To function completely, the flagellum requires over 40 different proteins. If a structure is so complex that all of its parts must initially be present in a suitably functioning manner, it is said to be irreducibly complex. All the parts of a bacterial flagellum must have been present from the start in order to function at all. According to evolutionary theory, any component which doesn't offer an advantage to an organism, i.e. doesn't function, will be lost or discarded. How a flagellum is used, however, adds an additional level of complexity to the picture.
Some bacteria have a single flagellum located at the end of a rod-shaped cell.
To move in an opposite direction, a bacterium simply changes the direction of rotation of the flagellum. Other bacteria have a flagellum at both ends of the cell and use one flagellum for going in one direction and the other for going in the opposite direction. A third group of bacteria has many flagella surrounding the cell. These flagella wrap themselves together in a helical bundle at one end of the cell and rotate in unison to move the cell in one direction. If the cell wants to change direction, the flagella unwrap themselves, move to the opposite end of the cell, reform the bundle, and again rotate in a coordinated fashion.