Viruses that infect bacteria, known as bacteriophages (phages), alternate between two life cycles: a lytic cycle, in which they infect a host cell, replicate and lyse the cell to release newly formed virions into the environment; and a lysogenic cycle, in which the phage genome integrates into the host DNA as a latent prophage. Some phages control this lysis–lysogeny switch using a peptide-based communication system known as the arbitrium system. Such systems, which are widespread in mobile genetic elements, comprise key genes (aimP, aimR, aimX). The arbitrium communication peptide (AimP) binds its cognate receptor (AimR) to repress aimX (a negative regulator of lysogeny), thereby promoting lysogeny. Until recently, arbitrium systems were thought to be highly specific, with each AimR responding exclusively to its cognate AimP. However, two recent back-to-back studies by Gallego-del-Sol et al. and Manley et al. challenge this view, providing conclusive evidence that arbitrium systems cross-communicate with each other and influence phages’ lysis–lysogeny decisions, with implications for microbial ecology.

In a parallel study, Gallego-del-Sol et al. combined structural, biochemical, genetic and ecological approaches to uncover the basis and implications of arbitrium crosstalk. Following a comprehensive analysis of clade 2 arbitrium systems, the authors identified key sequence constraints that limit AimP diversity and suggested the potential for cross-recognition. Using prophage induction assays and engineered chimeric phages, they demonstrated that crosstalk between arbitrium systems can be both symmetric (bidirectional signal recognition between phages) and asymmetric (unidirectional, with only one system responding to a non-cognate signal), depending on signal–receptor compatibility. They also showed that crosstalk occurs under physiologically relevant conditions. At a mechanistic level, thermofluor and isothermal titration calorimetry revealed that non-cognate peptides can bind AimR receptors with affinities comparable to cognate signals. High-resolution structural analyses further showed how conserved features within the AimR binding pocket enable cross-recognition while preserving selectivity. These molecular interactions were linked to functional outcomes: experiments in mixed lysogenic populations, polylysogens (cells carrying multiple prophages) and infection models established that arbitrium-mediated crosstalk reshapes lysis–lysogeny decisions and influences phage dynamics in complex microbial communities.