- Phage ecology
Bacteriophages (phages), potentially the most numerous "organisms" on Earth, are the virusesof bacteria(more generally, of prokaryotesThe term " prokaryotes" is useful to mean the sum of the bacteriaand archaeabacteriabut otherwise can be controversial, as discussed by [http://mmbr.asm.org/cgi/content/full/68/2/173#The_Dismantling_of_Bacteriology_and_a_Deconstruction_of_the_Procaryote Woese, 2004] ; see also pp. 103-104 of Woese, C. R. 2005. Evolving biological organization, p. 99-118. In J. Sapp (ed.), Microbial Phylogeny and Evolution Concepts and Controversies. Oxford University Press, Oxford. A history of the concept is provided by [http://www.im.microbios.org/0903/0903163.pdf Sapp, 2006] .] ). Phage ecology is the study of the interaction of bacteriophages with their environments. [This article on phage ecology was expanded from a stub during the writing of the first chapter of the edited monograph, "Bacteriophage Ecology" (forecasted publication date: March, 2008, Cambridge University Press), in order to be cited by that chapter especially as a repository of phage ecology review chapters and articles.] Phage ecology is increasingly an important component of sessions and symposiums associated with phage meetingsas well as general microbiological meetings.
Introduction to phage ecology
Vastness of phage ecology
obligate intracellular parasitesmeaning that they are able to reproduce only while infecting bacteria. Phages therefore are found only within environments that contain bacteria. Most environments contain bacteria, including our own bodies (there called normal flora). Often these bacteria are found in large numbers. As a consequence, phages are found almost everywhere.
rule of thumb, many phage biologists expect that phage population densitieswill exceed bacterial densities by a ratio of 10-to-1 or more (VBR or virus-to-bacterium ratio; see [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15109783&dopt=Abstract] for a summary of actual data). As there exist estimates of bacterial numbers on Earth of approximately 1030 [http://www.pnas.org/cgi/content/full/95/12/6578] , there consequently is an expectation that 1031 or more individual virus (mostly phage [http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=10704475] ) particles exist [http://www.phage.org/bgnws007.htm#submissions] , making phages the most numerous category of " organisms" on our planet.
Bacteria (along with
archaeabacteria) appear to be highly diverse and there possibly are millions of species [http://www.pnas.org/cgi/content/full/99/16/10494?ijkey=68dca75ae15798053a7e3b798295da11052ae938] . Phage-ecological interactions therefore are quantitatively vast: huge numbers of interactions. Phage-ecological interactions are also qualitatively diverse: There are huge numbers of environment types, bacterial-host types [http://www.pnas.org/cgi/content/full/103/32/12115] , and also individual phage types [http://www.pnas.org/cgi/content/full/99/22/14250] ).
Studying phage ecology
The scale of phage ecology is at once both exhilarating and intimidating. As a guiding principle toward understanding phage ecology we therefore seek generalizations, plus look to more established scientific disciplines for guidance, the most obvious being general
ecology. Toward that end we can speak of phage "organismal" ecology, population ecology, community ecology, and ecosystem ecology. Phage ecology from these perspectives will be described in turn (re: links in previous sentence).
Phage ecology also may be considered (though mostly less well formally explored) from perspectives of phage
behavioral ecology, evolutionary ecology, functional ecology, landscape ecology, mathematical ecology, molecular ecology, physiological ecology (or ecophysiology), and spatial ecology. Phage ecology additionally draws (extensively) from microbiology, particularly in terms of environmental microbiology, but also from an enormous catalog (90 years) of study of phageand phage-bacterial interactions in terms of their physiologyand, especially, their molecular biology.
Phage "organismal" ecology
Phage "organismal" ecology is primarily the study of the evolutionary ecological impact of phage growth parameters:
latent period, plus
**eclipse period (or simply "eclipse")
**rise period (or simply "rise")
*burst size, plus
**rate of intracellular phage-progeny maturation
*adsorption constant, plus
**rates of virion diffusion
**virion decay (inactivation) rates
host range, plus
**resistance to restriction
**resistance to abortive infection
*various temperate-phage properties, including
**rates of reduction to
**rates of lysogen induction
*the tendency of at least some phage to enter into (and then subsequently leave) a not very well understood state known (inconsistently) as pseudolysogeny [Pseudolysogeny references: Barksdale, L., and S. B. Ardon. 1974. Persisting bacteriophage infections, lysogeny, and phage conversions. Ann. Rev. Microbiol. 28:265-299; Miller, R. V., and S. A. Ripp. 2002. Pseudolysogeny: A bacteriophage strategy for increasing longevity in situ, p. 81-91. In M. Syvanen and C. I. Kado (eds.), Horizontal Gene Transfer. Academic Press, San Diego.]
Another way of envisioning phage "organismal" ecology is that it is the study of phage adaptations that contribute to phage survival and transmission to new hosts or environments. Phage "organismal" ecology is the most closely aligned of phage ecology disciplines with the classical molecular and molecular genetic analyses of bacteriophage.
From the perspective of ecological subdisciplines, we can also consider phage
behavioral ecology, functional ecology, and physiological ecology under the heading of phage "organismal" ecology. However, as noted, these subdisciplines are not as well developed as more general considerations of phage "organismal" ecology. Phage growth parameters often evolve over the course of phage experimental adaptation studies.
