Multi-Omic Analysis of Bacteriophage-Insensitive Mutants Reveals a Putative Role for the Rcs Two-Component Phosphorelay System in Phage Resistance Development in Erwinia amylovora

Phage therapy has garnered significant attention due to the rise of life-threatening multidrug-resistant pathogenic bacteria and the growing awareness of the transfer of resistance genes between pathogens. Considering this, phage therapy applications are being extended to target plant pathogenic bacteria, such as <i>Erwinia amylovora</i>, which causes fire blight in apple and pear orchards. Understanding the mechanisms involved in phage resistance is crucial for enhancing the effectiveness of phage therapy. Despite the challenges of naturally developing a bacteriophage-insensitive mutant (BIM) of <i>E. amylovora</i> (without traditional mutagenesis methods), this study successfully created a BIM against the podovirus ϕEa46-1-A1. The parent strain, <i>E. amylovora</i> D7, and the BIM B6-2 were extensively compared at genomic, transcriptomic, and phenotypic levels. The phenotypic comparison included the metabolic behavior, biofilm formation, and <i>in planta</i> evaluations of pathogenicity. The results revealed a mutation in strain B6-2 in the <i>rcsB</i> gene, which encodes a second regulator in the Rcs two-component phosphorelay system (TCS). This mutation resulted in significant changes in the B6-2 BIM, including downregulation of amylovoran gene expression (e.g., an average log₂ fold change of -4.35 across <i>amsA-L</i>), visible alterations in biofilm formation, increased sensitivity to antibiotics (22.4% more sensitive to streptomycin), and a loss of pathogenicity as assessed in an apple seedling virulence model in comparison to the wildtype strain. The findings presented in this study highlight the critical role of the Rcs phosphorelay system in phage resistance in <i>E</i>. <i>amylovora</i>. Based on these findings, we have proposed a model that explains the effect of the B6-2 <i>rcsB</i> mutation on the Rcs phosphorelay system and its contribution to the development of phage resistance in <i>E. amylovora</i>.