Virus: Structure, Replication, Cultivation: Microbiology

A virus is/are

• Obligate intracellular infective agent.         
• Contains only one type of nucleic acid (DNA or RNA).
• No metabolic activity of outside the living cell (due to lack the enzyme & cellular organization).

Largest virus:- Small pox(300nm).

Smallest virus:- Papo virus (20nm).

What is Viroids?

Viroids are ss circular RNA molecules lacking a protein coat (mostly Plant pathogenic)

What is Prions?

Prions are:

• Infectious proteins without nucleic acid
• Highly resistant to physical & chemical agents .
• Produce slow infection with long incubation period .
• Destruction of prions
  • Heat 360⁰ C for 1 hrs
  • Steam – 134-138⁰ C for 18 mints.
  • Chemicals – Sor. Hypochloride for 1 hrs
  • Other – bleach , phono 190% & I₂

• VIRUSES & CANCER:
  • Oncoviruses are Associated with the development of cancer.
  • Oncovirus produces a protein that bind to host tumor suppressor proteins & inactivation them.
  • Tumor suppressor proteins : regulate growth & initiate apoptosis.

DNA containing Virus & RNA containing Virus

Difference between prokaryotes (Bacteria) and virus

Structure of Virus

• Virion (Extracellular infections virus particles) with {nucleic acid + coat capsid=Nucleocapcid}
• Capcid made of copsomers (polypeptide)
• Function of capcid :- Impenetable shell around nucleic acid introduce viral genome into most cells.
• Envelope –Lipoprotein in nature
  • Protein (Viral encoded/derived)
  • Lipid part (Derived from host cell membrane or nuclear membrane)

Symmetry of virus

1. Icosahedral (cubical) symmetry
   • Rigid structure
   • Polygon with 12 vertice/corner & 20 facets (equilateral triangular)
2. Helical symmetry: helical or spiral  tube (Capsomers & Nucleic acid wound together)
3. Complex; eg Pox virus

REPLICATION CYCLE OF VIRUSES

1. Attachment /Adsorption:-

• Virion attach to host cells surface via ligands. HIV: Attachment b/w viral surface glycoprotein (gp 120) & CD-4 receptor on host cells .

2. Penetration & uncoating

• Enveloped virus: Envelop fuses with host cell membrane & enter into cell.
• Non-enveloped virus/ Naked virus:
  • In bacterial Cell: Inject nucleic acid into host cell & leaving capcid outside the cell .
  • In Eukaryotes cell:- By endocytosis – entire nucleocapcid gain entrance into host cell & capcid is being removed in cytoplasm of host cell.
• Uncoating – Process of stripping the virus nucleic acid from outer layer & capcid by lysosome enzyme of host cells.

3. Synthesis: Viral genume directs host machinery to shut down the normal cellular metabolism & direcd to synthesis of viral components & viral nucleic acid .

4. Assembly /maturation:- Virl nucleic acid & capcid assemble together to form daughter virus/virions.

5. Release:- Newly formed virions/viruses are released from host cell by budding process (enveloped virus) or lysing the host cell (Bacteriophage).

CULTIVATION OF VIRUS

The virus can multiply only in living cells & cannot be grown on any of the inanimate culture mediums.

1. Animal inoculation

2. Embryonated egg inoculation of hen

-> Choriollantoic membrane [CAM] – pox virus grow

-> Allantoic cavity (Influenza virus vaccine)

-> Amniotic Sac – influenza virus isolation.

