The significance of mutualistic phages for bacterial ecology and evolution

Bacteria and phages have traditionally been viewed as 'antagonists'. However, temperate phages can transfer genes, which can broaden their bacterial hosts' metabolic repertoire, confer or enhance virulence, or eliminate competing organisms, and so enhance bacterial fitness. Recent evidence shows that phages can also promote biofilm formation leading to population-level benefits for their bacterial hosts. Here, we provide a perspective on the ecological and evolutionary consequences for the bacteria interacting with phages, when phage and host interests are aligned. Furthermore, we examine the question whether bacterial hosts can lower immune barriers to phage infection, thereby facilitating infection by beneficial phages. Taking recent evidence together, we suggest that in many cases temperate phages are to be considered as being mutualistic as well as parasitic, at the same time. Temperate phages can provide a plethora of benefits to bacterial hosts, including the ability to respond rapidly to changes in the environment.Bacterial biofilms can benefit from lysogeny. When phages are induced in a few cells, the lysing cells release nutrients and extracellular DNA locally, which may strengthen biofilm structure. Further, lysis breaks biofilm bonds, allowing for enhanced cell dispersal.In bacterial competition, phages can facilitate bacteriocin release or act as 'weapons' against competitors themselves.Recent evidence indicates that bacterial populations may modulate phage infections, as individual cells may lose immunity or selectively allow infections to occur. The capacity of CRISPR-Cas immune systems to discriminate between lytic and temperate phages (based on gene expression) may enable bacterial hosts to selectively permit temperate phages to enter