Microbiology: A Very Short Introduction by Nicholas P. Money

Author:Nicholas P. Money
Language: eng
Format: epub
ISBN: 9780199681686
Publisher: OUP Oxford
Published: 2014-09-22T04:00:00+00:00

20. Cycle of viral replication in a host cell running from attachment to the cell surface, penetration by endocytosis, exposure of the viral genome, replication (synthesis) of the genome and capsid components, assembly of the next generation of virus particles, and release by lysis of the host cell

Two types of virus protein are synthesized by the host. These are proteins needed for copying the viral genome, and structural proteins that form the virus particles. Viral infections disrupt the normal functioning of the host cells and inhibit the usual production of host proteins in favour of generating the components needed for the new generation of viruses. In many infections the viral genome encodes proteins that sustain the parasitic relationship by ensuring that the host cell continues to operate as long as possible while it is engaged in virus production. Assembly of the protein capsids and packaging of the copies of the DNA or RNA genomes into the particles occurs inside the host cells. Once the assembly phase is completed, release of hundreds or thousands of viruses can occur by bursting of the host cell or by a more gentle form of escape via exocytosis. Tailed bacteriophages encode hydrolytic enzymes called endolysins that dissolve the peptidoglycan wall of their bacterial hosts.

Viruses that infect multicellular organisms use a variety of mechanisms to enter the tissues of their hosts before they penetrate susceptible cells. These include introduction through a wound in the skin of an animal or an abrasion on the surface of a plant. Insects and ticks can act as vectors for the transmission of animal and plant viruses from one host to another. Yellow fever is spread by mosquitoes and causes tens of thousands of deaths every year. Most cases of yellow fever occur in Africa. Other mosquito-borne viral diseases include dengue fever, which is a widespread tropical disease, and West Nile fever, first identified in Uganda, which was introduced into the United States in 1999. Yellow fever, dengue fever, and West Nile are all caused by RNA viruses classified as flaviviruses. Many other human viruses infect animal tissues directly following inhalation or via contact with infected body fluids through sexual activity or following injection with contaminated needles. Aphids are important vectors for the transmission of plant viruses.

Because they cannot be cultured on their own, virologists raise viruses in cultures of microorganisms, cultures of animal cells, and in whole animals and plants. To identify and to count bacteriophages, samples containing these viruses are mixed with pure cultures of bacteria and melted agar and poured on top of a solid agar surface. The melted agar solidifies after pouring and the bacteria divide and form an even lawn of cells within this top layer during an overnight incubation. Viruses that infect the cultured cells become visible as clear areas, or plaques, of dead bacteria. Each of the plaques can be initiated by a single virion as it multiplies through successive cycles of replication. The number of plaques, or plaque-forming units, and the dilution factor are used to calculate the number of viruses in the original sample.

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