Making the Best of Aggression: The Many Dimensions of Bacterial Toxin Regulation

Most bacteria use toxins to exclude competitors. As the synthesis and delivery of these molecules entail considerable costs for the producers, their expression is tightly regulated, often by molecular systems detecting physiological stresses or environment-specific cues. However, the ecological connection between such systems and competitive behaviors is not always clear. Here, we review the regulation of antibacterial toxins and propose a conceptual framework organizing the decision-making processes controlling toxin production. As bacteria are unable to precisely identify their competitors, we argue that toxin regulation primarily responds to cues directly or indirectly associated with the presence of competing strains. The density and fitness of the producing population also play a role in the decision-making process. Overall, we contend that optimal toxin production strategies involve monitoring of both self and foe.