Shigella flexneri (S. flexneri) is a Gram-negative bacterium that causes gastroenteritis and shigellosis in humans. In order to establish and maintain an infection, S. flexneri utilises a type three secretion system (T3SS) to deliver virulence factors called effector proteins into the cytoplasm of host cells, facilitating e.g. uptake into the host cell and escape from the endosome. Secretion through the T3SS is tightly regulated and is usually triggered by host-cell contact, but can also be artificially stimulated in vitro. In this assay, the dye Congo red is used to induce T3SS-dependent secretion of S. flexneri (Parsot et al., 1995) and secreted proteins are concentrated from the culture supernatant by precipitation with trichloroacetic acid. The assay presented here can easily be adapted to the secretion analysis of other bacteria utilising a T3SS, such as Salmonella typhimurium, which constitutively secrete when grown at 37 °C (Collazo et al., 1995; Pegues et al., 1995), or pathogenic species of Yersinia, where secretion can be induced by calcium deprivation (Heesemann et al., 1986; Forsberg et al., 1987).
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