Viruses and their importance
At a glance
• Humans • Other vertebrates • Invertebrates
Foot and mouth disease 2 • Fungi
Leatherjackets infected with Tipula iridescent virus • Bacteria
Delayed emergence of potato Damaged potato caused by tobacco rattle virus (spraing) caused by infection 3 tobacco rattle virus infection 3
Mushroom virus X4
Escherichia coli cell with phage T4 attached 5
Virology: Principles and Applications John B. Carter and Venetia A. Saunders 2007 John Wiley & Sons, Ltd ISBNs: 978-0-470-02386-0 (HB); 978-0-470-02387-7 (PB)
VIRUSES AND THEIR IMPORTANCE
At a glance (continued)
Some viruses are useful:
• • • • • • Phage typing of bacteria Sources of enzymes Pesticides Anti-bacterial agentsAnti-cancer agents Gene vectors
Viruses are parasites; they depend on cells for molecular building blocks, machinery and energy. Virus particles are small; dimensions range from approx. 20–400 nm.
A virus genome is composed of one of the following:
Photographs reproduced with permission of 1 WorldHealth Organisation. 2 Animal Sciences Group, Wageningen UR. 3 MacFarlane and Robinson (2004) Chapter 11 Microbe-Vector Interactions in Vector-Borne Diseases, 63rd Symposium of the Society for General Microbiology, Cambridge University Press. Reprinted with permission. 4 Warwick HRI. 5 Cornell Integrated Microscopy Center.
REASONS FOR STUDYING VIRUSES
Viruses are ubiquitous onEarth
Viruses infect all cellular life forms: eukaryotes (vertebrate animals, invertebrate animals, plants, fungi) and prokaryotes (bacteria and archaea). The viruses that infect prokaryotes are often referred to as bacteriophages, or phages for short. The presence of viruses is obvious in host organisms showing signs of disease. Many healthy organisms, however, are hosts of non-pathogenic virusinfections, some of which are active, while some are quiescent. Furthermore, the genomes of many organisms contain remnants of ancient virus genomes that integrated into their host genomes long ago. As well being present within their hosts, viruses are also found in soil, air and water. Many aqueous environments contain very high concentrations of viruses that infect the organisms that live in thoseenvironments. There is a strong correlation between how intensively a species is studied and the number of viruses found in that species. Our own species is the subject of most attention as we have a vested interest in learning about agents and processes that affect our health. It is not surprising that there are more viruses known that infect mankind than any other species, and new human virusescontinue to be found. The intestinal bacterium Escherichia coli has also been the subject of much study and many viruses have been found in this species. If other species received the same amount of attention it is likely that many would be found to be hosts to similar numbers of viruses. It is undoubtedly the case that the viruses that have been discovered represent only a tiny fraction of theviruses on the Earth. Most of the known plants, animals, fungi, bacteria and archaea have yet to be investigated for the presence of viruses, and new potential hosts for viruses continue to be discovered. Furthermore, the analysis of DNA from natural environments points to the existence of many bacterial species that have not yet been isolated in the laboratory; it is likely that these‘non-cultivable bacteria’ are also hosts to viruses.
lethal (e.g. rabies), and viruses also play roles in the development of several types of cancer. As well as causing individuals to suffer, virus diseases can also affect the well-being of societies. Smallpox had a great impact in the past and AIDS is having a great impact today. There is therefore a requirement to understand the nature of viruses, how...