Bacteria like Escherichia coli and Salmonella Typhimurium use flagella for locomotion. Each flagellum is run by a motor located in the cell body, with a long slender filament of 0.01-0.015mm long and 20nm in diameter attached to this motor. A swimming bacterium has all its filaments helical in shape involving a frequent change of their chirality, pitch and radius. To explain polymorphism, a new coarse-grained continuum rod theory with two molecular switches based on the quaternary structure of the filament is proposed. A phase diagram for filament shapes is calculated and the response of a filament to external moment and force is examined. To study the dynamics of the propagation of polymorphism in the filaments, a toy model with an external tensile shear fluid flow applied to a bistable cantilever 1D rod was modeled on the lines of phase transitions and simulations is performed using finite element techniques. Interesting results like nucleation of high-strain phase leading to either a trapped front separating two phases or uniform high-strain phase at steady state are observed.