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Chapter 13
Viruses, Viroids, and Prions
Part 1
General Characteristics of Viruses
• Very small in size
– Need an electron microscope to visualize and
determine viral sizes
– Passes through microbial filters (filterable
agent)
– Range from 20 - 100 nm in length
General Characteristics of Viruses
• Inert outside living host cells
• Obligatory intracellular parasites
– Viral nucleic acids only active inside a living
host cell
– Take over host’s metabolic machinery to multiply
itself
• Not all of them cause disease
– e.g. TT virus (TTV) discovered in 1997 is a
harmless symbiont (found in 2% of healthy
humans)
General Characteristics of Viruses
• Contain either DNA or RNA, not both
– Can be single-stranded or double-stranded
• Contain a protein coat that surrounds DNA
or RNA
– Some are enclosed by an envelope (composed
of lipids, proteins, and carbohydrate)
• Multiply Inside living cells by using the
host’s synthesizing machinery
General Characteristics of Viruses
• Directs synthesis of specialized structures
that can transfer the viral nucleic acid to
other cells
• Hard to treat
– Most antiviral drugs that would interfere with
viral multiplication would also interfere with
the functioning of the host cell
General Characteristics of Viruses
• Host range: spectrum of host cells the virus
can infect
• Most viruses infect only specific types of
cells in one host species (species specific)
• Host range is determined by specific host
attachment sites (viral receptors) and the
availability of cellular factors within the
(potential) host
General Characteristics of Viruses
• Host receptor site for bacteriophage (phage)
– part of the cell wall (bacterial), or sometimes
part of the fimbriae or flagella
• Host receptor site for animal viruses
– On the plasma membranes
• Viruses are classified by differences in the
structures of their protein coat
Viral Structure
• Virion: a complete, fully developed,
infectious viral particle composed of nucleic
acid and surrounded by a protein coat
– Vehicle of transmission from one host cell to
another
– Structures of protein coat used for viral
classification
Viral Structure
• Viral nucleic acid (either DNA or RNA)
– Can be single-stranded or double-stranded
– Can be linear or circular
– Can be in several separate segments (e.g.
influenza virus)
• Capsid: the protein coat of a virus that
surrounds the nucleic acid
– Each capsid composed of protein subunit
(capsomeres)
Viral Structure
– Arrangement of capsomeres characteristic of a
particular type of virus
– Structure of capsid determined by the viral
nucleic acid
– In a nonenveloped virus capsid protects nucleic
acid from nuclease enzymes in biological fluids
and promotes the virus’s attachment to
susceptible host cells
Viral Structure
• Envelope: an outer covering surrounding
the capsid of some viruses
– combination of lipid, proteins, and
carbohydrates
– Some animal virus take host cell’s plasma
membrane as envelope when they are released
from a host cell by an extrusion process
– Some envelopes may be covered by spikes
(carbohydrate-protein complexes used for
attachment to a host)
Viral Structure
• Mutation of viral surface proteins allows
viruses to escape antibodies made in an
infected host
– Cause reinfection with the same virus
– e.g. Influenzavirus (changes in its spikes)
General morphology
• Based on capsid architecture, viruses may
be classified into several different
morphological types
– Use electron microscopy and X-ray
crsytallography
• Helical viruses
– Resemble long rods; may be rigid or flexible
– e.g. Rabies and Ebola viruses
Helical virus
General morphology
• Polyhedral (many-sided) viruses
– Many animal, plant, and bacterial viruses
– Capsid is in the shape of an icosahedron (a
regular polyhedron with 20 triangular faces and
12 corners)
– e.g. Adenovirus and poliovirus
Polyhedral virus
General morphology
• Enveloped viruses
– Roughly spherical
– enveloped helical or enveloped polyhedral
viruses
– e.g. enveloped helical viruses: Influenzavirus
– e.g. enveloped polyhedral (icosahedral) virus:
herpes simplex virus
Enveloped virus
General morphology
• Complex viruses
– Viruses with complicated structures
– e.g. Bacterial viruses (bacteriophages) and
poxviruses (have several coats around the
nucleic acid)
Complex virus
Taxonomy of Viruses
• Oldest classification based on
symptomatology
• International Committee on the Taxonomy
of Viruses (ICTV) group viruses into
families based on:
– Nucleic acid type
– Strategy for replication
– Morphology
Taxonomy of Viruses
• Virus species: a group of viruses sharing
the same genetic information and ecological
niche (host range)
• Order names end in -ales
• Family names end in -viridae
• Genus names end in -virus
Taxonomy of Viruses
• No specific epithets (species) used for
viruses; use descriptive common names
– Subspecies are designated by a number
• Example
– Herpesviridae>Herpesvirus>Human herpes
virus 1 (HHV 1), HHV 2, HHV 3
– Retroviridae>Lentivirus>Human
immunodeficiency virus 1 (HIV 1), HIV 2
Isolation, Cultivation, and
Identification of Viruses
• Viruses must be grown in living cells
– Viruses that use bacterial cells as host easier to
grow using bacterial cultures than animal or
plant viruses
• Growing Bacteriophages in the Laboratory
– Grow in either suspensions of bacteria in liquid
medie or in bacterial cultures on solid media
– Solid media allows to detect and count viruses
using plaque method
Growing bacteriophages in the lab
• Plaque: a clearing in a bacterial lawn
resulting from lysis by phages
– Each plaque theoretically is formed from a
single virus in the initial viral suspension
• Plaque-forming units (pfu): the
concentration of viral suspensions measured
by the number of plaques
Growing animal viruses in the lab
• In living animals
– Some animal viruses can only be cultured in
living animals (mice, rabbits, and guinea pigs)
– Some human viruses cannot be grown in other
animals, or can be grown but do not cause
disease e.g. HIV 1 can infect Chimpanzees but
show no symptoms of the disease
– Simian AIDS (in green monkey) and feline
AIDS provide a model for studying human
AIDS
Growing animal viruses in the lab
• In Embryonated Eggs
– Inexpensive form of host and fairly convenient
– Virus is injected near the one most appropriate
for its growth
– Viral growth is signaled by the death of the
embryo, or by the formation of typical pocks or
lesions on the egg membranes
– Used to grow viruses for some vaccine (need to
watch out for allergic reaction to egg proteins)
Growing animal viruses in the lab
• In cell cultures (use animal and plant cells)
– Replaced embryonated eggs as the preferred
type of growth medium for many viruses
– More convenient to work with than whole
animals or embryonated eggs
– Primary cell lines: derived from tissue slices,
tend to die out after only a few generations
– Continuous cell lines: transformed cells that can
be maintained through an indefinite number of
generations; also known as immortal cell lines
Cell Culture
Growing animal viruses in the lab
– Some viruses have never been successfully
cultivated in cell culture
– Maintenance of cell culture lines requires
trained technicians
– Look for cytopathic effect (CPE) formed on the
monolayer of cells infected with virus
Cytopathic Effect (CPE)
Viral identification
• Cytopathic effects (CPE) in cell culture
• Serological tests
– Detect antibodies against viruses in patient
– Use antibodies to identify viruses in
neutralization tests, viral hemagglutination, and
Western blot
• Nucleic acids
– RFLPs (restriction fragment length
polymorphisms)
– PCR (West Nile encephalitis outbreak)