viruses, general Flashcards
do viruses have their own metabolism?
• Lack independent metabolism (can reproduce only after entering
a living host cell and using that cell’s biochemical machinery)
an “obligate intracellular parasite”
What is a virus?
“ a subcellular agent consisting of a nucleic acid core surrounded by a protein coat that must use the metabolic machinery of a living host to produce new virus particles”
What type of organisms can act as a host for viruses?
. Host range (spectrum of organisms able to act as host)
- animal, plant, bacteria, eucaryotic microbes
what type of nucleic acid do viruses have?
- DNA or RNA; single or double stranded
- a virus will have only one type of NA (DNA or RNA, never both)
what type of surface “coat” surrounding nucleic acid core?
- Naked protein coat (“capsid”) + glycoproteins (“spikes”)
- Enveloped capsid + lipid membrane + spikes
How does the taxonomy of viruses work?
takes into account the host, covering (naked or enveloped) and then dna or rna and double strand or single strand.
What are the events in the life cycle of a virus?
Events in the “Life Cycle” of a Virus
Outside of a host cell, viruses are inert and cannot reproduce To reproduce, all viruses undergo same basic life cycle: 1. Adsorption - virus attaches to host cell 2. Penetration / Uncoating - virus enters host cell and its nucleic acid is released from the capsid 3. Gene Expression - viral genes expressed to produce more viral NA and viral proteins 4. Assembly - new intact viruses assemble inside host cell 5. Release - virus particles exit host cell
What happens during adsorption of a DNA to a cell
- Adsorption
Requires receptors on eucaryotic
cell membrane to which viral
“spikes” can attach
Spike + receptor interaction partly
determines tissue specificity and
host range for a virus
(only cells carrying the corresponding
receptor can be infected)
Potential step at which infection
can be blocked
- drugs, vaccines, etc.
What happens during penetration/uncoating of the virus?
- Penetration / Uncoating Adsorption triggers a process that results in virus entry and
eventual “uncoating” of nucleic acid
“Endocytosis” (naked) vs. “Fusion” (enveloped) mechanisms
What happens during gene expression and assembly of the virus?
- Gene Expression
- Replication of viral NA and conversion of viral genetic info. into
new viral proteins - Often requires enzymes from the host cell
- Most common point of intervention by antiviral agents
4. Assembly - Production of new, intact virus particles inside a host cells by
combining viral capsid proteins, NA, etc. - Poorly understood mechanism
some spontaneous assembly?
requirement for host cell enzymes?
- Replication of viral NA and conversion of viral genetic info. into
What is the release of the virus like?
- Release
a) Naked Viruses
- virus particles accumulate in cytoplasm
- cell dies, ruptures, & releases virus
b) Enveloped Viruses
- Envelope acquired as virus exits host
(part of viral envelope is of host origin)
- Virus capsid “buds” out through membrane
host cell may survive
- Possible continual virus production and
release over time (“shedding”)
because of this there will be markers of the host on the virus if it envelopes back out of the cell.
What are the possible outcomes of viral infections?
Possible Outcomes of Viral Infections
- Lytic (Acute) Infection (eg. Rhinovirus - common cold)
Host cell is killed as mature virus released - Persistent (Chronic) Infection (eg. Hepatitis B, C)
Virus continually shed for long periods, but host cell survives Infectious virus particles constantly present in patient - Latent Infection (eg. Herpes simplex, Varicella zoster)
Viral genome integrates into host cell chromosome
(replicates with chromosome, but little gene expression) Net effect = no mature infectious virus produced or released But: Pathway to virus assembly, release, and host cell lysis
may be triggered at some future time
(How? Immune system stress? Other?)4. Host Cell Transformation
Formation of cancerous host cells by “tumor” viruses
(ie. viral infection results in uncontrolled host cell growth) Est. 15% of all human cancers may have a viral cause
eg. HTLV-1 T-cell leukemia
Human Papilloma virus cervical cancers Mechanisms of transformation varies:eg. Insertion of a viral “oncogene” into the host cell chromosome interferes with the regulation of host genes eg. Virus-encoded proteins destroy host proteins which normally function to regulate host cell growth (eg. inactivation of host p53 tumor-suppressor protein by HPV)
What are the problems associated with anti-viral antigens?
