BBSRC sLoLa grantThe spread of antimicrobial resistance (AMR) is a slow-moving pandemic that has been identified by the WHO as one of the top 10 threats facing humanity. Plasmids and other MGEs play a key role in the dissemination of AMR, but we have only a rudimentary understanding of the factors that determine if and how MGEs spread through microbial communities. It is generally assumed that bacterial immune systems are a major determinant, but existing studies are limited to only a few stand-alone defence systems. The development of novel bioinformatics approaches led to the discovery of dozens of formerly unknown defences, often clustered within genomic loci known as 'defence islands'. This suggests that bacterial genome defences consist of multiple integrated layers that act in concert to constrain MGE infections; analogous to how our own innate and adaptive immune systems work together to combat pathogen infections. While our preliminary data supports this hypothesis, systematic studies that rigorously examine this novel conceptual framework are lacking.Recent studies by our team members have shown that co-occurring defences can interact synergistically to provide high levels of multi-layered defence. This is extremely novel and raises many important questions. For example, how common is it for different combinations of defences to co-occur in a bacterial genome? What causes defences to interact synergistically or antagonistically? How is expression and activity of multi-layered defences orchestrated within a bacterial cell? And how do bacteria balance the need for strong multi-layered defence against the need to take up beneficial genes? We assembled a multi-disciplinary team of world-leading UK researchers to tackle some of the most pressing questions in the field of microbial genome evolution. Our ambitious goal to tease apart how complete, multi-layered, bacterial immune systems operate at the level of individual molecules, cells, populations and microbial communities requires complementary expertise and experimental capacity in bioinformatics, molecular microbiology, biochemistry, mathematical modelling, microscopy, and experimental evolution techniques. Our research program provides a new tier in our understanding of bacterial genome evolution and goes well beyond the frontiers of bioscience knowledge. The multidisciplinary approaches that are pioneered will transform our understanding of the role of bacterial immune systems in microbial genome evolution and will boost international competitiveness of UK Bioscience. This research falls in the BBSRC priority areas of integrative microbiome research, combatting AMR, systems approaches to biosciences and data driven biology. As far as we know no other team is engaged in addressing these important questions at a scale that is proposed here.