In the mid 1910s, when phage were first discovered, the concept of phage was very much a whole-culture phenomenon (like much of microbiologySummers, W. C. 1991. From culture as organisms to organisms as cell: historical origins of bacterial genetics. J. Hist. Biol. 24:171-190.] ), where various types of bacterial cultures (on solid media, in
broth) were visibly cleared by phage action. Though from the start there was some sense, especially by Fėlix d'Hėrelle, that phage consisted of individual " organisms", in fact it wasn't until the late 1930s through the 1940s that phage were studied, with rigor, as individuals, e.g., by electron microscopyand single-step growth experiments ( [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=11889095 example of latter] ). Note, though, that for practical reasons much of "organismal" phage study is of their properties in bulk culture (many phage) rather than the properties of individual phage virions or individual infections.
This somewhat whole-organismal view of phage biology saw its heyday during the 1940s and 1950s, before giving way to much more
biochemical, molecular genetic, and molecular biological analyses of phage, as seen during the 1960s and onward. This shift, paralleled in much of the rest of microbiology [http://mmbr.asm.org/cgi/content/full/68/2/173#The_Dismantling_of_Bacteriology_and_a_Deconstruction_of_the_Procaryote] , represented a retreat from a much more ecological view of phages (first as bacterial killers, and then as organismsunto themselves). However, the organismal view of phage biology lives on as a foundation of phage ecological understanding. Indeed, it represents a key thread that ties together the ecological thinking on phage ecology with the more "modern" considerations of phage as molecular model systems.
The basic experimental toolkit of phage "organismal" ecology consists of the single-step growth (or one-step growth; [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=11889095 example] ) experiment and the phage adsorption curve ( [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14660403&dopt=Citation example] ). Single-step growth is a means of determining the phage
latent period( [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14660403&dopt=Citation example] ), which is approximately equivalent (depending on how it is defined) to the phage period of infection. Single-step growth experiments also are employed to determine a phage's burst size, which is the number of phage (on average) that are produced per phage-infected bacterium.
The adsorption curve is obtained by measuring the rate at which phage
virionparticles (see [http://en.wikipedia.org/wiki/Virion#Structure] ) attach to bacteria. This is usually done by separating free phage from phage-infected bacteriain some manner so that either the loss of not currently infecting (free) phage or the gain of infected bacteria may be measured over time.
Phage population ecology
populationis a group of individualswhich either do or can interbreed or, if incapable of interbreeding, then are recently derived from a single individual (a clonal population). Population ecologyconsiders characteristics that are apparent in populations of individuals but either are not apparent or are much less apparent among individuals. These characteristics include so-called intraspecific interactions, that is between individuals making up the same population, and can include competition as well as cooperation. Competition can be either in terms of rates of population growth(as seen especially at lower population densities in resource-rich environments) or in terms of retention of population sizes (seen especially at higher population densities where individuals are directly competing over limited resources). Respectively, these are population-density independent and dependent effects.
Phage population ecology considers issues of rates of phage population growth, but also phage-phage interactions as can occur when two or more phage adsorb an individual bacterium.
Phage community ecology
communityconsists of all of the biological individualsfound within a given environment (more formally, within an ecosystem), particularly when more than one speciesis present. Community ecologystudies those characteristics of communities that either are not apparent or which are much less apparent if a community consists of only a single population. Community ecology thus deals with interspecific interactions. Interspecific interactions, like intraspecific interactions, can range from cooperative to competitive but also to quite antagonistic (as are seen, for example, with predator-prey interactions). An important consequence of these interactions is coevolution.
The interaction of phage with bacteria is the primary concern of phage community ecologists. Phage, however, are capable of interacting with species other than bacteria, e.g., such as phage-encoded
exotoxininteraction with animals[http://www.la-press.com/evolbio05.htm] . Phage therapyis an example of applied phage community ecology.
Phage ecosystem ecology
ecosystemconsists of both the bioticand abioticcomponents of an environment. Abiotic entities are not alive and so an ecosystem essentially is a communitycombined with the non-living environment within which that ecosystem exists. Ecosystem ecologynaturally differs from community ecologyin terms of the impact of the community on these abiotic entities, and "vice versa". In practice, the portion of the abiotic environment of most concern to ecosystem ecologists is inorganic nutrientsand energy.
Phage impact the movement of nutrients and energy within ecosystems primarily by lysing bacteria. Phage can also impact abiotic factors via the encoding of exotoxins (a subset of which are capable of solubilizing the
biological tissues of living animals[http://www.la-press.com/evolbio05.htm] ). Phage ecosystem ecologists are primarily concerned with the phage impact on the global carbon cycle, especially within the context of a phenomenon know as the microbial loop.
* [http://www.phage.org The Bacteriophage Ecology Group (BEG): Home of Phage Ecology and Phage Evolutionary Biology (www.phage.org)]
* [http://viruses.bluemicrobe.com/ The Virus Ecology Group (VEG)]
* An online, searchable phage ecology bibliography can be found [http://188.8.131.52/RIS/RISWEB.CGI here] (>6000 references).
* [http://cmol.nbi.dk/models/phage/phage.html An interactive model for an evolving ecology of phages and bacteria] .
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