The "Kill the Winner" hypothesis (KTW) is a model of population growth involving

prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1 ...

es and

protozoan Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...

s that links trophic interactions to biogeochemistry. It is based on the concept of prokaryotes taking one of two reactions to limited resources: "competition", that is, that priority directed to growth of the population, or a "winner"; and "defense", where the resources are directed to survival against attacks. It is then assumed that the better strategy for a

phage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacterio ...

, or virus which attacks prokaryotes, is to concentrate on the "winner", the most active population (possibly the most abundant). This tends to moderate the relative populations of the prokaryotes, rather than the "winner take all". The model is related to the

Lotka–Volterra equations The Lotka–Volterra equations, also known as the predator–prey equations, are a pair of first-order nonlinear differential equations, frequently used to describe the dynamics of biological systems in which two species interact, one as a pred ...

. Current understanding on KTW stems from our knowledge of lytic viruses and their host populations. The competition specialist, or “winner”, often corresponds to the most abundant population in the community. Their abundance and activity increase when the population competes for a shared limiting resource (e.g. phosphate) and win. The resource can exist as a free form or something that needs to be sequestered from biomass. Competition specialists (predators, grazers, parasites) are expected to dominate in oligotrophic environments, whereas they would be the losers in a eutrophic environment. The increased abundance and activity of the “winner” also increases viral predation. Defense specialists, tend to invest resources in avoidance strategies that may result in reduced growth and reproduction of the population; hence, the “loser” does not increase viral predation. Defense specialists are expected to dominate in eutrophic environments. KTW represents an idealized microbial food web with mathematical parameters that only account for viral predation we have studied ''in vitro''. It is related to Lotka-Volterra type equations. The KTW model is based on the assumption of stable environmental conditions and is widely applicable to different trophic levels and complex microbial systems; however, it may not always be correct. Because of its reliance of stable environmental conditions, it can only predict a small time point through a microbial community's history. It also disregards the fact that a prokaryotic species can be attacked by more than one virus population at a time. KTW will become more accurate, or even replaced, as more methodological limitations are explored for microbial communities. Piggyback-the-Winner (PTW) is a similar dynamic model of bacteria-virus interactions but incorporates the viral life cycle into the model. PTW model states that the nonlinear relationship between viruses and prokaryotes is observed due to viral dynamics being suppressed at high host densities and super-infection exclusion, rather than developed “resistance” as suggested by the KTW model.

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