Siderophores drive invasion dynamics in bacterial communities through their dual role as public good versus public bad

Microbial invasions can compromise ecosystem services and spur dysbiosis and disease in hosts. Nevertheless, the mechanisms determining invasion outcomes often remain unclear. Here, we examine the role of iron-scavenging siderophores in driving invasions of Pseudomonas aeruginosa into resident communities of environmental pseudomonads. Siderophores can be 9public goods9 by delivering iron to individuals possessing matching receptors; but they can also be 9public bads9 by withholding iron from competitors lacking these receptors. Accordingly, siderophores should either promote or impede invasion, depending on their dual effects on invader and resident growth. Using supernatant feeding and invasion assays, we show that invasion success indeed decreased when the invader was inhibited (public bad) rather than stimulated (public good) by the residents9 siderophores. Conversely, invasion success often increased when the invader could use its siderophore to inhibit the residents. Our findings identify siderophores as a major driver of invasion dynamics in bacterial communities.