2. Characteristics of Viruses
Obligate intracellular
Possess either DNA or RNA but never both.
Filterable- They are smaller than bacteria, passed trough
bacterial filters.
Can not be grown on artificial cell free media
Multiply by a complex method, not by binary fission as
seen in bacteria.
Do not have a proper cellular organization.
Do not have cell wall or cell membrane or cellular
organelles including ribosomes.
Lack enzymes necessary for protein and nucleic acid
synthesis.
Not susceptible to antibiotics.
4. MORPHOLOGY OF VIRUS
• Viruses possess a
Nucleic acid-
• DNA or RNA
• single or double stranded, circular or linear, segmented
or unsegmented
• surrounded by a protein coat called -----
Capsid
• composed of polypeptides called- capsomeres
• Have antigenic property and specific for each virus
• Specific arrangement of capsomeres called- Symmetry
– Icosahedral (cubical) symmetry
– Helical symmetry
– Complex symmetry
– Nucleic acid & capsid together k/s Nucleocapsid
5. MORPHOLOGY OF VIRUS
• Some viruses also have an outer envelope
surrounding the nucleocapsid
Lipoprorein in nature (Lipid+Protein)
Lipid part- derived from the host cell membrane
Protein part- virus coded, made up of peplomers,
which project as spikes on the surface of the
envelope-(e.g. Influenza viruses have hemagglutinin & neuraminidase
peplomers)
Peplomers are antigenic, bind to specific receptors on
the host cells.
Most Viruses are Enveloped Except
• DNA viruses- Parvovirus, Adenovirus & Papovavirus
• RNA viruses- Picornavirus, Hepatitis A virus & Hepatitis E
virus.
6. MORPHOLOGY OF VIRUS
• Size of the Viruses
Extremely small, vary from 20-400 nm in size.
Smallest- Parvovirus (20 nm)
Largest- Poxvirus (400nm).
7. MORPHOLOGY OF VIRUS
• Shapes of the Viruses
Most are roughly spherical, Except -----
• Rabies virus: Bullet shaped
• Rotavirus- wheel shaped
• Coronavirus-petal shaped peplomers
• Ebolavirus: Filamentous shaped
• Poxvirus: Brick shaped
• Adenovirus: Space vehicle shaped
• Astrovirus-star shaped peplomers
11. •
VIRAL REPLICATION
Viruses invade cells and use the
host cell's machinery to synthesize
more of their own macromolecules.
• Once inside the host virus will either go
into a
1. Lytic Cycle-
destroying the host cell during
reproduction.
or
• It will go into a
2. Lysogenic Cycle –
a parasitic type of partnership
with the cell
12. 10
Virus injects its
DNA
Virus DNA
commands host cell
to make new viral
parts
New viral parts
assembled
Cell lyses (breaks
apart) and new
viruses are
released
Virus attaches to
host cell.
LYTIC CYCLE
14. 13
• Does not begin immediately
• Viral DNA (called a prophage) attaches
to the host cell’s chromosomes – lies
dormant.
• The virus DNA integrates with the host DNA
and the host’s cell helps create more virus
DNA.
LYSOGENIC CYCLE
15.
16. Morphological Changes in the Host Cells
• Damage to host cell chromosomes: -
with HSV or adenovirus infection- Formation of chromatin rings
surrounding the nuclear membrane in host cells.
• Formation of inclusion body-
aggregates o f virions or viral proteins and other products of viral
replication.
characteristic of specific viral infections.
demonstrated in virus infected cells under the light microscope.
have distinct size, shape, location and staining properties to the
host cell.
lntracytoplasmic indusion bodies:
Acidophilic, seen as pink structures when stained with Giemsa or eosin
methylene blue stains (e.g. most pox.viruses and rabies
lntranuclear inclusion bodies:
basophilic in nature. Cowdry (1934) had classfied them into -----
– Cowdry type A inclusions: They are variable in size and have granular
appearance.
– Cowdry type B inclusions: They a re more circumscribed and multiple .
18. LABORATORY DIAGNOSIS
• Direct Demonstration of Virus
Electron microscopy
lmmunoelectron microscopy
Fluorescent microscopy
Light microscopy
• Histopathological staining: To demonstrate inclusion bodies
• lmmunoperoxidase staining
• Detection of viral antigens
ELISA, direct IF, ICT, flow through assays.
• Detection of the Specific Antibodies
Conventional techniques- HAI, neutralization test and CFT.
Newer diagnostic formats- ELISA, ICT, flow through assays.
• Molecular Methods to Detect Viral Genes
Nucleic acid probe-for detection of DNA or RNA by hybridization
PCR- for DNA detection by amplification
RT-PCR- for RNA detection
Real time PCR- for DNA quantification
Real time RT-PCR- for RNA quantification
• Isolation of Virus by
Animal inoculation
Embryonated egg inoculation
Tissue cultures: Organ c ulture, explant culture, cell line culture (primar y, secondary and continuous
cell lines).
22. Life cycle of bacteriophage
• Lytic cycle
• Lysogenic cycles
23. Bacteriophages
• Phages play an important role in the
transmission of genetic information between
bacteria by the process of transduction.
• They can carry antibiotic resistance genes or
virulence genes.
• Bacteria can be typed by phage typing
methods for epidemiological purposes
34. Poxviruses
• Large, being 250-300 nm by 300-350 nm in
size, and just visible under a light microscope
• Genus Orthopoxvirus includes the viruses
causing cowpox, vaccinia and variola
• The variola virus causes smallpox, eradicated
now.
• Poxvirus causing molluscum contagiosum.
35. • Long question
Laboratory diagnosis of viral infections.
• Short notes -----
1. Replication of viruses
2. Methods of detecting viral growth in cell
cultures
3. Inclusion bodies
36. MCQ
1. Which of the following viruses is/are enveloped?
a. Poliovirus
b. Adenovirus
c. Herpesvirus
d. Parvovirus 8 19
2. All of the following are RNA viruses except:
a. Human adenoviruses
b. Enterovirus
c. Coxsackievirus
d. Hepatitis A virus
3. All of the following viruses are transmitted by respiratory
route except:
a. Influenza virus
b. Rotavirus
c. Respiratory syncytial virus
d. Rhinovirus
37. MCQ
4. All of the following are intracytoplasmIC
inclusion bodies except
a. Negri bodies
b. Molluscum bodies
c. Cowdry type A inclusions
d. Guarnieri bodies
5. Which of the following vaccine is a killed
vaccine?
– a. Mumps vaccine
– b. Measles vaccine
– c. Rubella vaccine
– d. Semple vaccine