Anti-viral Agents
Problem: Viral life cycle is closely linked to host cell processes
(hard to target one without damaging the other)
Relatively few agents; None are truly “broad-spectrum”
(most are specific for a certain virus or groups of viruses)
All current anti-virals act at some point during viral life cycle
- Do not “kill” pre-existing virus (outside of a host cell)
- Gene expression step is most common target
- No activity against viruses in the latent state
Many viruses are not treatable with any type of anti-viral agent
(supportive therapy only)
What is the problem with laboratory diagnosis of viral infections?
Problems:
Symptoms of some viral diseases mimic other microbial diseases
- eg. viral sore throat vs. bacterial sore throat
Different types of viruses can produce very similar symptoms
Diagnostic methods based on culture (growth) are more difficult
for viruses than for bacteria
- viruses require a living host cell in order to “grow”
What are some diagnostic approaches to viruses?
- Molecular (PCR) tests Direct detection of viral DNA or RNA in patient’s specimen
- Rapid, sensitive, highly specific- Serology
- A.K.A. “ELISA” tests (enzyme-linked immunosorbent assay)- Difficult to distinguish current vs. past infection (antibodies persist)
- False negative results if blood is collected during “window” period (time period between first contact with a virus & when the immune system begins to produce antibodies)
- Virus Culture (Growth)
Tissue Culture Cells (human or animal tissue cells grown in vitro in
an artificial media)
Add patient specimen to TC cells
and allow virus to infect cells
Look microscopically for a
“cytopathic” effect on cells:
ie. Virus growth in cells causes
changes to cell morphology:
- Cell lysis, rounding, or fusion
- Inclusions form inside cells
Slow (2 – 7 days)
Many viruses cannot be
grown in vitro
- Direct Fluorescent Antibody” (DFA) Tests
Look for virus-infected cells in the host - Collect patient specimen likely to contain infected cells - Add antibodies (tagged with a fluorescent dye) that recognize viral proteins or structures (eg. spike glycoprotein) - Examine microscopically for binding of fluorescent Ab to infected cell Rapid (2 – 3 hours) Proper specimen collection is critical (need lots of infected cells)
What else about serology?
Compared to antigen or nucleic acid detection techniques, serology is slower, not as good for diagnosing acute infection, for two reasons:
- Specific antibody responses develop to a detectable level usually after several days of active infection. This window period may last for weeks.
- If antibodies are already present and IgM is unavailable, convalescent titers must be obtained and tested with the acute blood sample. This takes weeks and gives only a retrospective diagnosis.
What else about viral culture?
Although results take days to finalize this may be an important test when:
- We need to know whether the virus is viable. Antigen and NA tests detect only the presence of the antigen or DNA/RNA.
- There is treatment available and resistance is a possibility. Live virus is required for susceptibility testing.
How can viruses cause symptoms of disease in humans?
A. Viruses can cause symptoms of disease in a number of ways
Eg. Virus replication leads to host cell lysis
localized tissue damage immune system response (inflammation, etc.)
Eg. Viral glycoproteins inserted into host cell membrane
host cell surface altered triggers an immune response which kills cell
Note:
1. Many symptoms of a viral infection are due to the body’s own immune system response to the infection
2. Tissue damage caused by viruses often makes the host more susceptibility to infection by other microbes
eg. secondary bacterial infections
What about viruses spreading?
B. Viruses often do not stay
at site of first contact
with host
Spread via blood or
lymphatic system
May infect cells at other
sites as long as suitable
cell receptors present
where can viruses infect?
C. Viruses can infect
almost every
body organ
- Often, different
classes of viruses
may infect same
organ or tissue
eg. Nose / throat:
- Adenovirus
- Influenzae
- Rhinovirus
and others
