An Introduction to the VirusesNon-Living Etiologies

Viruses were too small to beseen with the first microscopes

The cause of viral infections was unknown for years

Louis Pasteur first proposed theterm virus

Viruses are considered filterable

Can infect every type of cellCannot exist independently from the host cell, so aren’t considered living thingsReferred to as infectious ParticlesObligate intracellular parasitesCannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses

Figure 6.1

Smallest infectious agents

Most are so small, they can only be seen with an electron Microscope

Viewing viruses

◦ Special stains and an electron microscope

◦ Negative staining outlines the shape

◦ Positive staining shows internal details

Contain only those parts needed to infect and control a host cell

◦ External coating Capsid Envelope- in 13 of the 20

families of animal viruses If no envelope, called naked

virus◦ Core

DNA RNAThe capsid and the nucleic acid

together are called the nucleocapsid.

Fully formed virus that is able to establish an infection in a host cell is termed a virion.

Figure 6.4

Many viruses (e.g. influenza and many animal viruses) have viral envelopes covering their protein capsids.

The envelopes typically are derived from portions of the host cell membranes (phospholipids and proteins), but include some viral glycoproteins.

Functionally, viral envelopes are used to help viruses enter host cells.

Glycoproteins on the surface of the envelope serve to identify and bind to receptor sites on the host's membrane.

The viral envelope then fuses with the host's membrane, allowing the capsid and viral genome to enter and infect the host.

Figure 6.10

Figure 6.8

Protects nucleic acids

Help introduce the viral DNA or RNA into a suitable host cell

Stimulate the immune system to produce antibodies that can protect he host cells against future

infections

Note: The capsid surrounds the virus and is composed of a finite number of protein subunits known as capsomeres, which usually associate with, or are found close to, the virion nucleic acid.

Number of viral genes is small

They only have the genes necessary to invade host

cells and redirect their activity

Two Types of Viruses:

DNARNA

ssDNA dsDNA

◦ linear◦ circular

Mostly single-stranded◦ Positive-sense RNA:

genomes that are ready for immediate translation into proteins

◦ Negative-sense RNA: genomes have to be converted into the proper form to be made into proteins

Segmented- individual genes exist on separate pieces of RNA

Main criteria◦ Structure◦ Chemical composition◦ Similarities in genetic makeup

International Committee on the Taxonomy of Viruses, 2000◦ 3 orders◦ 63 famillies “-viridae”◦ 263 genera “-virus”

Some virologists use a species naming system, but it is not an official designation

The host cell is absolutely necessary for viral multiplication

Bacteriophage Most contain dsDNA Often make the

bacteria they infect more pathogenic for humans

Icosahedral capsid head containing DNA Central tube surrounded by a sheath Neck Base plate Tail pins Fibers

Attachment

Virus attaches to specific receptor sites on the host bacterium .

The bacteriophage attach to the bacterial cell wall.

Specific strains of bacteriophages can only adsorb to specific strain of host bacteria. This is known as viral specificity .

Viruses are Host specific

Penetration

The virus "drills" a hole in the bacterial wall and the virus injects its genome into the bacterial cytoplasm.

Some phages accomplish this by contracting a sheath which drives a hollow tube into the bacterium.

Viral Component Replication

Enzymes coded by the phage genome shut down the bacterium's cellular activities.

The phage replicates its genome and uses the bacterium's metabolic machinery to synthesize phage enzymes and phage structural components (nucleic acids, proteins and enzymes.

Viral Assembly

Viral proteins are synthesized and self-assembled into viral components such as head, tail, and tail fibers.

Maturation

The phage parts assemble around the genomes.

As the viral components are assembled into new viral particals are made, an enzyme, lysozyme, is released weakening the cell wall of the host bacterium.

Release

Usually, a phage-coded lysozyme breaks down the bacterial peptidoglycan causing osmotic lysis and release of the intact bacteriophages .

The new viruses are released and are now able to infect a new host, therefore, starting the entire cycle all over again.

Figure 6.17

Figure 6.18

Figure 6.19