Counting to two: how phages decide between lysis and lysogeny

Upon infecting a bacterium, temperate phages must decide between killing the cell to reproduce (lysis) or entering a symbiotic lifestyle (lysogeny). This choice is often informed by the cell’s state, as well as the number of infecting phage particles (MOI). Since phage gene copy numbers scale identically with MOI, an MOI-dependent decision requires a fast-acting asymmetry between the lytic and lysogenic pathways. We introduce a minimal model suggesting that only a handful of coupling mechanisms can produce such an asymmetry; for instance via a host protease, kinase, or RNase acting on one pathway. By distilling complex regulatory networks to their essential components, our model clarifies the logic of lysis-lysogeny decision mechanisms across phage species.