Antibacterial microcins self-assemble to promote biofilm formation in Acinetobacter baumannii.

The bacterial pathogen Acinetobacter baumannii forms a resilient biofilm, enhancing bacterial survival in hostile environments. Although biofilm structure is primarily built from polysaccharides and nucleic acids, specific proteins are often essential for its development and integrity. Here, we identify microcins as a driver of biofilm formation in A. baumannii. Microcins are small (<10 kDa), secreted, antimicrobial proteins that typically mediate bacterial competition, yet their function beyond the antibacterial is unclear. We demonstrate that A. baumannii microcins use conserved sequence motifs to self-assemble into amyloid fibers, promoting biofilm formation. Scanning electron microscopy reveals microcin fibers extending from A. baumannii, suggesting a structural role as a biofilm scaffold. We also identify proteins required for microcin's antibacterial function that are dispensable for biofilm formation, further distinguishing these two activities. Our findings show that microcins directly promote biofilm development and broaden understanding of the diverse functions of these understudied proteins.