Viruses- classification, multiplication and cultivation.pptx

The detailed slideshare about viruses, classification of viruses, multiplication of viruses cycles including lytic cycle, lysogenic cycle and cultivation of viruses

1. Viruses: Introduction, classification, replication and cultivation Prepared by: Ms. Aaditi K. Kamble. Assistant Professor (DRA)

2. Viruses can be seen by the electron microscope . 10 to 100 times smaller than most bacteria (20 to 300 nm). They can grow only in animal or plant cells or in microorganisms - obligate intracellular parasites. Viruses largely lack metabolic machinery of their own to generate energy or synthesize proteins . They depend on the host cells to carry out these vital functions. The viral genetic material is either DNA or RNA, but the virus does not have both . Host cells have both DNA and RNA. Nucleic acid is enclosed in a highly specialised protein coat which protects the genetic material when the virus is outside any host cell and serves as a vehicle for entry into another specific host cell. The structurally complete, mature and infectious virus is called the ' virion ’. They multiply by a complex process and not by binary fission. They are unaffected by antibacterial antibiotics . Viral diseases range from minor ailments such as the common cold to highly fatal diseases such as AIDS or yellow fever. Viruses may cause mumps, rabies, influenza, measles or herpex simplex . Viruses

3. Viruses multiply only in cells of particular species and thus they are divided into three main classes: Bacterial viruses ( phagineae ): They have DNA and are called bacteriophages or simply, phages . Plant viruses ( phytophagineae ): They have RNA and infect potato, sugarcane, tobacco, cucurbitis and any other higher plant. Animal viruses ( zoophagineae ): They usually have DNA but may also have RNA and infect man, pigeon, parrot, dog, cow, arthropods etc. They have also been classified as dermotropic (skin-tissue), neurotropic (nerve-tissue), viscerotropic (digestive tract) and pneumotropic (respiratory system) depending upon organs that they infect. Classification has been developed based upon physicochemical properties. summarised as follows: Primary characteristics: Chemical nature of nucleic acid : RNA or DNA, single or double stranded, single or segmented genome, (+) or (-) strand, molecular weight. Structure of virion : Helical, icosahedral or complex, naked or enveloped, complexity, number of capsomers for icosahedral ( polyhederal ) virions, diameter of nucleocapsids for helical viruses. Site of replication : Nucleus or cytoplasm. (B ) Secondary characteristics: Host range: Host species, specific host tissues or cell types. Specific surface structures : e.g. antigenic properties. Mode of transmission : e.g. feces . Classification:

4. MORPHOLOGY OF VIRUSES A virion is a complete, fully developed viral particle composed of nucleic acid surrounded by a coat that protects and serves as a vehicle of transmission from one host cell to another host cell. Viruses are not cellular and therefore do not have a nucleus, cytoplasm or cell membrane . Virus contains a single kind of nucleic acid, either DNA or RNA , which is the genetic material. The nucleic acid of a virus can be single stranded or double-stranded . Depending on the viruses, the nucleic acid can be linear or circular. In some viruses (e.g. influenza), the nucleic acid is in separate segments. The nucleic acid of viruses is surrounded by a protein coat called the 'capsid ’. Each capsid is composed of protein subunits called 'capsomeres ’. In some viruses, the proteins composing the capsomeres are of a single type or many types may be present. Capsid is covered by an envelope, which consists of combination of lipids, proteins & carbohydrates . Some animal viruses are released from the host cell and coats the virus with a layer of the host cells plasma membrane, that layer becomes the viral envelope .

5. Depending on the virus, envelopes may or may not be covered by spikes, which are carbohydrate protein complexes that project from the surface of the envelope. Some viruses attach to host cells by means of spikes. This is one of the characteristics used for identification of viruses. Viruses whose capsids are not covered by an envelope are known as 'naked viruses' or 'non-enveloped viruses’. The capsid of a naked virus protects the nucleic acid from nuclear enzymes in biological fluids and promotes virus attachment to susceptible host cells. Capsid also serves as a vehicle of transmission from one host to another.

6. Viruses may be classified into different types, depending on the capsid structure: long rods that may be rigid or flexible. The nucleic acid is found within a hollow, cylindrical capsid , which is helical. The genetic material single-stranded RNA (some cases ssDNA) bound into the protein helix by interactions between the negatively charged nucleic acid and positive charges on the protein. The length of a helical capsid is related to length of the nucleic acid . many-sided or polyhedral . icosahedron, a regular polyhedron with 20 triangular faces and 12 corners. The capsomers of each face form an equilateral triangle. In icosahedral capsids the nucleic acid is tightly packed in a central core , forming a pool of parallel loops around a cylindrical hole. roughly spherical. When helical or polyhedral viruses are enclosed by envelopes, they are called enveloped helical and enveloped polyhedral viruses. envelope is responsible for protection of viruses from enzymes and chemicals. tadpole shaped, hexagonal head and a cylindrical tail. head consists of a tightly packed core of nucleic acid surrounded by a protein coat/ capsid. size of the head varies from 28 nm to 100 nm. The tail is composed of a hollow core , and a terminal base plate which has attached to it prongs, tail fibers or both.

