STK-mediated FadR phosphorylation regulates the acid resistance and virulence of Streptococcus suis

The phagolysosomes of macrophages play a crucial role in eradicating pathogenic microorganisms, but bacteria have evolved sophisticated mechanisms to survive in the acidic environment of phagolysosomes, leading to host infection and subsequent dissemination. However, it is largely unknown how bacteria sense the extracellular stimuli and regulate their acid tolerance capacity to resist the killing by host immune cells. Here, we report the new substrate FadR of the serine/threonine kinase (STK) in Streptococcus suis serotype 2 (SS2) and demonstrate that the phosphorylation site is Thr230. Notably, FadR phosphorylation significantly enhances the acid resistance of SS2, leading to an increase in the lethality of SS2 in mice, and a marked increase in bacterial load in the blood and various organs, and more severe pathological changes in various organs of the mice. Interestingly, this study further indicated that FadR protein can bind to the promoter of arginine deiminase (adi), and FadR phosphorylation enhances its binding ability to the adi promoter and increases adi transcription levels. The increase of ADI in SS2 promotes the metabolism of arginine and increases the ammonia content, thus enhancing the acid resistance and intracellular survival capacity of the bacteria in macrophages. Altogether, the research reveals an acid resistance regulatory mechanism that bacteria can utilize the STK-FadR signaling axis to sense changes in the external acidic environment, and then manipulate the ADI system to enhance bacterial resistance to acidic environment or host immunity.