Endogenous superoxide is a key effector of the oxygen sensitivity of a model obligate anaerobe

Significance Microbes display profound differences in their tolerance for oxygen, and this trait organizes the structure of many microbial communities. However, the molecular basis of oxygen sensitivity is not well understood. In this study we determined that Bacteroides thetaiotaomicron , an abundant member of the human intestinal flora, is incapacitated by superoxide stress when it enters a fully oxic environment. The key difference from oxygen-tolerant bacteria lies not in its defensive systems, nor in the nature of the affected enzymes, but in the rate of endogenous oxidant formation. Anaerobes thrive in oxygen-poor environments because they deploy low-potential electron-transfer pathways; these results suggest that an ancillary effect is the reactivity of these pathways with oxygen, thereby generating enough reactive oxygen species to preclude oxic growth.