One of the central systems responsible for bacterial motility is the flagellum. 
The bacterial flagellum is a macromolecular protein complex that is more than 
five times the cell length. Flagella-driven motility is coordinated via a 
chemosensory signal transduction pathway, and so bacterial cells sense changes 
in the environment and migrate towards more desirable locations. The flagellum 
of Salmonella enterica serovar Typhimurium is composed of a bi-directional 
rotary motor, a universal joint and a helical propeller. The flagellar motor, 
which structurally resembles an artificial motor, is embedded within the cell 
envelop and spins at several hundred revolutions per second. In contrast to an 
artificial motor, the energy utilized for high-speed flagellar motor rotation is 
the inward-directed proton flow through a transmembrane proton channel of the 
stator unit of the flagellar motor. The flagellar motor realizes efficient 
chemotaxis while performing high-speed movement by an ingenious directional 
switching mechanism of the motor rotation. To build the universal joint and 
helical propeller structures outside the cell body, the flagellar motor contains 
its own protein transporter called a type III protein export apparatus. In this 
chapter we summarize the structure and assembly of the Salmonella flagellar 
motor complex.