Abstract
Peritrichously flagellated bacteria, especially the two we will discuss, Salmonella typhimurium and its kissing cousin Escherichia coli, assemble about 10 flagella over their cell surfaces. Bacterial flagella differ in every aspect but name from eukaryotic flagella (undulipodia). In the eukaryotic flagellum, dynein motors powered by ATP generate moving bends by stepping along linear microtubular tracks. The eukaryotic flagellum undulates like a snake inside its cellular membrane. In contrast the bacterial flagellum works like a power boat with a rotary motor turning a rigid, helical propeller. The source of energy is not ATP but rather the electromotive gradient of protons or sodium ions across the cell's membrane. A working bacterial flagellum contains about 20 proteins, over an order of magnitude fewer than the number of protein species comprising the eukaryotic undulipodia, and not surprisingly it is about an order of magnitude smaller in its dimensions. (For a review of the bacterial flagellum, see9).
In E. coli and S. typhimurium, flagella turning at speeds of 18,000 rpm push cells at 30 μm/s, but the speed records are set by motors in other bacteria that turn at rates exceeding 100,000 rpm and push cells at hundreds of micrometers per second. What is all the more remarkable is that unlike kinesin, ncd, and myosin, flagellar motors can run in both directions, i.e., clockwise (CW) and counterclockwise (CCW). These motors appear to be more powerful than myosin, kinesin, and ncd motors (Table 1), and they also deliver constant torque of 4500 pN nm (1) at speeds over 6000 rpm, whereas force falls off with velocity for myosin, kinesin, and ncd. Only at high speeds does torque in flagellar motors decrease linearly with speed (2).