**2.4 Type III secretion system for colonization and injection of effectors into eukaryotic host cells**

The Type III secretion system (T3SS) is a multicomponent device translocating various effectors directly into the neighbouring eukaryotic host cells and is found in many *Vibrio* species, including *V. cholerae* [109]. Many non O1/O139 strains, which can lead to severe diarrhea even though they do not produce the CT and toxin coregulated pilus (TCP), possess a T3SS [110, 111]. Unlike the pandemic strains, the diarrhea induced by non O1/O139 strains shows damages to the intestinal epithelium [112]. The T3SS would in fact be essential for intestinal colonization and invasive diarrhea in those strains. The T3SS is composed of a basal structure that shares similarities with the flagella, and a needle, connecting the bacterial cytoplasm to the eukaryotic cell using a pore at its end [113]. While the structure of the T3SS is conserved among Gram-negative bacteria, the effectors encoded differ from one another, subsequently to the intended host. In *V. cholerae*, the translocated effectors are multiple, and their accumulation disrupts host cellular processes with a key role in the early stages of infection, such as cytoskeletal rearrangement and cytotoxicity, resulting in intestinal epithelial damages and colonization of the gut [109, 114].

#### *2.4.1 Structure and secretion through the T3SS*

The T3SS is a multicomponent apparatus spawning both bacterial membranes. While the effectors' secretion through T3SS is Sec independent, the translocation of the membrane components of the injectosome requires it [113]. The T3SS uses ATP for the active translocation of the effectors through both bacterial membranes directly into host cell cytoplasm. The T3SS consists of an injectosome with structural and genetic homology to the flagellum and a molecular syringe, the structure has been reviewed elsewhere [113]. In brief, the syringe connects the membrane complex to the host cell cytoplasm. It is composed of (i) a basal needle, (ii) a tip and, at its end, (iii) a translocation pore. The membrane complex is composed of an assembly of concentric rings creating a channel through both bacterial membranes. It includes an outer membrane ring connected, in the periplasm, to the inner membrane ring, in addition to a cytoplasmic portion, made of a cytoplasmic ring and an ATPase complex. The exact T3SS assembly in *V. cholerae* has not been studied yet, and some components remain to be identified [109].
