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What is a bacteriophage?

Discovered in 1915 by Frederick W. Twort, and independently in 1917 by Félix d’Herelle (a then somewhat obscure researcher at the Pasteur Institute in Paris), bacteriophages are viruses which attack only bacteria. They are ubiquitous and in numbers difficult to comprehend, but estimated at around 1031 in total. Their importance in ecosystems, particularly marine ecosystems, has only recently been comprehended.

In the simplest terms, bacteriophages or "phages" are viruses which attack only bacteria. Like all viruses, bacteriophages share broad similarities of structure and function. There are differences, however, based on the particular strain of virus or the mode of reproduction (lytic or lysogenic, or a combination of these). All phages are obligate parasites of bacteria. All, in some form or another, possess genes (which code for structural proteins as well various enzymes) enclosed in a capsid, as well as tails and other structures used to penetrate the host.

Phages can be divided according to what form their genetic material is stored (RNA or DNA, single-stranded or double-stranded, circular or linear). There are some general patterns and loose rules regarding relationships between lifestyle and structure. Filamentous phages (which lack tails and appear as thin tubes) are always lysogenic. T4, the most commonly recognized phage (at least to non-specialists), exhibits a prolate head and a contractile tail in the classical “spaceship” form, often (and mistakenly) seen as the epitome of the viral shape. T4 is lytic, but not all lytic phages exhibit a similar structure. Many are spherical or icosahedral, lacking any discernable tail. These globular phages tend to possess the greatest quantity of genetic information, but again, this is not a concrete pattern.

Marine phages

If they can be described as being “alive,” marine phages, although invisible and essentially unnoticed by scientists until very recently, are the most abundant and diverse form of life on the planet. They influence biogeochemical cycles globally, provide and regulate microbial diversity, cycle carbon through marine food webs, and are essential in preventing bacterial population explosions.

Aquatic phages in general are evident in historical accounts: the mythical healing powers of the Ganges River, for example, were not fictional. However, their curative effects were less the benevolence of a Hindu deity than bacteriolytic phages.

Applications

As scientists begin to understand phages, their potential becomes clearer. Marine cyanophages can be used to prevent or reverse eutrophication, for example, if properly utilized against blue-green algae.

References

Resch A, Fehrenbacher B, Eisele K, Schaller M, Gotz F (2005) Phage release from biofilm and planktonic Staphylococcus aureus cells. FEMS Microbiol Lett 1:89-96.

Kellogg C, Rose J, Jiang S, Thurmond J, Paul J (1995) Genetic diversity of related vibriophages isolated from marine environments around Florida and Hawaii, USA. Mar Ecol Prog Ser 120:89-98.

Waldor M, Friedman D, Adhya S eds. (2005) Phage ecology and bacterial pathogenesis. In Phages: their role in bacterial pathogenesis and biotechnology pp 66-84. Washington DC: ASM Press.

Wommack K, Hill T, Muller A, Colwell R (1996) Effects of sunlight on bacteriophage viability and structure. Appl Environ Microbiol 62:1336-1341.