Systematic functional assessment of anti-phage systems in their native host.

Bacterial resistance to bacteriophages (phages) relies on two primary strategies: preventing phage attachment and blocking post-attachment steps. These post-attachment mechanisms are mediated by diverse defence systems, including DNA-degrading systems such as restriction-modification and CRISPR-Cas, along with abortive infection systems that induce cell death or dormancy. Computational analyses suggest that bacterial genomes encode multiple defence systems, which may act synergistically to enhance phage resistance. However, the regulation, interactions and ecological roles of these systems in native hosts remain poorly understood. This study explored the role of eight predicted defence systems in the clinical isolate NILS69 of by testing its susceptibility to 93 phages. Infectivity and adsorption assays using mutants defective in these systems revealed that only PD-T4-3 and restriction-modification systems restricted phages that were able to adsorb. The restriction-modification system acted via a predicted type IV endonuclease and was also able to limit plasmid conjugation if the plasmid was transferred from a donor strain lacking a methylase, which is the hallmark of type I, II or III restriction-modification systems. Other defence systems showed no detectable activity, likely owing to phage specificity, environmental regulation or cofactor requirements. These findings underscore the need for further studies to investigate the regulation and ecological roles of bacterial defence systems in their native host contexts.This article is part of the discussion meeting issue 'The ecology and evolution of bacterial immune systems'.