Joshi, K.; Fong, W. Y.; Blanc, M.-P.; Guerra, F. E.; Fang, F. C.

Salmonella enterica encounters acid stress during gastrointestinal transit and within the phagosomal environment of macrophages. Acid stress resistance has been well characterized in Salmonella enterica serovar Typhimurium, but comparative studies in the human-adapted Salmonella enterica serovar Typhi are limited. We compared the growth of S. Typhimurium 14028s and S. Typhi Ty2 at pH values ranging from 3-8 and observed that Salmonella enterica serovar Typhimurium exhibits enhanced growth at pH 4.5 compared to S. Typhi. Comparative transcriptomic profiling of S. Typhimurium and S. Typhi at pH 4.5 and 7.5 identified numerous differentially expressed acid-induced genes (DEGs), including genes encoding membrane proteins (OmpC, PhoE, HydB), a transcriptional regulator (RpoS), and stress response proteins (YciG, STM14_1829, YmdF). Targeted deletion of selected genes in S. Typhimurium significantly suppressed growth at acidic pH, confirming their role in acid stress resistance. These resistance mechanisms are compromised in S. Typhi due to pseudogenization. Heterologous expression of pseudogenized genes in S. Typhi restored acid tolerance. Collectively, these findings suggest that S. Typhi has lost the ability to withstand acid stress due to genomic decay and the loss of multiple genes essential for acid survival in S. Typhimurium, reflecting divergent evolutionary paths in these two serovars.