Bacteriophages benefit from mobilizing pathogenicity islands encoding immune systems against competitors

Bacteria encode sophisticated anti-phage systems that are diverse and versatile and display high genetic mobility. How this variability and mobility occurs remains largely unknown. Here, we demonstrate that a widespread family of pathogenicity islands, the phage-inducible chromosomal islands (PICIs), carry an impressive arsenal of defense mechanisms, which can be disseminated intra-and inter-generically by helper phages. These defense systems provide broad immunity, blocking not only phage reproduction, but also plasmid and non-cognate PICI transfer. Our results demonstrate that phages can mobilize PICI-encoded im-munity systems to use them against other mobile genetic elements, which compete with the phages for the same bacterial hosts. Therefore, despite the cost, mobilization of PICIs may be beneficial for phages, PICIs, and bacteria in nature. Our results suggest that PICIs are important players controlling horizontal gene trans-fer and that PICIs and phages establish mutualistic interactions that drive bacterial ecology and evolution.

Automated summary

This study discovers a important anti-phage system in bacteria called phage-inducible chromosomal islands (PICIs), which can be disseminated intra-and inter-generically by helper phages and provide broad immunity. Results show that phages can mobilize PICI-encoded immunity systems to use them against other mobile genetic elements, which compete with the phages for the same bacterial hosts. This finding has significant implications for phages, bacteria, and horizontal gene transfer control.