replicate.
Growing animal viruses
1. In vivo- live animals
2. bird embryos – chicken, duck; intact, selfsupporting unit, sterile, self-nourished
3. In vitro- cell culture-makes it possibloe to
propagate most viruses (sterile conditions,
special media with correct nutrients) pg
176
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Fig. 6.22
Technician fertilizing chicken eggs with influenza virus, first
step in preparing vaccines
Monolayer of cells
Plaques show signs of
infection
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Fig. 6.23a
Petri dish of E. coli bacteria show macroscopic plaques
Other noncellular infectious agents
1. prions - misfolded proteins, contain no nucleic acid
– cause spongiform encephalopathies – holes in the brain
– common in animals
•
•
•
scrapie in sheep & goats
bovine spongiform encephalopathies (BSE), aka mad cow disease
humans – Creutzfeldt-Jakob Disease
2. viroids - short pieces of RNA, no protein coat
– only been identified in plants, so far
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Diagnosis of viral diseases
• More difficult than other agents
• Consider overall clinical picture
• Take appropriate sample
– Infect cell culture- look for characteristic
cytopathic effects
– Screen for parts of the virus
– Screen for immune response to virus
(antibodies)
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•
•
•
•
Cytopathic changes
Immunofluorescence techniques
Electron microscope
Screen samples for presence of indicator
molecules (antigens) with polymerace chain
reaction (PCR)
• Certain infections require cell culture, embryos, or
animals for definitive diagnosis
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Diagnosis Fig. 6.24a
Diagnosis