Bacteriophages mobilize bacterial defense systems via lateral transduction

To counter challenges from bacteriophages (phages), bacteria use defense mechanisms that can reside on mobile genetic elements or within chromosomes. These immune systems are easily gained and lost, allowing adaptation to threats. However, the mechanism of mobilization of chromosomally encoded defense genes remains poorly understood. Here, we show that phage- and phage-inducible chromosomal island (PICI)-mediated lateral transduction (LT), a highly efficient horizontal gene transfer mechanism, facilitates the transfer of these defense genes between bacteria. Using several bacterial models, we demonstrate that defense systems are often positioned near phage or PICI attachment sites, allowing them to exploit LT for their mobility. In addition, LT diversifies defense genes carried by prophages and PICIs, driving immune system evolution and turnover. These processes provide phage resistance to new bacterial hosts and profoundly affect population genomics. Our findings reveal LT as a crucial mechanism shaping bacterial evolution and influencing the trajectory of pathogenic clones in nature.