Systematic functional assessment of antiphage systems in their native host

Bacterial resistance to bacteriophages (phages) relies on two primary strategies: preventing phage attachment and blocking post-attachment steps. These latter mechanisms are mediated by defence systems, including DNA-degrading systems such as Restriction- Modification (RM) and CRISPR-Cas, as well as abortive infection systems that induce cell death or dormancy. Computational studies suggest that bacterial genomes encode several 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 E. coli by testing its susceptibility to 93 phages. Infectivity and adsorption assays using mutants defective in these systems revealed that only PD-T4-3 and RM systems restricted phages able to adsorb. The RM 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 RM systems. Other defence systems showed no detectable activity, likely due 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.