Flagellar Motor Based Micro Hybrid Devices
S. Tung, J.-W. Kim

Flagellar motors belong to the molecular motor group that is responsible for a wide range of mechanical movements on the cellular level [1]. A single flagellar motor can generate a power output of 10-15 W [2]. It can also produce a torque level of 4 nN-nm, which is two hundred times higher than that of an F1-ATPase. Flagellar motors are found in nature, used by many species of bacteria such as Escherichia coli and Salmonella typhimurium to rotate the flagellar filaments on their bodies for locomotion purpose (Figure 1). Each motile E. coli cell has an average of 6 helical flagellar filaments that arise at random points on the cell body [3]. Each flagellar filament is about 20 nm in diameter and 10 μm long. It is driven at the base by a rotary motor, which rotates the filament at a speed of about 100 rps during free swimming. Normally, each flagellar filament turns counterclockwise about 80% of the time. However, mutations that affect the chemotactic signaling pathway can lock the motors in one direction of rotation.

The high power and high torque capability of tethered flagellar motors suggest the possibility of using flagellar motors as microfluidic actuators in MEMS type devices.

- General about flagellar motors -
Structural diversity of bacterial flagellar motors

The flagellum consists of a motor also known as the basal body, a flexible linker termed the hook, and a filament that behaves as a helical propeller and is typically many times the length of the bacterium itself. The motor converts ion flux across the cytoplasmic membrane into a torque that rotates the flagellum. In some organisms such as Escherichia coli, counterclockwise rotation generates thrust that propels the cell forward. Signals from chemoreceptor arrays (Briegel et al, 2009) modulate the probability of the flagellar motor reversing direction (to spin clockwise) (Hazelbauer et al, 2008), which causes the bacterium to randomly re-orient.

Although all bacterial flagella share similarities in structure and mechanism, extensive variation in number, placement and usage exist between species. While cells such as Vibrio cholerae and Caulobacter crescentus, for example, exhibit a single polar flagellum, others, including Salmonella enterica and E. coli distribute several or many flagella around their periphery. Under different conditions, some bacteria alternate between polar and lateral flagellar systems (McCarter, 2004). The flagella of spirochaetes do not typically pierce the outer membrane, but instead remain in the periplasm where they rotate and/or contort the cell (Murphy et al, 2006; Liu et al, 2009; Kudryashev et al, 2010). Flagellar rotation is powered either by a protonmotive force (e.g., E. coli) or by a sodium-motive force (e.g., V. cholerae). The ‘run and tumble’ swimming mode that switches between clockwise and counter-clockwise rotations is best known, but other bacteria function differently, for example by varying the speed of a unidirectional motor. The molecular architectures of the helical flagellar filaments are also diverse (Galkin et al, 2008).