Change of waveform in bacterial flagella : the role of mechanics at the molecular level

Abstract The flagella of Salmonella and other bacteria are constructed from molecules of the protein flagellin in a way which permits relatively easy transition between members of a family of distinct stable left and right-handed helical waveforms. Changes of waveform, particularly between “normal” (left-handed) and “curly” (right-handed) play an important role in the switch from smooth swimming to tumbling in chemotaxis. This paper establishes some mechanical properties of model flagella built from bi-stable subunits, which in turn clarifies the mechanics of the changes of waveform which occur, in a viscous fluid environment, at various points in the swimming cycle. Available data on the joining of different helical waveforms in a single filament, supplemented by information on the way in which helical filaments flatten down in preparation for electron microscopy, are well-fitted by the mechanical behaviour of an assembly of mechanical subunits having some simple distinctive design features. The same arrangement makes possible an explanation for the formation of flagellar-like but straight polymers from Salmonella flagellin in the presence of high concentrations of NaCl.