Our study showed that bile-mediated modulation of bacterial metabolism involves induction of various metabolic processes, e.g., anaerobic respiration dependent on nitrate. We observed that the activation of the nitrate metabolism-related genes fnr and narL is notably higher in the bile-tolerant WT strain compared to the bile-sensitive ΔcspE strain. Consequently, the WT strain displays lower amounts of reactive oxygen species and higher survival compared to the ΔcspE strain during bile stress.

Salmonella Typhimurium is an enteric pathogen that is highly tolerant to bile. Next-generation mRNA sequencing was performed to analyze the adaptive responses to bile in two S. Typhimurium strains: wild type (WT) and a mutant lacking cold shock protein E (ΔcspE). CspE is an RNA chaperone which is crucial for survival of S. Typhimurium during bile stress. This study identifies transcriptional responses in bile-tolerant WT and bile-sensitive ΔcspE. Upregulation of several genes involved in nitrate metabolism was observed, including fnr, a global regulator of nitrate metabolism. Notably, Δfnr was susceptible to bile stress. Also, complementation with fnr lowered reactive oxygen species and enhanced the survival of bile-sensitive ΔcspE. Importantly, intracellular nitrite amounts were highly induced in bile-treated WT compared to ΔcspE. Also, the WT strain pre-treated with nitrate displayed better growth with bile. These results demonstrate that nitrate-dependent metabolism promotes adaptation of S. Typhimurium to bile.