A Fluorescent Reporter Reveals On/Off Regulation of the Shigella Type III Secretion Apparatus during Entry and Cell-to-Cell Spread

Franc¸ ois-Xavier Campbell-Valois, Pamela Schnupf, Giulia Nigro, Martin Sachse, Philippe J. Sansonetti, and Claude Parsot

Cell Host & Microbe (15), 177–189, February 12, 2014

Speaker: Yi-Wen Liu (劉怡彣)                              Time: 14:00~15:00, Mar. 14, 2014

Commentator: Dr. Ching-Hao Teng (鄧景浩 老師)   Place: Room 601

Abstract

Several Gram-negative pathogenic bacteria use a type III secretion system (T3SS) to inject proteins into host cells. However, mechanisms regulating the T3SS activity are less known. Detecting and monitoring the T3SS of intracellular bacteria within intact host cells has been challenging. Shigella flexneri is a Gram-negative pathogen. They use their T3SS to enter epithelial cells and cause bacillary dysentery in humans. The authors want to design a transcription-based secretion activity reporters (TSARs) based on the combination of MxiE-dependent promoters controlling expression of a fast-maturing GFP to sense the recent activity of the S. flexneri T3SS in both fixed and live samples. They found that the T3SS is active during entry, inactive in the cytoplasm of host cells, and reactivated during cell-to-cell spread. Further analyses of the TSAR signal in wild type, cell-to-cell spread defected strain and nonmotile bacteria, found that T3SA was less active innonmotile bacteria, but the cell-to-cell spread defected bacteria had high TSAR signal. This result indicates that the T3SA is still functional in non-spreading bacteria. The authors ultimately found out that the entry vacuole and protrusions formed during spreading are the unique sites of T3SS-mediated secretion. Results reported herein suggest that the major factor controlling the activity of the T3SS in S. flexneri is its interaction with plasma membrane-derived compartments, which it manipulates by provoking the rupture of the host membrane in a T3SS and translocators-dependent process. Thus, the T3SS is subject to a tight on/off regulation within the bacterial intracellular niche.

References

1. Cornelis, G.R. (2006). The type III secretion injectisome. Nat. Rev. Microbiol. 4, 811–825.

2.     Kane, C.D., Schuch, R., Day, W.A., Jr., and Maurelli, A.T. (2002). MxiE regulates intracellular expression of factors secreted by the Shigella flexneri 2a type III secretion system. J. Bacteriol. 184, 4409–4419.