The sulfur assimilation pathway mitigates redox stress from acidic pH in Salmonella Typhi H58

Salmonella enterica serovar Typhi is the causative agent of typhoid fever, a human-restricted systemic infection. The rapidly disseminating multi-drug resistant H58 subclade is endemic in Africa, Asia, and Oceania, yet little is known regarding its intracellular behavior and virulence properties. It was of interest to understand the H58 response to host cell infection in terms of its response to acid stress and subsequent changes in gene regulation. We compared the H58 response in vitro and during infection of THP-1 human macrophages with the well-characterized response of Salmonella Typhimurium, which causes gastroenteritis. In S. Typhimurium infections, bacteria reside in an acidic intracellular vacuole and become acidified, driving the expression of pathogenicity island 2 genes (SPI-2). SPI-2 virulence factors modify the vacuole and enable bacterial replication. In response to acid stress, the sulfur assimilation pathway is highly upregulated and unique to H58. Replacing the Typhi cysK promoter with the Typhimurium promoter resulted in a cysK gene that was upregulated by acid stress in the H58 background, highlighting the differential regulation between the two serovars. In acidic conditions, H58 experienced much greater redox stress compared with S. Typhimurium, and the sulfur assimilation pathway was required to mitigate the redox stress. Higher redox stress modified the transcriptional regulator SsrB, resulting in diminished secretion of the SPI-2 virulence factor SifA. Our results highlight significant differences between S. Typhi H58 and S. Typhimurium and emphasize the importance of studying S. Typhi strains directly to understand their unique behavior during pathogenesis.