Post genome uptake into a now phage-infected bacterium, a temperate phage must literally decide between displaying two distinct life styles: lytic cycle versus lysogenic cycle.
The regulatory mechanisms underlying the lytic-lysogeny decision can be described as giving rise to a bistable switch. The molecular details of this mechanisms are best worked out for the temperate phage lambda (λ).
Under certain circumstances this decision is based towards lysogenic cycles, such as when multiplicities of phage adsorption per infected bacterium are somewhat higher than one. In these instances it is a reasonable inference that other bacteria are experiencing similar levels of phage infection and therefore that bacteria which are susceptible to the phage in question are declining in density within the environment (due to phage infection). Under such circumstances, lysogeny would appear to be the preferred strategy, as consistent with the so-called "hard times" hypothesis. By contrast, when uninfected bacteria are abundant then lytic infections along with subsequent production of phage-virion progeny would appear to be the more advantageous strategy since bacterial acquisition is dependent upon phages displaying productive infections.
In bacteriophage lambda (λ) the lytic-lysogeny decision is dependent on levels of the lambda protein CII that are found within infected bacterium. High levels results in little production of the CI repressor and consequently a lytic cycle. Lower levels, by contrast, allow CI production and a resulting tripping of the switch to a lysogenic cycle. The actual decision is made quite far into the lambda infection, occurring after about 10 min under typical laboratory growth conditions. Factors that seem to bias the lytic-lysogeny decision for phage lambda towards lysogeny include lower temperatures, less nutrient-rich (or simply "poorer") media, or high multiplicities of phage adsorption.
Though not necessarily universally appreciated, in fact most lytic-lysogeny decisions, at least in the laboratory, seem to result in lytic cycles rather than lysogenic cycles. In other words, temperate phages in many instances can be viewed first as lytic phages and only relatively rarely do they display lysogenic cycles. To a degree this makes sense ecologically since the fact that a phage has found a bacterium is, on average, consistent with a potential for the virion progeny of those phages to also find a bacterium to infect.
References:
Little, J. W. (2005). Lysogeny, prophage induction, and lysogenic conversion. In: Phages: Their Role in Bacterial Pathogenesis and Biotechnology. Waldor, M. K., Friedman, D. I., and Adhya, S. L. (eds), ASM Press, Washington, DC, pp. 37-65.
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