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

Acinetobacter baumannii is highly drug-resistant and persists in diverse environments, making it a challenging nosocomial pathogen. A. baumannii forms robust biofilms, a fact that enables it to persist in clinical settings and facilitate bacterial community interactions. Here we report that a recently reported microcin is required for biofilm formation in A. baumannii. Microcins are small (<10kDa) secreted antimicrobial proteins that have been reported to mediate bacterial competition. Interestingly, microcins have never been documented as being related to biofilm production. We identify that this microcin forms amyloid fibers in biofilms and purified form, a function that is critical to biofilm formation. Scanning electron microscopy (SEM) shows this microcin present in fibrils surrounding A. baumannii. We also validate and identify new proteins that are required for microcin antimicrobial activity and report that the microcin kills target cells by disrupting the bacterial inner membrane. This study reports the first instance of a microcin exhibiting dual functionalities, one associated with antimicrobial activity against prey strains and the other essential for biofilm formation in A. baumannii.