Pathogenic Diversification of the Gut Commensal Via Acquisition of a Second Type III Secretion System

is a Gram-negative bacterium found in a wide variety of water and land environments and organisms. It has been isolated as part of the gut microbiome of animals and insects, as well as from stool samples of patients with diarrhea. Specific strains encode gene homologs of virulence factors found in other pathogenic members of the same Enterobacterales order, such as serovar Typhimurium and Whether these genes are also pathogenic determinants in is not known. Here we have used 205/92, a clinical isolate, with and infection models to investigate -host interactions at the cellular level. Our particular focus was 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. T3SS is encoded on a large plasmid that is present in a subset of strains, which are primarily isolates from diarrheal patients. Using a combination of electron and fluorescence microscopy and gentamicin protection assays we show that 205/92 is internalized into eukaryotic cells, rapidly 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 human intestinal epithelial cells as compared to macrophages. In a bovine ligated intestinal loop model, colonizes the intestinal mucosa, inducing mild epithelial damage with negligible fluid accumulation. No overt role for T3SS or T3SS was seen in the calf infection model. However, T3SS was required for the rapid killing of . We propose that the acquisition of two T3SS by horizontal gene transfer has allowed to diversify its host range, from a highly virulent pathogen of insects to an opportunistic gastrointestinal pathogen of animals.