Pulcherrimin: a bacterial swiss army knife in the iron war

Abstract Siderophores are soluble or membrane-embedded molecules that play a major role in Fe acquisition by microorganisms. Pulcherriminic acid (PA) is a compound produced by different microbes that sequesters Fe in the precipitated pulcherrimin, but which role in Fe homeostasis remains elusive. Using Bacillus subtilis (PA producer) and Pseudomonas protegens as a competition model, we demonstrated that PA is involved in a yet undescribed Fe-managing system. When challenged by a competitor, PA production creates a local Fe(III) source, which can be retrieved via the bacillibactin siderophore produced by B. subtilis. Furthermore, precipitation of Fe(III) as pulcherrimin prevents oxidative stress in bacterial competition by restricting the Fenton reaction and deleterious ROS formation. Together, our findings uncover that PA is at the core of a counterintuitive Fe management strategy that capitalizes on controlled Fe precipitation when challenged by a competitor. This makes PA a unique and multifunction tool in the iron war.