The EAL ‐like protein STM 1697 regulates virulence phenotypes, motility and biofilm formation in S almonella typhimurium

The ubiquitous second messenger c‐di‐ GMP regulates the switching of bacterial lifestyles from motility to sessility and acute to chronic virulence to adjust bacterial fitness to altered environmental conditions. Conventionally, EAL proteins being c‐di‐ GMP phosphodiesterases promote motility and acute virulence phenotypes such as invasion into epithelial cells and inhibit biofilm formation. We report here that in contradiction, the EAL ‐like protein STM 1697 of S almonella typhimurium suppresses motility, invasion into HT ‐29 epithelial cell line and secretion of the type three secretion system 1 effector protein SipA , whereas it promotes rdar biofilm formation and CsgD expression. STM 1697 can, however, functionally replace the EAL ‐like protein STM 1344 and vice versa, whereby both proteins neither degrade nor bind c‐di‐ GMP . Like STM 1344, STM 1697 suppresses the transcription of class 2 and class 3 flagella regulon genes by binding to FlhD , a component of the master regulator of the flagella regulon FlhD 4 C 2 and act additively under numerous conditions. Interestingly, the interaction interface of STM 1697 with FlhD 2 is distinct from its paralogue STM 1344. We predict that the stand alone EAL domain proteins STM 1697 and STM 1344 belong to a subclass of EAL domain proteins in S . typhimurium , which are all involved in motility, biofilm and virulence regulation through interaction with proteins that regulate flagella function.