SigA forms amyloid fibrils and supports the competition of Shigella with the microbiota

Shigella sonnei is an enteropathogen that causes bacillary dysentery. During the first step of its virulence cycle, it must outcompete the resident microbiota to establish its own niche. Here we report that SigA, the sole SPATE (Serine Protease Autotransporter of Enterobacteriaceae) family member in S. sonnei, plays an indirect but central role in this process. A genome-wide analysis showed that the SPATE family includes SigA, Pic, SepA, and Sat. We demonstrated that SigA self-assembles into amyloid fibrils (F-SigA) independently of its protease activity. F-SigA remains associated with the S. sonnei surface in vitro and in vivo. Purified F-SigA fibrils have a diameter of 17.7 {+/-} 3.2 nm, and their amyloid organization was confirmed using specific markers and biochemical methods. F-SigA is secreted into the lumen in vivo and localizes to the surface of the colonic epithelium. We found that colicin E1 (ColE1) interacts with F-SigA amyloid fibrils, and that F-SigA-ColE1 complexes display antimicrobial activity that promotes S. sonnei competition with other bacteria. Because Pic, another Shigella SPATE, also forms amyloid fibrils, we anticipate that this virulence mechanism may be relevant across a wide range of Shigella strains and enterobacteria and may serve additional roles during the Shigella virulence cycle.