A Strategy of Vibrio vulnificus to Turn off Flagellar Motility in the Presence of Glucose

Abstract

To survive in a continuously changing environment, bacteria sense concentration gradients of attractants or repellents, and purposefully migrate until a more favourable habitat is encountered. While glucose is known as the most effective attractant, the flagellar biosynthesis and hence chemotactic motility has been known to be repressed by glucose in some bacteria. To date, the only known regulatory mechanism of the repression of flagellar synthesis by glucose is via downregulation of the cAMP level, as shown in a few members of the family Enterobacteriaceae. In this thesis, it has been shown in Vibrio vulnificus, motile and curved rod-shaped halophilic bacterium with a single polar flagellum, the glucose mediated inhibition of flagellar motility operates by a completely different mechanism. In the presence of glucose, glucose-specific enzyme IIA (EIIAGlc) of the phosphoenolpyruvate: sugar phosphotransferase system is dephosphorylated and inhibits the polar localization of FapA (flagellar assembly protein A) by sequestering it from the flagellated pole. A loss or delocalization of FapA results in a complete failure of flagellation and motility. However, when glucose is depleted, EIIAGlc is phosphorylated and releases FapA such that free FapA can be localized back to the pole and trigger flagellation. The ligand fishing experiment revealed that FapA interacts with not only EIIAGlc but also a polar landmark protein, HubP (hub of the pole), which anchors client proteins to the cell poles and modulates the localization of the chromosome origin, chemotactic signaling proteins and flagellum. In the absence of HubP, FapA is diffused throughout the cytoplasm, indicating that proper polar targeting of FapA depends on HubP. HubP competes with dephosphorylated EIIAGlc for binding to FapA to regulate the early stage of flagellar assembly. Together, these results suggest that dephosphorylated EIIAGlc inhibits flagellation by sequestrating FapA from polar localized HubP in the presence of glucose and thereby enables V. vulnificus cells to adapt to and stay in a glucose-rich environment.