Lecture 1 and 2 - Introduction Flashcards
What does virus mean in latin?
Slimy liquid or poison
What is a virus?
Virus: submicroscopic filterable (< 0.2 µ) infectious agent multiply only in living cells
What kind of parasites are viruses?
Viruses are obligate parasites that are metabolically inert when they are outside their hosts.
What does a virus rely on?
Viruses rely, to varying extents, on the metabolic processes of their hosts to reproduce themselves.
Do viruses have cellular organelles?
Viruses: Do not have cellular organelles (mitochondria, chloroplasts, Golgi, and endoplasmic reticulum or ribosomes.
What do viruses have in terms of biology?
Viruses: have nucleic acids in the form of DNA or RNA, never both except Retroviruses
What is a virion?
Virions are the mature extracellular infectious viral particle produced by the infected cells under the control of the genetic materials of the virus.
What is a viroid?
Viroid: an infectious entity affecting plants, smaller than a virus and consisting only of nucleic acid without a protein coat.
What is a satellite or defective virus?
Satellite or Defective Viruses: Viruses require a second virus (helper virus) for replication
Ex: Hepatitis delta virus requires the presence of HBV to complete its replication cycle.
What is the living criteria of viruses?
- Control and divert cellular function for their own purposes.
- Can multiply: viral genome, genetic code, new viral components are synthesized and assembled within the infected host cell.
- Viruses may undergo mutation.
What is the nonliving criteria of viruses?
Viruses are not able to reproduce on their own
• Viruses must infect other cells in order to perpetuate their life cycle
• Viruses can be crystallized but no other cells can be crystallized
• Viruses do not have energy generating (ATP) machinery
• Viruses do not have metabolic machinery
• Viruses do not have the necessary machinery for generating nucleic acid or protein
• Viruses do not contain cytoplasm or any cellular organelles
• Viruses may contain membranous envelope derived from the infected host cells during viral exit
What are the reasons for the non life viral theory?
Viruses are considered to be non-living because:
- Are not cells
- Do not grow or respond to their surroundings
- Can not make food, take in food, or produce wastes
- They do not respond to stimuli
What are the reasons for the life viral theory?
have only two fundamental characteristics of the living system
- the presence of nucleic acid as genetic material
- the ability to replicate and produce their own copies.
What is the conclusion found between the living and nonliving viral theories?
Viruses can only multiply in another cell. • Conclusion: viruses are nonliving outside the cells but live inside the cell
What are some characteristics of viruses that they do not share with other microorganisms?
Viruses do not:
- isolate/ propagate on synthetic media
- reproduce with binary fission
- contain both DNA and RNA
- have presence of energy machinery
- have metabolism
- have a size > 300 nm
- cannot be detected by light microscope ( Most viruses can only be seen under EM except in cases of giant viruses as Mimivirus can be seen by light microscope)
- have a cell wall.
Viruses do have IFNs sensitivity.
What determines the mutation rate of viruses?
Mutation rates are primarily determined by the type of viral nucleic acids
• RNA polymerase fidelity and transcriptional proofreading (DNA/RNA) polymerases
What is the criteria of classification of viral order ?
Group of virus families sharing certain common characters
What is criteria of classification of viral family?
A group of genera sharing certain common characters with each others
What is criteria of classification of viral sub-family?
- A group of genera sharing certain common characters
- Used only when it is needed to solve a complex hierarchical problem
What is criteria of classification of a viral genus?
A group of related species that share some significant properties Usually only differ in host range and virulence
What is criteria of classification of a viral species?
Consist of as few words must not consist only of a host name
What are the physical properties used as criteria in viral classification?
Physical properties of viruses
- Type of viral NA
- The strandedness of the NA (ss or ds)
- Viral size
- Symmetry of viruses
- Total number of Capsomeres per virus
- Presence or absence of the viral envelope
What are the chemical properties used as criteria in viral classification?
II. Chemical properties of the viruses
- Heat sensitivity
- pH sensitivity
What are the biological properties used as criteria in viral classification?
