CpxA Deletion Is Associated with Altered Polymyxin B Susceptibility and Flagellar-Associated Transcriptional Remodeling in Salmonella Typhimurium

Abstract

Polymyxin B is considered a last-resort antibiotic for multidrug-resistant or extensively drug-resistant Gram-negative bacterial infections. Understanding the determinants of polymyxin B susceptibility in Salmonella is important for public health. Phenotypic analyses showed that deletion of cpxA was associated with altered stress tolerance and a one-dilution difference in the reported polymyxin B MIC, whereas the ΔcpxAΔarcB double mutant exhibited broader growth and stress-related defects. Benjamini–Hochberg-controlled RNA sequencing under polymyxin B exposure identified 60 differentially expressed genes (35 with higher and 25 with lower expression in ΔcpxA), including 12 flagellar-associated genes and significant enrichment of flagellar functions. RT-qPCR showed significantly higher relative expression of four selected FDR-significant flagellar-associated genes in the ΔcpxA strain; fliC, fliS, fliK, and flgE were directionally concordant with RNA-seq, and expression returned toward WT-like levels in the complemented strain. Representative micrographs illustrated differences in extracellular filamentous appearance. These findings associate cpxA deletion with selective flagellar-associated transcriptional changes under polymyxin B exposure, accompanied by a one-dilution reported MIC difference and qualitative differences in extracellular filamentous appearance.