Overexpression of <i>Escherichia coli yaiX</i> Confers Multidrug Resistance and Enhances Virulence in the Silkworm Infection Model

The emergence of bacteria with both antimicrobial resistance and high virulence has become a global health concern, underscoring the urgent need to elucidate the molecular basis underlying these traits. Here, we employed the silkworm (Bombyx mori) infection model, which is suitable for high-throughput screening, together with an Escherichia coli library containing plasmid clones of all genes from strain W3110, to identify genes whose overexpression enhances virulence. We found that overexpression of the uncharacterized protein YaiX promoted bacterial proliferation in silkworms and increased host lethality. Compared with the empty-vector control, the YaiX-overexpressing strain exhibited resistance to multiple antimicrobial agents with diverse mechanisms of action, including β-lactams, tetracyclines, fluoroquinolones, aminoglycosides, cationic surfactants, and hydrogen peroxide. Sequence analysis revealed that amino acids 18-52 of YaiX contain a transferase hexapeptide domain predicted to form a left-handed parallel β-helix. Overexpression of YaiX mutants lacking regions outside this domain conferred ampicillin resistance, whereas deletion of the hexapeptide domain abolished this phenotype. RNA sequencing and GO enrichment analyses further indicated that YaiX overexpression altered the expression of genes encoding RNA-binding proteins and porins. These findings suggest that YaiX overexpression, through its hexapeptide domain, modulates gene expression and contributes to both multidrug resistance and enhanced virulence in E. coli.