Vi-suppressed wild strain Salmonella typhi cultured in high osmolarity is hyperinvasive toward epithelial cells and destructive of peyer's patches

Salmonella typhi GIFU10007-3 which lost a viaB locus on its chromosome beca me highly invasive in our previous study. To investigate the phenomenon, we controlled Vi expression in wild strain S. typhi GIFU10007, and studied th e invasive phenotype both in vitro and in vivo. When the wild strain of S. typhi was cultured in 300 mM NaCl containing Luria-Bertani broth (LBH), the expression of Vi antigen was suppressed, but secretion of invasion protein s (SipC, SipB and SipA) was increased. In this condition, wild strain S, ty phi became highly invasive toward both epithelial cells and Ill cells of ra t Peyer's patches. When GIFU10007 was cultured under conditions of high osm olarity, the bacteria disrupted Fever's patches and induced massive bleedin g in these structures only 20 min after inoculation into the ileal loop. In contrast, Vi-encapsulated wild strain GIFU10007 cultured under low osmolar ity was not destructive, even after 60 min. To understand the role of the t ype III secretion system under conditions of high osmolarity, we knocked ou t the invA and sipC genes of both GIFU10007 acid GIFU10007-3. Neither invA nor sipC mutants could invade epithelial cells or M cells in a high osmolar ity environment. Our data show that the highly invasive phenotype was only expressed when the wild strain S. typhi was cultured under high osmolarity, which induced a state of Vi suppression, and in the presence of the type I II secretion system.