The landscape of lysogeny across microbial community density, diversity and energetics.

Lysogens are common at high bacterial densities, an observation that contrasts with the prevailing view of lysogeny as a low-density refugium strategy. Here, we review the mechanisms regulating lysogeny in complex communities and show that the additive effects of coinfections, diversity and host energic status yield a bimodal distribution of lysogeny as a function of microbial densities. At high cell densities (above 106  cells ml-1 or g-1 ) and low diversity, coinfections by two or more phages are frequent and excess energy availability stimulates inefficient metabolism. Both mechanisms favour phage integration and characterize the Piggyback-the-Winner dynamic. At low densities (below 105  cells ml-1 or g-1 ), starvation represses lytic genes and extends the time window for lysogenic commitment, resulting in a higher frequency of coinfections that cause integration. This pattern follows the predictions of the refugium hypothesis. At intermediary densities (between 105 and 106  cells ml-1 or g-1 ), encounter rates and efficient energy metabolism favour lysis. This may involve Kill-the-Winner lytic dynamics and induction. Based on these three regimes, we propose a framework wherein phage integration occurs more frequently at both ends of the host density gradient, with distinct underlying molecular mechanisms (coinfections and host metabolism) dominating at each extreme.