A dual-function microcin in Acinetobacter baumannii facilitates biofilm formation and interbacterial competition

Acinetobacter baumannii is a frequent cause of drug-resistant nosocomial infections. Its ability to persist in harsh environments facilities its spread and is supported by formation of robust biofilms. Biofilms are comprised of communities of bacteria encased in a matrix of exopolymeric substances. Microcins are small (<10kDa) secreted antimicrobial proteins that mediate bacterial competition but have not been associated with biofilms directly. Here we describe the first dual action microcin from A. baumannii that has both antibacterial activity and is required for biofilm formation. We show the ability of this microcin to support biofilm formation is independent of its antibacterial action, and how this microcin forms amyloid fibers in secreted and purified forms, a function that is critical to biofilm formation. Scanning electron microscopy (SEM) shows this microcin surrounding A. baumannii in fibers suggesting it may act as support for polymetric biofilm components. Finally we map the molecular pathway used by this microcin to inhibit prey cells, which uncovers a new host protein that facilities microcin mediated-membrane disruption. This study reports the first instance of a multifunctional microcin, which uncovers new roles for this understudied class of antibacterial proteins. Overall design: RNA seq profiling of A. baumannii 17978 with and without the MccAB1 microcin gene. RNA profiles were collected from stationary phase after 24 hours of growth.