ABSTRACTShigella flexneriis an intracellular bacterial pathogen that invades epithelial cells in the colonic mucosa, leading to bloody diarrhea. A previous study showed thatS. flexneriforms biofilms in the presence of bile salts, through an unknown mechanism. Here, we investigated the potential role of adhesin-like autotransporter proteins inS. flexneribiofilm formation. BLAST search analysis revealed that theS. flexneri2457T genome harbors 4 genes,S1242,S1289,S2406, andicsA, encoding adhesin-like autotransporter proteins. Deletion mutants of theS1242,S1289,S2406andicsAgenes were generated and tested for biofilm formation. Phenotypic analysis of the mutant strains revealed that disruption oficsAabolished bile salt-induced biofilm formation. IcsA is an outer membrane protein secreted at the bacterial pole that is required forS. flexneriactin-based motility during intracellular infection. In extracellular biofilms, IcsA was also secreted at the bacterial pole and mediated bacterial cell-cell contacts and aggregative growth in the presence of bile salts. Dissecting individual roles of bile salts showed that deoxycholate is a robust biofilm inducer compared to cholate. The release of the extracellular domain of IcsA through IcsP-mediated cleavage was greater in the presence of cholate, suggesting that the robustness of biofilm formation was inversely correlated with IcsA processing. Accordingly, deletion oficsPabrogated IcsA processing in biofilms and enhanced biofilm formation.