Viruses: Are noncellular, non-cytoplasmic or acellular infectious agents

Virology: The study of viruses

Virologists: Scientists who study viruses

Virus particle (virion): extracellular form of a virus

The concept of virus

1. Edward Jenner (1798), introduced the term virus in microbiology.
2. Virus in Greek means poison.
3. Edward Jenner noticed that milk maids who infected with cowpox develop immunity against smallpox.
4. He inoculated a boy with the vesicle fluid taken from the hand of infected maid.
5. The boy developed sustained immunity against smallpox.

Discovery of Viruses

Martinus Beijerinck (1897)
i. showed that causative agent of tobacco mosaic disease was still infectious after filtration
ii.referred to as filterable agent which coined from the latin name “virus” meaning poison

Frederick Twort (1915)
i. first to isolate viruses that infect bacteria (bacteriophages or phages)

Wendell Stanley (1935)
i. crystallized sap from sick tobacco plants
ii.He discovered viruses were made of nucleic acid and protein

GENERAL STRUCTURE AND FUNCTION OF VIRUSES

1. They are generally smaller than bacteria, and this can pass through bacteriological filter. A Virion size range is ~10-400 nm

2. Viruses are acellular, non-cytoplasmic infectious agents. They lack cellular organelles, such as mitochondria and ribosomes

3. All viruses are obligate parasites and can multiply only within the living host cells and are highly resistant to antimicrobial agents and extremes of physical conditions.

4. Viruses do not multiply in chemically defined media.

5. Viruses do not undergo binary fission.

6. Viruses have a very simple structure. The core of the viruses is made upon of nucleic acid, which is surrounded by a protein coat called capsid.

7. Viruses contain only one type of nucleic acid, either DNA or RNA, but never both. Some circular, but most linear. The infectious property of a virus is due to its nucleic acid

8. The capsid or protein coat is made up of many identical protein sub-units called capsomeres. The capsomeres are composed of either one or several type of proteins and glycoproteins.

9. The host specificity of virus is due to proteins of the capsid.

10. Outside the host cell, they are non-living. Thus, viruses show characters of both living and non-living.

11. Some viruses have additional lipoprotein envelope , composed of virally coded protein and host lipid. The viral envelope is covered with glycoprotein spikes

12. Some viruses have enzymes inside the virion. All ss- RNA viruses with negative polarity have the enzyme transcriptase ( RNA dependent RNA polymerase) inside virions.

Non-living Characters of Viruses:

Following characters of viruses assign them as non-living:
(a) They can be crystallized.
(b) Outside the cell, they behave like inert chemicals.
(c) They do not show growth, development, nutrition, reproduction, etc.
(d) They can be precipitated.

Living characters of viruses:

(a) They multiply within host cells.
(b) They possess genetic material, either DNA or RNA.
(c) There are definite races or strains.
(d) They exhibit mutations.
(e) Lack enzymes for most metabolic processes.
(f) Lack machinery for synthesizing proteins.
Because of the above reasons, viruses form unique bridge between living and non-living things.

Morphology (Symmetry)

Based on capsid architecture, although enveloped viruses end up being approximately spherical.

1. Helical, non-enveloped
2. Helical, enveloped

Envelopes form when viral glycoproteins and oligosaccharides associate with the plasma membrane of the host cell.
All envelopes have a phospholipid bilayer with embedded proteins.

1. Polyhedral (Icosahedral), non-enveloped
2. Polyhedral (Icosahedral), enveloped
   Polyhedral means many sides (most are icosahedral - 20 triangular faces and 12 corners)
3. Complex - Virions composed of several parts, each with separate shapes and symmetries. Bacterial viruses contain complicated structures, icosahedral heads and helical tails.
   This symmetry is also exhibited by poxvirus and rhabdovirus. This group comprises all those viruses which do not fit into either of the above two groups.

Viral Taxonomy

• Family names end in -viridae
• Genus names end in -virus
• Viral species: A group of viruses sharing the same genetic information and ecological niche (host).
• Common names are used for species
• Subspecies are designated by a number

CLASSIFICATION OF VIRUSES

(1) Nucleic acid type: RNA or DNA; single-stranded or double-stranded; strategy of replication.
(2) Size and morphology, including type of symmetry, number of capsomeres, and presence of membranes.
(3) Presence of specific enzymes, particularly RNA and DNA polymerases, and neuraminidase.
(4) Susceptibility to physical and chemical agents, especially ether.
(5) Immunologic properties.
(6) Natural methods of transmission.
(7) Host, tissue, and cell tropisms.
(8) Pathology; inclusion body formation.
(9) Symptomatology.

VIRAL REPLICATION

• A bacteriophage (informally, phage) is a virus that infects and replicates within a bacterium.
• The infection cycle viruses was first worked out in bacteriophages.
• Animal virus infections can be either lytic or lysogenic.

five Steps of Lytic Cycle

1. Attachment to the cell
2. Penetration (injection) of viral DNA or RNA
3. Replication (Biosynthesis) of new viral proteins and nucleic acids
4. Assembly (Maturation) of the new viruses
5. Release of the new viruses into the environment (cell lyses)

Bacteriophage Replication

• Bacteriophage inject their nucleic acid
• They lyse (break open) the bacterial cell when replication is finished

Lytic Cycle Review

Attachment: Phage attaches by tail fibers to host cell
Penetration: Phage lysozyme opens cell wall,tail sheath contracts to force tail core and DNA into cell
Biosynthesis: Production of phage DNA and proteins
Maturation: Assembly of phage particles
Release Phage: lysozyme breaks cell wall

Lysogenic Cycle

• Phage DNA injected into host cell
• Viral DNA joins host DNA forming a prophage
• Viral DNA (part of prophage) may stay inactive in host cell for long periods of time
• Replicated during each binary fission
• Over time, many cells form containing the prophages
• When an activation signal occurs, the phage DNA starts replicating

Lytic and Lysogenic Cycles