O2-dependent incapacitation of the Salmonella pathogenicity island 1 repressor HilE

For successful colonization, pathogenic bacteria need to adapt their metabolism and virulence functions to challenging environments within their mammalian hosts that are frequently characterized by low oxygen (O₂) tensions. Upon oral ingestion, the human pathogen <i>Salmonella enterica</i> serovar Typhimurium (<i>S</i>. Typhimurium) is exposed to changing O₂ and pH levels. Low concentrations of O₂, which can enhance the virulence of enteroinvasive pathogens, facilitate the expression of the type three secretion system (T3SS-1) encoded by the <i>Salmonella</i> pathogenicity island 1 (SPI-1) that is critical for enteroinvasion and pathogenicity of <i>S.</i> Typhimurium. To study the impact of key environmental cues of the intestine when <i>Salmonella</i> encounter enterocytes, we established an <i>in vitro</i> growth model, which allows shifting the concentration of O₂ from 0.5% to 11% and the pH from 5.9 to 7.4 in the presence of acetate and the alternative electron acceptor nitrate. Compared to normoxia, hypoxia elevated the expression of SPI-1 genes encoding T3SS-1 translocators and effectors, which resulted in higher invasion and effector translocation in epithelial cells. While hypoxia and pH shift only marginally altered the gene expression of SPI-1 regulators, including the SPI-1 repressor <i>hilE</i>, hypoxia and pH shift completely incapacitated HilE in a post-translational manner, ultimately promoting SPI-1 activity. From these findings, we conclude that O₂-dependent HilE function allows for ultrasensitive adaptation of SPI-1 activity in environments with varying O₂ availability such as the intestinal tract.