CONTACT OF SHIGELLA WITH HOST-CELLS TRIGGERS RELEASE OF IPA INVASINS AND IS AN ESSENTIAL FUNCTION OF INVASIVENESS

The invasion of colonic epithelial cells by Shigella, an early essenti al step for causing bacillary dysentery, is mediated by the IpaB, IpaC and IpaD proteins, Secretion of the Ipa proteins from Shigella requir es functions encoded by the mxi and spa loci. In this study, we show t hat contact between the bacteria and epithelial cell triggers release of the Ipa proteins into the external medium, which results in a rapid decrease in levels of Ipa proteins presented on the cell surface. Whe n the bacteria were used to infect polarized Caco-2 cells, release of Ipa proteins occurred efficiently from bacteria interacting with the b asolateral surface rather than with the apical surface. Moreover, the interaction of bacteria with components of the extracellular matrix, s uch as fibronectin, laminin or collagen type IV, also stimulates the r elease of Ipa proteins. The release of Ipa proteins from Shigella requ ired the surface-located Spa32 protein encoded by one of the spa genes on the large plasmid.