Elucidating the biological mechanisms underlying the motility of flagellated bacteria by understanding torque generation

There is an imperative for the understanding of the physiology of motility of flagellated bacteria such as Campylobacter jejuni (C.jejuni), Salmonella enterica (S.enterica) which cause hospitalisations and death worldwide, as well as bacteria such as Caulobacter crescentus (C.crescentus) which undergo significant shape shifting during their life cycle are now widely studied. In particular, it is important to understand the molecular mechanisms of bacterial motility as motility plays an important role in the infection of hosts.

Bacterial movement is determined by activity of the motor embed in the membrane of the bacteria, the hook attaches the motor to the tail like structure known as a flagellum. Biologically, motility is known to be initiated within the flagellum by the interaction between two structures, the stator and the C-ring. The stator consists of two proteins, a pentameric motility protein A (5motA) and a dimeric motility protein B (2motB) is thought to move clockwise. Then, the C-ring follows movement initiated by the stator but in a counter clockwise direction like a cogwheel. Mechanistically, motA is known to interact with the flagellar motor switch protein FliG protein within the C-ring.

In bacteria such as S.enterica and E.coli the exact number of stators that effects movement differs depending on the environment of the bacteria. For example, under high-viscous (i.e. high resistance) environment more stators are thought to be active than under low-viscous (i.e. low resistance) environment. Next, the direction of movement of the flagellum, which propels the bacteria forward, is thought to be affected by the local milieu. The default direction of movement of the flagellum is counter clockwise, however, when local stimuli are encountered ('chemotaxis') the direction of movement can change to clockwise.

Current literature on the biology underlying bacterial motility has important limitations. In particular, the following remains incompletely understood. First, no studies have reported how the proteins 5MotA-2MotB interact with the C-ring via the FliG C-terminal in C. jejuni, although possible models have been presented. Second, it remains incompletely understood whether the availability of stators is static (non-changing) or a dynamic (changing) process in C. jejuni. The latter is relevant to gain insight in the evolution of bacteria, since stators can be visualised in C. jejuni (a more complex motor) but not in S.enterica or E.coli (a simple motor). Third, it remains unknown how the stators are physically assembled around the C-ring in S.enterica. Fourth, it remains unclear how 5MotA-2MotB interact with the C-ring via the FliG C-terminal in C. jejuni when C.jejuni is rotating clockwise as oppose to counterclockwise rotation. In view of the above, the present thesis investigated the following aims.

In addition to this, I have taken on two collaboratory projects which focus on other proteins in the motor. First, the protein FlaY is thought to be a critical protein, determining whether motility is possible. However, the exact localisation of FlaY within the flagellum remains incompletely unknown. Second, the localisation of so-called anti-break protein (0996) which mediates the rotation of C.jejuni motor.