Model for Polymorphic Transitions in Bacterial Flagella

TL;DR

This paper introduces a new continuum rod model for bacterial flagella that explains their polymorphic transformations by incorporating molecular switches and phase diagrams, advancing understanding of bacterial motility.

Contribution

The authors develop a coarse-grained continuum rod theory with molecular switches to explain flagellar polymorphism, incorporating phase diagrams and responses to external forces.

Findings

Model predicts shape transitions under mechanical loads.

Phase diagram characterizes polymorphic states.

Explains effects of environmental changes on filament shape.

Abstract

Many bacteria use rotating helical flagellar filaments to swim. The filaments undergo polymorphic transformations in which the helical pitch and radius change abruptly. These transformations arise in response to mechanical loading, changes in solution temperature and ionic strength, and point substitutions in the amino acid sequence of the protein subunits that make up the filament. To explain polymorphism, we propose a new coarse-grained continuum rod theory based on the quaternary structure of the filament. The model has two molecular switches. The first is a double-well potential for the extension of a protofilament, which is one of the eleven almost longitudinal columns of subunits. Curved filament shapes occur in the model when there is a mismatch strain, i.e. when inter-subunit bonds in the inner core of the filament prefer a subunit spacing which is intermediate between the two spacings favored by the double-well potential. The second switch is a double-well potential for twist, due to lateral interactions between neighboring protofilaments. Cooperative interactions between neighboring subunits within a protofilament are necessary to ensure the uniqueness of helical ground states. We calculate a phase diagram for filament shapes and the response of a filament to external moment and force.