III. Biological properties of viruses
- Cytopathology (CP)
- Site of replication (IC or IN)
- Inclusion bodies
- Hemagglutination property
- Antigenic properties
Parts of a virus: What is the inner core? What is inside this region?
Inner core: (Nucleic acids, NA) -Always occupy the central region of the virus
- Usually coiled to packed into a small space
- May be double-stranded (ds) or single-stranded (ss)
What are the double stranded viruses? single stranded?
- ds: most of the DNA viruses; exception; Parvovirdae and Circovirdae (ss)
- ss: most RNA viruses; exception; Reoviridae and Birnavirdae (ds)
- Sometimes segmented: Influenza viruses, Reovirues, and Birnaviruses
Parts of a virus: What is the outer membrane? What is its role/ what does it contain?
Outer membrane: (Viral capsids)
- Surrounds the NA
- composed of repeated subunits called capsomeres
Parts of a virus: What is the additional outer membrane? What is its role/ what does it contain?
-Lipo-protein layer acquired from the cell membrane of the infected cells
-Usually covered with some projections called peplomers
Examples of enveloped viruses: Coronaviridae, Influenza viruses
What do you call non enveloped viruses?
naked viruses.
What are the 3 types of symmetry classifications of viruses?
- Hellical
- Icosahedral
- Complex
What is the capsid? What are capsomeres?
Capsid: the protein shell surrounding the nucleic acid
Capsomeres-: the structural subunits of the capsid
What is the function of viral capsids?
- Protects viral genome
- Serves as an attachment protein (only in the case of naked non enveloped viruses) and bind the virus to susceptible host cells thereby promotes virus entry into cells
- Stimulate the immune response to produce antibodies against the virus
- Facilitates the assembly and packaging of the viral genetic materials
What does it mean for a virus to have helical symmetry?
- Capsomers arranged around the NA in a helical fashion
- Capsomers attached directly to the helix of NA
- Most of the helical viruses are enveloped, and all are RNA viruses
- Examples: TMV
What does it mean for a virus to have icosahedral or cubic symmetry?
- Capsomeres arranged around the NA in a icosahedral fashion
- Capsomers form a regular icosahedron composed of 20 equilateral triangular facets
- Two types: pentagons or hexagons at vertices
- Examples: Adenovirus, Picornavirus, Papovavirus, herpes
What does it mean for a virus to have complex symmetry?
-Mixture of asymmetric NA in an icosahedral fashion in a helical fashion and symmetric structures
Poxvirus has a unique complex structure
- Examples: Poxviruses and Bacteriophages
What kind of virus is this?
Naked icosahedral
What kind of virus is this?
Naked helical
What kind of virus is this?
Enveloped Hellical
What kind of virus is this?
Enveloped helical
What is a viral envelope?
Envelope: the outer shell surrounding the viral nucleocapsid in case of enveloped
viruses
What is the structure of a viral envelope?
• Structure of the viral envelope:
- Lipid bilayer derived from the host cell membrane during the virus exit from the cell
- Glycoproteins
What is the function of the viral envelope?
Functions of viral envelope:
- Protects viral nucleocapsid - Mediates viral attachment and entry into the host cells
- Viral attachment proteins as
- HA in case of influenza viruses
- Spike in case of Coronaviruses
- Mediates viral entry into the host cells
- Fusion protein as in case of the New Castle Diseases virus
What are enveloped viruses sensitive to?
Clinical applications: enveloped viruses are sensitive to lipid solvents and
common disinfectants
Can a virus have more than one type of peplomers? Can they have more than one function?
yes
yes
What are the 2 functions of HN peplomers in paramyxoviruses?
1- Hemagglutination of the RBCs of some animals species
2- Neuraminidase causes elution of the agglutinated RBCs and facilitates the virus entry
What is the function of HA peplomers in influenza viruses?
Hemagglutinates some RBCs
What are other functions of peplomers? Do they have some antigenic properties?
- Some viral peplomers induce fusion of various cells together to form syncytium
- Viral peplomers have some antigenic properties
What is a +Ve sense genome?
