Modulation of Shigella virulence in response to available oxygen in vivo

Benoit Marteyn, et al. 2010. Nature. 465, 355-358

Speaker: Chung-Han Shieh（謝宗翰）                       Time: 15:00~16:00, Nov. 17, 2010

Commentator: Dr. Jiunn-Jong Wu (吳俊忠博士)         Place: Room 601

Abstract:

Shigella species comprise gram-negative bacteria that can invade into intestinal epithelial cells and cause dysentery. The invasion of epithelial cells by this bacterium is governed by its type three secretion system (T3SS). T3SS forms a syringe-like structure that can inject effectors, like Ipa proteins, into the host cells. To seek for the genes in S.flexneri involved in the invasion of host cells, the authors generated a transposon mutant bank, and in which a fnr mutant showing significantly decreased invasion of HeLa cells was identified. FNR (fumarate and nitrate reduction) has been known as a regulator of anaerobic metabolism. The authors then demonstrated in a rabbit infection model that FNR was essential for the invasion of S. flexneri into intestinal epithelial cells. They further found that the secretion of effectors by T3SS was upregulated and the needle length varied in the fnr mutant. In addition, the needles of the wild-type strain grown in the anaerobic conditions were significantly longer than those grown in the aerobic conditions. It has been shown previously that both spa32 and spa33 are required for the effectors secretion and needle length regulation, therefore, the authors checked the expression levels of spa32 and spa33 in the fnr mutant. They found that loss of FNR resulted in increased transcriptional levels of spa32 and spa33 and that FNR could bind to the promoters ofthese two genes. Compared to the wild-type strain, deletion of the FNR-binding sequences in spa32 and spa33 (p＃32/p＃33) lead to T3SS effector secretion in the absence of O₂. However, the invasion ability of p＃32/p＃33 mutant was much worse than the wild-type strain under the anaerobic condition. They then detected the presence of O₂ at a concentration sufficient to activate the secretion of T3SS effectors on the cell surface by microelectrode. In a clamp vascular rabbit model, they also detected a zone above the villi which contained sufficient oxygen concentration to attract S. flexneri. Collectively, their data suggest that Shigella might be primed in the anaerobic environment to elongate the needle of T3SS first and then delivered the maximal amount of effectos in the presence of O₂ to result in bacterial invasion into the host cells.

References:

1.      Magdalena, J. et al. Spa32 regulates a switch in substrate specificity of the type III secreton of Shigella flexneri from needle components to Ipa proteins. J. Bacteriol. 184, 3433–3441 (2002).

2.      Tamano, K., Katayama, E., Toyotome, T. & Sasakawa, C. Shigella Spa32 is an essential secretory protein for functional type III secretion machinery and uniformity of its needle length. J. Bacteriol. 184, 1244–1252 (2002).

3.      Bahrani, F. K., Sansonetti, P. J. & Parsot, C. Secretion of Ipa proteins by Shigella flexneri: inducer molecules and kinetics of activation. Infect. Immun. 65, 4005–4010 (1997).