Emergence of critically buckled motile helices under stress.

Bacteria under external stress can reveal unexpected emergent phenotypes. We show that the intensely studied bacterium can transform into long, highly motile helical filaments poized at a torsional buckling criticality when exposed to minimum inhibitory concentrations of several antibiotics. While the highly motile helices are physically either right- or left-handed, the motile helices always rotate with a right-handed angular velocity [Formula: see text], which points in the same direction as the translational velocity [Formula: see text] of the helix. Furthermore, these helical cells do not swim by a "run and tumble" but rather synchronously flip their spin [Formula: see text] and thus translational velocity-backing up rather than tumbling. By increasing the translational persistence length, these dynamics give rise to an effective diffusion coefficient up to 20 times that of a normal cell. Finally, we propose an evolutionary mechanism for this phenotype's emergence whereby the increased effective diffusivity provides a fitness advantage in allowing filamentous cells to more readily escape regions of high external stress.