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

Area of Science:

• Bacteriology
• Microbial Ecology
• Genetics

Background:

• Bacteria employ diverse defense systems against bacteriophages (phages), including DNA degradation and abortive infection.
• Multiple defense systems may function synergistically, but their regulation and ecological roles in native hosts are poorly understood.

Purpose of the Study:

• To investigate the function of eight predicted defense systems in the clinical isolate *Escherichia coli* NILS69 against 93 phages.
• To identify which defense systems are active against phages that successfully adsorb to the bacterial cell.

Main Methods:

• Generated mutants of *E. coli* NILS69 lacking specific defense systems.
• Performed infectivity and adsorption assays using 93 different phages.
• Analyzed the activity of the restriction-modification system in plasmid conjugation.

Main Results:

• Only the PD-T4-3 and restriction-modification systems effectively restricted phages that had adsorbed.
• The restriction-modification system, via a type IV endonuclease, also inhibited plasmid conjugation.
• Other tested defense systems showed no detectable activity, possibly due to phage specificity or environmental factors.

Conclusions:

• The restriction-modification system and PD-T4-3 are significant contributors to phage resistance in *E. coli* NILS69.
• Further research is needed to elucidate the regulation and ecological significance of bacterial defense systems in their natural contexts.