Viruses are submicroscopic infectious agents that can only replicate inside living host cells. They contain nucleic acid surrounded by a protein coat and have no cell structure of their own. Viruses range in size from 20-250nm and are classified based on their nucleic acid type and composition. They are obligate intracellular parasites and depend on host cell machinery for replication. Viruses are cultivated using methods like cell culture, organ culture, and animal inoculation to study their growth and properties. Viral replication involves adsorption, penetration, uncoating, biosynthesis, assembly, and release of new virus particles that infect other host cells.

1. General characteristics of
viruses
Anju Rana

2. Sub microscopic entity consisting of a single nucleic acid
surrounded by a protein coat and capable of replication
only within the living cells of bacteria, animals or plants.
Definition

3. History: In 1892, Dmitri Ivanovsky from Russia discovered viral
particles and that too long before the discovery of electron microscope.
His discovery paved the way to modern field of microbiology known as
virology.
Martinus Beijerinck confirmed the findings of Ivanovsky and used the
word virus to describe this new agent of disease.

4. Characteristics of virus
 Viruses have an inner core of nucleic acid surrounded by protein
coat known as an envelope
 Most viruses range in sizes from 20 – 250 nm
 Viruses are inert (nucleoprotein ) filterable Agents
 Viruses are obligate intracellular parasites
 Virus occupy a space in between living and non-living, because
they are crystallisable and non-living outside the body of host.
 Viruses depend fully on the host’s cell machinery to continue their
life – metabolically inefficient.
 They are responsible for a number of dreadful diseases in human
and plants.

6. In 1971, Wildy has put efforts to introduce scientific classification of
viruses. The criteria that help in grouping the animal viruses are:
• The type of nucleic acid
• Chemical composition of the virus particle
• Susceptibility to physical and chemical agents
• Size
• Design and construction of the virus
• Antigenic characters

7. Properties Bacteria Viruses
Size 1000 nm
Bacterial filters can hold
them back
20-300 nm
Easily pass through
bacterial filters
Visible…. Under light microscope Electron microscope is
required
Living/non-living Living Living only inside the
host cell
Can be crystallised
Cell wall Peptidoglycan or
lipopolysaccharide
Devoid of cell wall.
Protein coat may be
present.
Cellular
arrangement and
organisation
Unicellular or branched
chains
Cellular organisation is
present
No cellular
arrangement
Absent
Nucleic acid
Ribosomes
Primitive type of RNA and
DNA lies freely in
cytoplasm
Present
Either DNA or RNA
enclosed in protein coat
Absent
Multiplication Binary fission or primitive
sexual or asexual form
Capable of multiplying on
their own
Complicated way of
multiplication
Need host’s machinery
to multiply
Exists as… Saprophytes, parasites or
commensals
Essentially exist as
intracellular parasites

8. Transmission Transmitted through various
routes of infections like air,
food, water etc.
Can be easily
transmitted from
infected host to a
healthy person
Importance Bacteria are economically
and industrially important to
man-kind
Viruses are pathogenic.
Bacteriophages,
virions, prions are
useful in
biotechnological
studies.
Treatment Antibiotics prove beneficial
in treatment
Do not respond to
antibiotics
Examples Streptococcus, Closteridium,
Corynebacterium etc.
AIDS virus,
chickenpox virus,
measles, mumps virus
Disease Localised to systemic
infections
Systemic infection
mostly

9. Structure
Viruses exists in two states – extracellular and intracellular.
Virion is the extracellular state and a protein coat surrounds the nucleic acid. Viruses contain
either DNA or RNA which could be single or double stranded. The capsid with nucleic acid is
called as nucleocapsid. Some viruses have phospholipid envelope.
The capsid is composed of capsomeres which are made of protein subunits.
The envelope is either lipid derived from host cell membrane or viral
glycoprotein in nature. The envelope has glycoprotein subunits in the form of
projecting spikes known as peplomers. The influenza virus has two types of
peplomers: neuraminidase and haemagglutinins. The chemical, biological and
antigenic properties of viruses are due to envelop.

10. Intracellular state of the virus exists as only nucleic acid
devoid of capsid. The viruses may be helical, icosahedral or
spherical in shape. Some viruses may be complex in
structure.

11. Cultivation of viruses
Some of the methods used to grow viruses are:
Animal inoculation
Embryonated eggs
Tissue or cell culture

12. Main requirements for cell culture are:
 An artificial supply of amino acids, carbohydrates, vitamins, minerals,
hormones, growth factors
 Oxygen or carbon dioxide gas
 Equipment to regulate pH, temperature and osmotic pressure of the growth
environment.
 Storage facility to store cells at a temperature of -130oC with the help of
glycerol as protective agent - cryopreservation.

13. Organ culture: Minced parts of the organs can be maintained in laboratories by
keeping them in preservatives that could maintain their original architecture and
function. The organ culture technique is useful in cultivation of viruses that are
highly specific and prefer certain organ to grow.
Primary cell culture: These cells are at a stage when freshly isolated from the
tissue and cultured to form a monolayer until cell division stops due to contact
inhibition, also known as confluence. After this the cells need to be subcultured to a
new container having fresh medium so that further growth could occur. Examples
are monkey kidney cells, chick embryo cell cultures and human embryonic kidney
cell cultures.

