Pathogenic diversification of the gut commensal via acquisition of a second type III secretion system.

is a Gram-negative bacterium found in various water and land environments and organisms, including insects and mammals. Some strains encode gene homologs of virulence factors found in pathogenic Enterobacterales members, such as serovar Typhimurium and . Whether these genes are pathogenic determinants in is not known. In this study, we investigated -host interactions at the cellular level, focusing on the role of two type III secretion systems (T3SS) belonging to the Inv-Mxi/Spa family. T3SS is widespread in spp. and encoded on the chromosome. A large plasmid that is present in a subset of strains, primarily isolated from diarrheal patients, encodes for T3SS. We show that 205/92 is internalized into eukaryotic cells, lyses its internalization vacuole, and proliferates in the cytosol. This triggers caspase-4-dependent inflammasome responses in gut epithelial cells. The requirement for the T3SS in entry, vacuole lysis, and cytosolic proliferation is host cell type-specific, playing a more prominent role in intestinal epithelial cells than in macrophages or insect cells. In a bovine ligated intestinal loop model, colonizes the intestinal mucosa and induces mild epithelial damage with negligible fluid accumulation in a T3SS- and T3SS-independent manner. However, T3SS was required for the rapid killing of . We propose that the acquisition of two T3SS has allowed to diversify its host range, from a highly virulent pathogen of insects to an opportunistic gastrointestinal pathogen of animals.