Morphology and Elastohydrodynamics of Helical Bacteria and Spirochetes

Date of Completion

January 2012


Biology, Cell|Biology, Microbiology|Biophysics, General




The motility of bacteria plays an important role in their reproduction, energy balance, and predator avoidance, and the study of cell motility can also provide insights on the modes (and prevention) of infection by bacteria. Various aspects of cell motility are investigated in this dissertation, including low-Reynolds number elastohydrodynamics, swimming kinetics, swimming efficiency, and cell morphology. Three swimmers are studied in the dissertation: a Purcellian model swimmer, and the bacteria Spiroplasma and Borrelia burgdoferi. ^ The low-Reynolds number motions of Purcell's three-link swimmer, and of a closely-related two-paddle swimmer, are investigated and compared using slender-body theory and resistive-force theory. The results are compared (in the case of the three-link swimmer) with the resistive-have calculations of Becker, Koehler and Stone (BKS). In particular, we examine the effect of hydrodynamic interaction and slenderness on the displacement and efficiency of the swimmers. ^ Spiroplasma swimming is studied with a simple model based on resistive-force theory. Specifically, we consider a bacterium shaped in the form of a helix that propagates traveling-wave distortions which flip the handedness of the helical cell body. We treat cell length, pitch angle, kink velocity, and distance between kinks as parameters and calculate the swimming velocity that arises due to the distortions. In addition, the left-handed and right-handed helices are concatenated by kinks during the swimming process of Spiroplasma, and the block angle between these helices with reversed chiralities is solved for by using a combination of differential geometry and perturbation theory. ^ As a unique group of motile bacteria, spirochetes are distinguished by their helical and flat-wave shapes and the location of their flagella, which reside within the tiny space between the bacterial cell wall and the outer membrane (i.e., the periplasm). In Borrelia burgdoferi rotation of the flagella produces cellular undulations that drive swimming. Flagellar shape changes due to the forces and torques that act between the flagella and the cell body are investigated. ^