14. The primary cell cultures are of two types:
 Adherent cells, need anchorage to adhere for growth. The adherent type
of cells are mostly derived from tissues of the organs like, kidneys.
 Suspension cells, are anchorage independent and remain suspended in
the medium. The suspension cells are derived from cells of blood
system like lymphocytes.

15. Cell lines: Cell lines are the cells left in the primary culture vessels after
subculture. These are also known as subclones. The life span of the cells
derived from primary subculture is limited and in later stages, cells with
highest capability to grow predominate. This results in genotypic as well as
phenotypic variation in the cells.
 Cell strain: If a population of cell line is selected from the culture because
of some positively required characters for studies and are further grown in
artificial medium, these cell lines are called as cell strains. The genotypical
changes are the basis of selection from parent cell lines.
 Finite cell lines: Normally the cells divide for a limited number of times –
usually 5-10 divisions. These cells are called as finite cell lines and their
growth is limited due to contact inhibition, availability of nutrients in
medium. These cell lines are anchorage dependent, and grow slowly
(doubles in 24-96 hours).

16.  Continuous cell lines: Sometimes the cells have capability to
divide infinitely and grow as continuous cell lines. This
happens due to spontaneous mutations or virtual addition of
a chemical in the medium in which cells are growing.
 The transformation of cell lines under laboratory conditions
gives rise to continuous cell lines. They acquire quality of
anchorage independence, and become free of confluence.
Hela cell lines that are derived from carcinoma cells of
cervix are an example of this type of cell lines. Growth rate
is increased (doubles in 12-24 hours).

17. Secondary cell cultures: When an inoculum
from primary cell culture is subcultured it is
termed as secondary cell culture. The
secondary sub cultures are done to refresh
the growth medium and provide new space
for the cells to grow.

18. Monolayer cultures: They are the cell lines that are only
one cell in thickness and cover the bottom of the vessel as
a monolayer.
Suspension culture: The cells which are non-adherent
type and grow in suspension are easy to harvest, such
cultures are called as suspension, e.g. cells of leukaemia.
Explant cultures: Fragments of minced tissues are
embedded in plasma clots and grown as explants. Such
explant cultures are used for isolation of adenoviruses.

19. • Detection of viral growth in lab
Cytopathic effect on right side contd……

20. • Transformation: When tumour forming viruses grow in a medium they induce
cell transformation and loss of contact inhibition. In this case the growth of
virus is in the form of piles as is seen in herpesviruses, papovaviruses and
retroviruses.
• Interference: The growth of a virus can be checked by observing the competitive
interference from another virus growing in same medium. A known dose of
cytopathogenic virus is introduced into the culture. Original virus replicating in
the cells will prevent the growth of cytopathogenic virus and no cytopathic
effect is seen in the cell culture.
• Haemadsorption: The red blood cells adhere to the cells infected with viruses.
Therefore this property can be used to detect viral growth.
• Flourescent antibody technique: Viral growth cab detected by direct as well as
indirect fluorescent antibody technique. The method is sensitive and fast.
• Electron microscope can be utilised to see the viral growth in infected cells.
• Enzymes: The enzymes like reverse transcriptase can be analysed and this gives
an idea of viral growth indirectly.

21. Replication of viruses

22. The whole process of viral replication can be divided into following phases:
• Adsorption: The viruses adhere to a host cell. For this it comes in contact with
cell by random collision and gets attached to the cell with the help of specific
receptors on the surface of host cell. The viruses have ligands that recognise
these receptors.
• Penetration: The viruses enter the cells by different mechanisms depending on
the type of host cell. In case of bacteria, the bacteriophage is unable to
penetrate it. The reason lies in the special composition of bacterial cell wall
and its rigidity. The virus introduces only its nucleic acid into the bacterial cell.
• Uncoating: Under this process, the outer layer and capsid are get rid of by the
virus. In uncoating, the lysosomal enzymes secreted by host cell and
proteolytic enzymes secreted by the virus help. In the end the viral nucleic acid
is released into the cell.
• Biosynthesis: Viral nucleic acid and capsid are synthesised during this phase.
Enzymes and regulatory proteins needed in the various stages of the
replication are synthesised. The regulatory proteins shut down the normal
metabolism of the cell and direct it to produce the viral components.

23. Contd..
• Assembly: After the replication of viral proteins, nucleic acids and other
parts; assembly of components occurs in the cytoplasm or nucleus of the
host cell. Picornaviruses and poxviruses are assembled in nucleus whereas
herpesviruses and adenoviruses are assembled in cytoplasm.
• Release/egress: The non-enveloped viruses are released by lysing host
cell as is done by bacteriophages; whereas the enveloped viruses are
released by reverse phagocytosis or budding like myxoviruses. The outer
covering of the virus is derived from the host cell membrane while
egression. A large number of virus particles are released from one
infected cell.