Deciphering the Causes of IbfA-Mediated Abortive Infection in the P22-like Phage UAB_Phi20

The study of bacterial defense mechanisms against phages is becoming increasingly relevant due to their impact on the effectiveness of phage therapy. Employing a multifaceted approach that combines bioinformatics, molecular microbiology, TEM microscopy, and conventional microbiology techniques, here, we identify the <i>ibfA</i> gene as a novel defense factor targeting the virulent phage UAB_Phi20, acquired by <i>Salmonella</i> Typhimurium through lateral transfer on the IncI1α conjugative plasmid pUA1135 after oral phage therapy in broilers. IbfA, a two-domain protein containing ATPase and TOPRIM domains, significantly reduces UAB_Phi20 productivity, as indicated by decreased EOP, ECOI, and a diminished burst size, potentially reducing cellular viability without causing observable lysis. Our results indicate that IbfA enhances the transcription of early genes, including the antirepressor <i>ant</i>, which inhibits the C2 repressor of the lytic cycle. This may cause an imbalance in Cro/C2 concentration, leading to the observed reduction in the transcription of late genes encoding structural and cellular lysis proteins, and resulting in the abortion of UAB_Phi20 infection.