Yersinia pestis requires the 2-component regulatory system OmpR-EnvZ to resist innate immunity during the early and late stages of plague.

Plague is transmitted by fleas or contaminated aerosols. To successfully produce disease, the causal agent (Yersinia pestis) must rapidly sense and respond to rapid variations in its environment. Here, we investigated the role of 2-component regulatory systems (2CSs) in plague because the latter are known to be key players in bacterial adaptation to environmental change. Along with the previously studied PhoP-PhoQ system, OmpR-EnvZ was the only one of Y. pestis' 23 other 2CSs required for production of bubonic, septicemic, and pneumonic plague. In vitro, OmpR-EnvZ was needed to counter serum complement and leukocytes but was not required for the secretion of antiphagocyte exotoxins. In vivo, Y. pestis lacking OmpR-EnvZ did not induce an early immune response in the skin and was fully virulent in neutropenic mice. We conclude that, throughout the course of Y. pestis infection, OmpR-EnvZ is required to counter toxic effectors secreted by polymorphonuclear leukocytes in the tissues.
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