-> Yolk Sac Inoculation (Chlamydia & rickettsiae)

3. Tissue culture

• Organ culture
• Explant culture
• cell -> (Primany, diploid & cont. cell culture)

HIV virus

AIDS:Acquired immunodeficiency syndrome

• AIDS:Acquired immunodeficiency syndrome virus, enveloped.
  • HIV-1 – Worldwide
  • HIV-2- African country

• HIV Virus:
  • Enveloped; 2 identical copies of ss, +ve sense RNA & Reverse transcriptase enzyme.
  • Structural gene – gog,pol& env
  • Envelop – gp 120
  • Nucleocapsid – p24
• Malignancies associated with HIV :-
• Kaposi’s sarcoma
• β – cell lymphoma/Non- Hodgkin lymphoma
• Retroviruses possess reverse trancriptase (RNA directed DNA polymerase) enzyme which prepares a DNA copy of the RNA genome in host cell. The presence of enzyme reverse transcriptase is a characteristic feature.
• Human immunodeficiency virus (HIV), the causative agent of AIDS, belongs to retroviruses.
• HIV genome contains the three structural genes (gag, pol and evn). Five non-structural genes ( tat , rev , nef, vif and upr ) are present in both HIV-1 and HIV-2. Other these, HIV – 1 contains vpu and HIV-2 has vpx. The products of these genes, both structural and non-structural , act as antigens. Infection person’s serum contains antibodies to these antigens. Detection of these antigens and antibodies is of great importance in the diagnosis and prognosis of HIV infections.
• There are three modes of transmission of HIV infection. These are sexual contact, parenteral and perinatal.
• HIV infects principally the CD4 lymphocytes. The infection causes damage to T helper (T4) lymphocytes. T4 cells are depleted in numbers and the T4:T8 (helper: suppressor) ratio is reversed.
• When CD4+ cell fall down below 200 per mm³, the titer of virus increases markedly and there is irreversible breakdown of immune defence mechanisms . most of the patients with HIV disease die of infections other than HIV e.g.  opportunistic infection and malignancies. AIDS is the end stage of HIV infection.
• Laboratory diagnosis of HIV infection includes specific tests for HIV and tests for immunodeficiency. Specific tests include antigen (p24) detection, virus isolation , detection of viral nucleic acid and antibody detection .
• The p24 antigen is the earliest virus marked to appear in the blood. Virus isolation, detection of viral nucleic acid by polymerase chain reaction (PCR) and p24 antigen detection are useful for diagnosis in window period . HIV infection persons remain negative for antibodies during window period.
• Demonstration of antibodies is the simplest and most commonly employed technique for diagnosis. It may take several weeks to months for antibodies to appear after infection. The diagnosis of HIV infection is made by detecting serum antibodies to viral proteins, both core (p24) or envelope (gp120, gp41). There are two types of serological tests available for antibody detection – screening tests and supplemental tests and supplemental tests.
• Screening (E/R/S) tests include ELISA, rapid tests and simple tests. Western blot test and indirect immunofluoresecnce test are supplemental tests used for HIV antibody detection.
• ELISA is the method most commonly used.
• There are three strategies (strategy 1 to 3) for HIV testing in India.

Important Key point about Virus

• Virus – host interaction may cause different effects, ranging from no apparent cellular damage to rapid call destruction.
• Some viruses (e.g. poliovirus) cause cell death (cytocidal infection). Other may cause cellular proliferation or malignant transformation (oncognic viruses). In some instancse, viruses remain as latent infections (herpes simplex virus) whereas other produce some morphological change in cell to from inclusion bodies (rabies virus).

• Inclusion bodies are virus-specific intracellular globular masses which are produced during replication of virus in host cells.
• Viruses may enter the body through respiratory tract, skin, genital tract, conjunction, of congenitally.
• Interferons (IFNs) are a family of Glycoprotein produced by host cell on induction of viral microorganisms : These interferons have antiviral activity . They are classified into three types namely IFN-α, IFN-β and IFN-γ.
• Laboratory diagnosis of viral infections depends on direct demonstration of virus and its components, isolation of virus, and detection of the specific antibodies.
• Poxviruses are the largest and the most complex of all viruses. They are brick-shaped.
• Retroviruses:- RNA [Characteristic Reverse transcriptase enzyme &  prepare DNA copy form RNA genome]

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