Modifying and reacting to the environmental pH drives bacterial interactions

Microbes usually exist in communities consisting of myriad different but interacting species. These interactions are typically mediated through environmental modifications; microbes change the environment by taking up resources and excreting metabolites, which affects the growth of both themselves and also other microbes. A very common environmental modification is a change of the environmental pH. Here we show that changing and reacting to the pH leads to a feedback loop that can determine the fate of bacterial populations. We find experimentally that these pH changes can decide the fate of bacterial populations; they can either facilitate or inhibit growth, and in extreme cases will cause extinction of the bacterial population. Moreover, modifying the pH can determine the interactions between different species. Understanding how single species change the pH and react to it allowed us to estimate their pairwise interaction outcomes. Those interactions lead to a set of generic interaction motifs—bistability, succession, murder suicide, and mutualism—that may be independent of which environmental parameter is modified and thus reoccur in different microbial systems.