Single cell, super-resolution imaging reveals an acid pH-dependent conformational switch in SsrB regulates SPI-2

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Abstract

After Salmonella is phagocytosed, it resides in an acidic vacuole. Its cytoplasm acidifies to pH 5.6; acidification activates pathogenicity island 2 (SPI-2). SPI-2 encodes a type three secretion system whose effectors modify the vacuole, driving endosomal tubulation. Using super-resolution imaging in single bacterial cells, we show that low pH induces expression of the SPI-2 SsrA/B signaling system. Single particle tracking, atomic force microscopy, and single molecule unzipping assays identified pH-dependent stimulation of DNA binding by SsrB. A so-called phosphomimetic form (D56E) was unable to bind to DNA in live cells. Acid-dependent DNA binding was not intrinsic to regulators, as PhoP and OmpR binding was not pH-sensitive. The low level of SPI-2 injectisomes observed in single cells is not due to fluctuating SsrB levels. This work highlights the surprising role that acid pH plays in virulence and intracellular lifestyles of Salmonella; modifying acid survival pathways represents a target for inhibiting Salmonella.

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All data generated or analysed during this study are included in the manuscript and supporting files. Source data files have been provided for all figures

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Andrew Tze Fui Liew

Mechanobiology Institute, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Yong Hwee Foo

Mechanobiology Institute, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Yunfeng Gao

Mechanobiology Institute, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Parisa Zangoui

Mechanobiology Institute, National University of Singapore, Singapore, Singapore

Competing interests
The authors declare that no competing interests exist.
Moirangthem Kiran Singh

Mechanobiology Institute, National University of Singapore, Singapore, Singapore

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The authors declare that no competing interests exist.
Ranjit Gulvady

Institut Curie, Paris, France

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The authors declare that no competing interests exist.
Linda J Kenney

Department of Microbiology and Immunology, University of Illinois-Chicago, Chicago, United States

For correspondence
kenneyl@uic.edu

Competing interests
The authors declare that no competing interests exist.

"This ORCID iD identifies the author of this article:" 0000-0002-8658-0717

Funding

National Institutes of Health (AI-123640)

Linda J Kenney

Veteran's Affairs (IOBX-000372)

Linda J Kenney

The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.

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This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.