Strand of viral RNA that can immediately serve as a template mRNA for protein synthesis during the process of translation.
What is a -Ve sense genome?
Strand of viral RNA that must first be converted into complementary positive sense strands before creating protiens.
What is an ambisense viral genome?
An RNA viral genome that is part of positive and in part of negative polarity.
Both nucleic acid strands encode protiens.
What is the function of viral genetic materials?
- Carries the genetic blueprint for the progeny production
- Codes for the synthesis of viral enzymes required for viral replication.
- Codes for the synthesis of viral structural proteins required for viral assembly
Do all DNA viruses have Ds genomes?
• All DNA viruses have ds genomes except Parvoviridae and circoviridae
Do all RNA viruses have SS genomes?
All RNA viruses have SS genomes except Reoviridae and Birnaviridae
What is the optimal pH for growth and multiplication of viruses?
Neutral
What happens to viruses in extremely acidic/ extremely alkiline ph?
media are deleterious to most viruses
Can any virus survive a pH of 10?
pH 10: is detrimental to most viruses
- Exception: African swine fever virus (ASFV) – stable at pH 4-13
What are the strategies of viral inactivation?
- Denaturing of viral protiens by:
- Phenol derivatives (Lysol) -Ammonium compounds (Roccal) -Sodium hypochlorite (Clorox)/oxidizing agent -Hydrogen peroxide/oxidizing agent
- Iodophors (mixture of iodine and surfactants)- oxidizing agent
- Inactivation of viral protiens
- First: soaking the viral solution in (1 N NaOH)
- Second: autoclaving for 1.5 hrs
• Denaturing both viral nucleic acids and proteins
- Formaldehyde (1-5%), - Glutaraldehyde (2%)
- Ethylene oxide (fumigation) – extremely toxic (must be used in a sealed chamber).
- UV light: cross links the pyrimidine bases (C & T/U)
• Dissociation of viral envelopes:
- 70 % alcohol mixed with water result in lipid disruption and denaturing of proteins
What is some beneficial roles of viruses?
Baculoviruses: modified by genetic engineering to express some other viral proteins for vaccine purposes as well as development of some novel diagnostic assays
- Poxviruses and Adenoviruses used as viral vectors
- Lentiviruses: modified to insert some foreign genes of interest into cells for research purposes and gene therapy
- Bacteriophages: used in the control of some bacterial infections
What are bacteriophages?
- Viruses specifically infect bacteria
- They are obligate intracellular parasites
- Infect various bacterial species, but most extensively investigated phages infect enteric bacteria, E. coli, and Salmonella
- Phages can be DNA or RNA and either double or single-stranded.
What is the structure of a bacteriophage?
• They have a capsid (head), a sheath, and tail fibers, which are contractile. The
capsid has the DNA. The tail fibers attach to the bacterial cell surface and
inject the DNA into the cell.
How do bacteriophages replicate?
The Lytic Cycle: the host bacterial cell is lysed (lytic phages)
The Lysogenic Cycle: host bacterial cell remains intact and alive
the life cycle results in a stable genetic relationship with the host. The phages are called temperate phages
What is phage theory?
Phage therapy is the therapeutic use of lytic bacteriophages to treat bacterial infections
How specific are phages? How is this adventagous?
Most phages are specific to one species of bacteria, and many are only able to lyse
specific strains within a species.
• The limited host range can be advantageous, in principle, as phage therapy results in less harm to the normal body flora and ecology than commonly used antibiotics
How are lytic phages similar to antibiotics?
• Lytic phages are similar to antibiotics in that they have remarkable antibacterial activity • Therapeutic phages have some advantages over antibiotics
phages have been reported to be more effective than antibiotics in treating certain infections in humans and experimentally infected animals.
When has lytic phages been more effective than antibiotics?
- Staph aureus phages treating patients with purulent disease of the lungs
- phages used to reduce e coli on hide sufaces prior to slaughter
True or False: Phages are preferable to antibiotics, because bacteria cannot develop a resistance to phages.
FALSE
Bacteria can also develop resistance to phages
