Page 241 - Review of Medical Microbiology and Immunology ( PDFDrive )

mebooksfree.com
mebooksfree.com
mebooksfree.com

mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com infected brain tissue. A vacuolated (spongiform) appear- mebooksfree.com
mebooksfree.com
PART III Basic Virology
230
in normal cells is protease-sensitive, whereas the prion
ance is found, without inflammatory cells. Prion proteins
protein in infected cells is protease-resistant, probably
because of the change in conformation.
in infected brain tissue form rod-shaped particles that are
morphologically and histochemically indistinguishable
The observation that the prion protein is the product of
from amyloid, a substance found in the brain tissue of
a normal cellular gene may explain why no immune
response is formed against this protein (i.e., tolerance
individuals with various central nervous system diseases
occurs). Similarly, there is no inflammatory response in
(as well as diseases of other organs).
mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com • Some viruses produce antigenic variants of their surface proteins mebooksfree.com
PEARLS
Virus Size & Structure
that allow the viruses to evade our host defenses. Antibody
• Viruses range in size from that of large proteins (~20 nm) to
against one antigenic variant (serotype) will not neutralize a dif-
that of the smallest cells (~300 nm). Most viruses appear as
ferent serotype. Some viruses have one serotype; others have
spheres or rods in the electron microscope.
multiple serotypes.
• Viruses contain either DNA or RNA, but not both.
• All viruses have a protein coat called a capsid that covers the
Viral Envelope
genome. The capsid is composed of repeating subunits called
capsomers. In some viruses, the capsid is the outer surface, but
in other viruses, the capsid is covered with a lipoprotein enve-
derived from the host cell and proteins encoded by the virus.
lope that becomes the outer surface. The structure composed • The viral envelope consists of a membrane that contains lipid
Typically, the envelope is acquired as the virus exits from the
mebooksfree.com mebooksfree.com mebooksfree.com Prions mebooksfree.com mebooksfree.com mebooksfree.com
of the nucleic acid genome and the capsid proteins is called
cell in a process called budding.
the nucleocapsid.
• Viruses with an envelope are less stable (i.e., they are more eas-
• The repeating subunits of the capsid give the virus a symmetric
ily inactivated) than naked viruses (those without an envelope).
appearance that is useful for classification purposes. Some viral
In general, enveloped viruses are transmitted by direct contact
nucleocapsids have spherical (icosahedral) symmetry,
via blood and body fluids, whereas naked viruses can survive
whereas others have helical symmetry.
longer in the environment and can be transmitted by indirect
means such as the fecal–oral route.
• All human viruses that have a helical nucleocapsid are envel-
oped (i.e., there are no naked helical viruses that infect humans).
Viruses that have an icosahedral nucleocapsid can be either
enveloped or naked.
• Prions are infectious particles composed entirely of protein.
Viral Nucleic Acids
They have no DNA or RNA.
• They cause diseases such as Creutzfeldt-Jakob disease and
• The genome of some viruses is DNA, whereas the genome of
disease. mebooksfree.com
mebooksfree.com
mebooksfree.com mebooksfree.com mebooksfree.com • Prion proteins are encoded by a cellular gene. When these mebooksfree.com
kuru in humans and mad cow disease and scrapie in animals.
others is RNA. These DNA and RNA genomes can be either
These diseases are called transmissible spongiform enceph-
single-stranded or double-stranded.
alopathies. The term spongiform refers to the spongelike
• Some RNA viruses, such as influenza virus and rotavirus, have a
appearance of the brain seen in these diseases. The holes of the
segmented genome (i.e., the genome is in several pieces).
sponge are vacuoles resulting from dead neurons. These dis-
• All viruses have one copy of their genome (haploid) except
eases are described in Chapter 44.
retroviruses, which have two copies (diploid).
proteins are in the normal, alpha-helix configuration, they
Viral Proteins
are nonpathogenic, but when their configuration changes to
• Viral surface proteins mediate attachment to host cell recep-
a beta-pleated sheet, they aggregate into filaments, which
tors. This interaction determines the host specificity and
disrupts neuronal function and results in the symptoms of
organ specificity of the virus.
• The surface proteins are the targets of antibody (i.e., antibody • Prions are highly resistant to inactivation by ultraviolet
mebooksfree.com mebooksfree.com mebooksfree.com • Because they are normal human proteins, they do not elicit mebooksfree.com
mebooksfree.com
mebooksfree.com
bound to these surface proteins prevents the virus from attaching
light, heat, and other inactivating agents. As a result, they
to the cell receptor). This “neutralizes” (inhibits) viral replication.
have been inadvertently transmitted by human growth hor-
mone and neurosurgical instruments.
• Viruses also have internal proteins, some of which are DNA or
RNA polymerases.
an inflammatory response or an antibody response in
• The matrix protein mediates the interaction between the viral
humans.
nucleocapsid proteins and the envelope proteins.
mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com