Roles of Histidine Kinase Gene yycG in the Pathogenicity of Listeria monocytogenes

As an opportunistic foodborne pathogen, Listeria monocytogenes could successfully switch self-characteristics in response to environmental conditions. In L. monocytogenes, sixteen two-component systems (TCSs) have been found to contribute the bacteria to sensing and adapting to various conditions. Our previous genomic study showed that mutation was observed in the histidine kinase gene yycG between Lm850658 and M7 among the sixteen TCSs. The YycFG TCS was found playing important roles in many other Gram-positive bacteria. While the roles of YycFG TCS in L monocytogenes remain poorly known. In this study, we aimed to determine whether yycG play roles in pathogenicity of L. monocytogenes. We created a histidine kinase gene yycG deletion strain (Delta yycG) based on the wild type strain Lm850658 and a complemental strain (Delta yycG). We found the yycG deletion significantly impaired the growth ability and mobility, but enhanced the hemolysis ability in vitro. In addition, infection assays on cell and mice model showed that Delta yycG exhibited significantly defected in infection ability and virulence. All these phenotypes of the Delta yycG could be reversed largely to the levels of the wild type strain Lm850658 by gene complementation. Cell wall-associated and secreted protein analysis showed that the secreted content of Delta yycG was significantly increased. And western blotting revealed that Internalin protein B (InIB) and Listerolysin O (LLO) was markedly increased in the secreted fractions of Delta yycG, which might be responsible for decreased adhesion and invasion ability and increased hemolytic activity, respectively. Overall, we found the histidine kinase yycG played important roles in pathogenicity of L. monocytogenes for the first time. Further investigation is needed to explore how the YycFG TCS modulates the growth, mobility, cell surface proteins translocation and virulence.