Transcriptomic Analysis of Salmonella Typhimurium After Exposure to Blue Light

Antimicrobial blue light (aBL) at 405 nm reduces the viability of foodborne pathogens, including Salmonella. Visible light, including blue light, can kill bacteria by a mechanism that involves reactive oxygen species (ROS) generated after excitation of endogenous photosensitizers. However, the molecular response of Salmonella to aBL is largely unknown. This study was undertaken to characterize differential gene expression in Salmonella Typhimurium strain 14028s following sublethal exposure to 405 nm aBL and to evaluate the phenotypic relevance of selected upregulated genes. Cell suspensions were exposed to aBL (252 J/cm²) at 21°C, and RNA was extracted from aBL-treated and untreated bacterial suspensions. Libraries were constructed, and RNA-seq was performed on an Illumina NextSeq 2000 with paired-end sequencing. Differential gene expression analysis was performed using DESeq2, with fold changes (|log₂FC| > 1, FDR < 0.05) considered differentially expressed. RNA-seq analysis revealed significant changes in gene expression patterns following sub-lethal aBL exposure. Overall, 238 genes were upregulated, and 1,077 genes were significantly downregulated (|log₂FC| >1, FDR<0.05). Functional annotation analysis indicated that 24 differentially expressed genes (DEGs) were primarily involved in catalytic activity. Notably, katG, which encodes a catalase enzyme involved in hydrogen peroxide hydrolysis and overall oxidative stress defense, was markedly upregulated (log₂FC =4.5). The expression of four genes in the L-ascorbate dissimilation operon (ulaBEGR) and two genes involved in oxidative stress response (ahpF, ahpC) was significantly increased. Mutants lacking katG, ulaB, ulaE, ulaG, or ulaR were more sensitive to aBL treatment than the wild-type control strains (P<0.05). This RNA-seq analysis generated valuable findings about the molecular mechanisms underlying Salmonella's response to aBL exposure. Understanding the aBL antimicrobial mechanisms may contribute to the development of more effective strategies for enhancing food safety by identifying factors or hurdle treatments that could help overcome bacterial responses to aBL.