(A) In the healthy colon, a group of "good" gut bacteria (Clostridia) produce butyrate, which renders the mucosal surface of the intestine hypoxic (low in oxygen). (B) S. Typhimurium (Salmonella) uses its virulence factors to induce host responses that eliminate Clostridia, reducing the concentration of butyrate in the gut. In the absence of butyrate, mature cells in the colon increase the availability of oxygen (O2) in the lumen of the intestine.  Image from:  \cite{Rivera_Ch_vez_2017}