Lose to win: marT pseudogenization in Salmonella enterica serovar Typhi contributed to the surV-dependent survival to H2O2, and inside human macrophage-like cells

Abstract The difference in host range between Salmonella enterica serovar Typhimurium ( S . Typhimurium) and Salmonella enterica serovar Typhi ( S . Typhi) can be partially attributed to the gain of functions, to the loss of functions (i.e. pseudogenization), or to a combination of both processes. As previously reported, the loss of functions by pseudogenization may play a role in bacterial evolution, especially in host-restricted pathogens such as S . Typhi. The marT-fidL operon, located at the SPI-3, encodes the MarT transcriptional regulator and a hypothetical protein (i.e. FidL) with no significant similarities to known proteins, respectively. Even though predicted S . Typhimurium FidL exhibit 99.4% identity with S . Typhi FidL, marT has been annotated as a pseudogene in S . Typhi. In this work, we found that S . Typhi expressing S . Typhimurium marT-fidL exhibited an increased accumulation of reactive oxygen species (ROS), leading to a decreased survival in presence of H 2 O 2 . Moreover, we found that that the presence of a functional copy of S . Typhimurium marT-fidL in S . Typhi resulted in a repression of surV (STY4039), an ORF found in the S . Typhi SPI-3 but absent from S . Typhimurium SPI-3, that contribute to the resistance to H 2 O 2 by decreasing the accumulation of ROS. Finally, we observed that the presence of S . Typhimurium marT-fidL in S . Typhi negatively affected the survival inside macrophage-like cells, but not in epithelial cells, after 24 h post infection. Therefore, this work provides evidence arguing that marT pseudogenization in Salmonella Typhi contributed to the surV -dependent survival against H 2 O 2 , and inside human macrophage-like cells. This is a good example of how the loss of functions ( marT pseudogenization) and the gain of functions (presence of surV ) might contribute to phenotypic changes improving virulence.