Oxidative stress changes interactions between two bacterial species

Knowing how species interact within microbial communities is crucial to predicting and controlling community dynamics, but interactions can depend on environmental conditions. The stress-gradient hypothesis (SGH) predicts that species are more likely to facilitate each other in harsher environments. Even if the SGH gives some intuition, quantitative modeling of the context-dependency of interactions requires understanding the mechanisms behind the SGH. In this study, we show with both experiments and a theoretical analysis, that varying the con-centration of a single compound, linoleic acid, modifies the interaction between two bacterial species from competitive at a low concentration, to facilitative at higher concentrations where linoleic acid becomes toxic for one of the two species. We demonstrate that the mechanism behind facilitation is that one species is able to reduce Reactive Oxygen Species (ROS) that are produced spontaneously at higher concentrations of linoleic acid, allowing for short-term rescue of the species that is sensitive to ROS and longer coexistence in serial transfers. In our system, competition and facilitation between species can occur simultaneously, and changing the concentration of a single compound can alter the balance between the two.