بسم الله الرحمن الرحمن

By Dr.Abeer Abd Elfattah Prof. of Microbiology and Immunology BASIC VIROLOGY By Dr.Abeer Abd Elfattah Prof. of Microbiology and Immunology

GENERAL PROPERTIES OF VIRUSES Viruses are small infectious agents. They are obligate intracellular parasites; therefore, extracellular viruses are metabolically inert.

Properties that differentiate viruses from other microorganisms 1- Size: viruses are smaller than other organisms (20-300 nm). 2- Genome: the genome of viruses is either DNA or RNA; it contains only one type.

3- Metabolically: viruses have no metabolic activity outside susceptible host cells. They do not possess active ribosomes or protein-synthesizing apparatus.

Viruses can multiply only in living cells. When viruses enter a host cell, they introduce their genetic material, DNA or RNA, inside the host cell. The virus directs the host cell metabolism to form new copies of viral genes and viral proteins.

STRUCTURE OF VIRUSES 2. A protein coat (capsid) Viruses consist of: 1. A core of nucleic acid (genome) 2. A protein coat (capsid) Both nucleic acid and the protein coat form the nucleocapsid. 3. An envelope is present in some viruses.

Virion It is the intact virus particle. In non-enveloped viruses it is composed of nucleic acid and protein coat. In enveloped viruses it is composed of nucleic acid, protein coat and the envelope.

Nucleic Acid (viral core or viral genome) It is either DNA or RNA and it may be either double-stranded (ds) or single-stranded (ss). The viral genome is responsible for infectivity.

All DNA viruses are ds except Parvoviruses, which are ss, while all RNA viruses are ss except Reoviruses, which are ds.

All RNA viruses have non-segmented genomes except Influenza, Rota, and Bunya viruses, which are segmented. Single-stranded RNA viruses are divided into positive (+) sense and negative (-) sense.

The RNA of the (+) sense genome is infectious (functions as a mRNA in the infected cell) while the RNA of the (-) sense genome is not infectious (the virion carries an RNA polymerase that transcribes the RNA into mRNA).

Capsid (Protein Coat) It is the protein coat that encloses the genome and it is made up of protein subunits called: capsomeres. The arrangement of these capsomeres gives the virus its symmetry. It protects the genetic material from the effect of nucleases.

It mediates the attachment of the virus to specific receptors on the host cell surface in non-enveloped viruses. It includes important antigens, which induce neutralizing antibodies and activate cytotoxic T lymphocytes (CTLs) to kill virus-infected cells.

Envelope It is a lipoprotein membrane surrounding some viruses. Viruses acquire this membrane from host cell membranes (either cytoplasmic or nuclear) during budding from the infected host cell. The lipid component is derived from the host cell membrane and the glycoprotein component is virus specific.

Enveloped viruses are more sensitive to heat, detergents and lipid solvents (as alcohol or ether) than non- enveloped viruses. Disruption of virus envelope results in virus inactivation.

Virus-encoded glycoproteins are exposed on the surface of the envelope e.g. haemagglutinin in influenza virus.

They mediate the attachment of the virus to specific receptors on the host cell surface in enveloped viruses and are the target of humoral immune response.

Atypical virus like particles and Prions Defective viruses: They are composed of viral nucleic acid and proteins, but cannot replicate without a “helper” virus, which provides the missing function.

Pseudoviruses: They contain host cell DNA instead of viral DNA within the capsid. Viroids: They consist of a single molecule of circular RNA without a protein coat or envelope.

Prions: These are the smallest known infectious particles. They are composed solely of proteins with no detectable nucleic acid.

SHAPES OF VIRUSES The arrangement of viral capsomeres gives the virus structure its symmetry.

1-Icosahedral (cubic) symmetry: The virus capsomeres are arranged in 20 triangles with the approximate outline of a sphere

2-Helical symmetry: The capsomeres are arranged in a hollow coil

3- Complex structure: Some viruses are more complicated in structure

4-Head and tail viruses: They look like a sperm e.g Bacteriophages.

Types of symmetry of virus particles

REACTION TO PHYSICAL & CHEMICAL AGENTS

Physical agents 1. Heat: Destroy viral infectivity at 50-60 ˚C for 30 minutes, except hepatitis B virus. 2. Cold: Viruses are stable at low temperature. Most viruses can be stored at – 40˚C or preferably at – 70 ˚C.

Some viruses are inactivated by the process of freezing and thawing. Lyophilization (drying from frozen state under vacuum) can preserve viruses at 4˚C for many years. 3. Radiation: Ultraviolet rays, x-rays, and γ rays inactivate viruses.

Chemical agents 1. pH: Viruses are usually stable between pH values of 5.0 and 9.0. Some viruses e.g. Enteroviruses are resistant to acidic conditions. All viruses are destroyed by alkaline conditions.

2. Ether, alcohols and Detergents: Dissolve the viral envelope causing inactivation of enveloped viruses. 3. Oxidizing Agents :Viruses are inactivated by chlorine, iodine, and H2O2.

destroys viral infectivity (by reacting with nucleic acid) 4. Formaldehyde: destroys viral infectivity (by reacting with nucleic acid) without affecting viral antigenicity; therefore it has been used in the production of inactivated viral vaccines.

5. Salts: Some viruses (Picornaviruses and Reoviruses) can be stabilized by salts; e.g. MgCl2 (1 mol/L) Thus used in stabilization of live attenuated vaccine of poliomyelitis to be maintained potent for weeks at high temperature in tropics.

6. Phenols: Most viruses are resistant to phenolic compounds. 7. Glycerol: Glycerol (50%) destroys bacteria, but preserves viruses. Therefore, it is used to decontaminate viral preparations. 8. Antibiotics: They have no effect on viruses.

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