Vibrio species are Gram-negative, rod-shaped bacteria that live in aqueous environments. Several species, such as V. harveyi, V. alginotyticus and V. splendidus, are associated with diseases in fish or shellfish. In addition, a few species, such as V. cholerae and V. parahaemolyticus, are risky for humans due to infections from eating raw shellfish infected with these bacteria or from exposure of wounds to the marine environment. Bacterial flagella are not essential to live in a culture medium. However, most Vibrio species are motile and have rotating flagella which allow them to move into favorable environments or to escape from unfavorable environments. This review summarizes recent studies about the flagellar structure, function, and regulation of Vibrio species, especially focused on the Na+-driven polar flagella that are principally responsible for motility and sensing the surrounding environment, and discusses the relationship between flagella and pathogenicity.

Significance: Stator is the energy-converting membrane protein complex in the flagellar motor. Its ion-conducting activity is only activated when incorporated into the motor, but the mechanism for assembly-coupled activation remains a mystery. In this study, we solved the structure of a C-terminal fragment of the sodium-driven stator protein PomB (PomB _C ), the region responsible for anchoring the stator unit, at 2.0-Å resolution. In vivo disulfide cross-linking studies of PomB double-Cys mutants and their motility assay suggested that the N-terminal region of PomB _C changes its conformation, which is expected for MotB, the counterpart of PomB in the proton-driven Salmonella motor, in the final step of the stator assembly around the rotor.

The bacterial flagellar motor switches rotational direction between counterclockwise (CCW) and clockwise (CW) to direct the migration of the cell. The cytoplasmic ring (C-ring) of the motor, which is composed of FliG, FliM, and FliN, is known for controlling the rotational sense of the flagellum. However, the mechanism underlying rotational switching remains elusive. Here, we deployed cryo-electron tomography to visualize the C-ring in two rotational biased mutants in Vibrio alginolyticus. We determined the C-ring molecular architectures, providing novel insights into the mechanism of rotational switching. We report that the C-ring maintained 34-fold symmetry in both rotational senses, and the protein composition remained constant. The two structures show FliG conformational changes elicit a large conformational rearrangement of the rotor complex that coincides with rotational switching of the flagellum. FliM and FliN form a stable spiral-shaped base of the C-ring, likely stabilizing the C-ring during the conformational remodeling.

The stator proteins PomA and PomB form a complex that couples Na ⁺ influx to torque generation in the polar flagellar motor of Vibrio alginolyticus . This stator complex is anchored to an appropriate place around the rotor through a putative peptidoglycan-binding (PGB) domain in the periplasmic region of PomB (PomB _C ). To investigate the function of PomB _C , a series of N-terminally-truncated and in-frame mutants with deletions between the transmembrane (TM) segment and the PGB domain of PomB was constructed. A PomB _C fragment consisting of residues 135 to 315 (PomB _C5 ) formed a stable homodimer and significantly inhibited the motility of wild-type cells when overexpressed in the periplasm. A fragment with an in-frame deletion (PomB _ΔL ) of up to 80 residues retained function, and its overexpression with PomA impaired cell growth. This inhibitory effect was suppressed by a mutation at the functionally critical Asp (D24N) in the TM segment of PomB, suggesting that a high level of Na ⁺ influx through the mutant stator causes the growth impairment. The overproduction of functional PomA/PomB _ΔL stators also reduced the motile fractions of the cells. That effect could be slightly relieved by a mutation (L168P) in the putative N-terminal α-helix that connects to the PGB domain without affecting the growth inhibition, suggesting that a conformational change of the region including the PGB domain affects stator assembly. Our results reveal common features of the periplasmic region of PomB/MotB and demonstrate that a flexible linker that contains a “plug” segment is important for the control of Na ⁺ influx through the stator complex as well as for stator assembly.