7. LIFE CYCLE OF BACTERIOPHAGES Bacteriophages exhibit two different types of life cycles: Lytic or virulent cycle: In virulent cycle, there is intracellular multiplication of phages followed by lysis and release of progeny virions. This is called lytic cycle. The multiplication cycle of these phages like that of all viruses, occurs in five stages. Attachment or adsorption Penetration Biosynthesis of phage components Maturation Release of progeny phage particles. Replication cycle Eclipse phase Latent period Rise period

8. Temperate or avirulent or lysogenic cycle: In lysogenic cycle the phage DNA becomes integrated with the bacterial genome, replicating synchronously without any cell lysis. The integrated phage nucleic acid is known as the ' prophage ’. The prophage behaves like a segment of the host chromosome and replicates synchronously in the bacterial cell. This is called ' lysogeny ' and a bacterium that carries a prophage within its genome is called a 'lysogenic bacterium’. A lysogenic bacterium is resistant to reinfection by the same or related phages . This is known as 'superinfection immunity ’. A rare spontaneous event or the action of UV light or certain chemicals can lead to the excision of the phage DNA and to initiation of the lytic cycle . This is known as 'spontaneous induction of prophage'.

9. CULTIVATION OF VIRUSES Viruses are obligate intracellular parasites, which cannot be grown on inanimate culture media. Three methods are used for the cultivation of viruses. Laboratory animals: Reed and colleagues (1900) used human volunteers for their pioneering work on yellow fever. Human volunteers are used only when no other method is available and viruses are harmles s. Landsteiner and Popper (1909) used monkeys for isolation of the poliovirus . However, due to their cost and risk to handlers, monkeys find less application in virology. The poliomyelitis virus after intraspinal/ intracerebral inoculation in monkeys causes paralytic disease. The use of white mice , pioneered by Theiler (1903) extended the scope of animal inoculation. Mice are still the most widely employed animals in virology . Infant (suckling) mice are very susceptible to coxsackie and arboviruses. Other animals such as rabbits, guinea pigs and ferrets are used in some situations . Small pox virus may be inoculated in the scarified skin or cornea of rabbits. The growth of the virus in inoculated animals may be indicated by death, disease or visible lesions. Animal inoculation is also used for the study of pathogenesis, immune response and epidemiology .

10. 2. Embryonated eggs: The embryonated hen's egg was first used for cultivation of viruses by Goodpasture (1931) and the method was further developed by Burnet . The embryonated egg offers several sites for the cultivation of viruses. Fertile chicken eggs incubated for 5 to 12 days can be inoculated through the shell aseptically . The opening may be sealed with paraffin wax and the egg incubated at 36°C for the time required for the growth of the virus. The duration of incubation may depend on the type of viruses and the route of inoculatio n. Inoculation into the allantoic cavity provides a rich yield of influenza and some paramyxoviruses . Inoculation into the amniotic sac is employed for the primary isolation of the influenza virus . Yolk sac inoculation is used for the cultivation of some viruses, chlamydiae and rickettsiae . Yellow fever (17 D strain) and rabies (Flury strain) are other vaccines produced from chick embryo and vaccinia and herpes viruses from the chorioallantoic membrane.

11. 3. Tissue culture: The first application of tissue culture in virology was by Steinhardt and colleagues (1913) who maintained vaccinia virus in fragments of rabbit cornea . The turning point which made tissue culture, the most important method for the cultivation of viruses was the demonstration by Enders, Weller and Robbins (1949) for poliovirus. There are mainly three types of tissue cultures: ( i ) Organ culture: Organ cultures are useful for the isolation of some viruses which appear to be highly specialised parasites of certain organs e.g. tracheal ring organ culture is employed for the isolation of coronavirus. (ii) Explant culture: Minced tissue may be grown as explant embedded in plasma clots. This is not useful in virology. In the past, adenoid tissue explant cultures were used for adenovirus. (iii) Cell culture: This is a very popular and useful technique routinely used for cultivation of viruses. Tissues are dissociated into the component cells by the action of proteolytic enzymes such as trypsin and mechanical shaking. The cells are washed, counted and suspended in a growth medium and distributed in Petri plates, test tubes or bottles. The cells adhere to the glass surface and grow out to form a monolayer sheet.

12. MULTIPLICATION OF HUMAN VIRUSES Virus depends on the synthetic machinery of a host cell for replication because it lacks biosynthetic enzymes. Early studies on viral replication employed the bacteriophage as a model. There are some similarities in pattern of multiplication of bacterial and animal or human viruses. Homework Write a detailed note on Lytic cycle Lysogenic cycle Multiplication of human virus