Abstract
A large variety of motile bacterial species exhibit collective motions while inhabiting liquids or colonizing surfaces. These collective motions are often characterized by coherent dynamic clusters, where hundreds of cells move in correlated whirls and jets. Previously, all species that were known to form such motion had a rod-shaped structure, which enhances the order through steric and hydrodynamic interactions. Here we show that the spherical motile bacteria Serratia marcescens exhibit robust collective dynamics and correlated coherent motion while grown in suspensions. As cells migrate to the upper surface of a drop, they form a monolayer, and move collectively in whirls and jets. At all concentrations, the distribution of the bacterial speed was approximately Rayleigh with an average that depends on concentration in a non-monotonic way. Other dynamical parameters such as vorticity and correlation functions are also analyzed and compared to rod-shaped bacteria from the same strain. Our results demonstrate that self-propelled spherical objects do form complex ordered collective motion. This opens a door for a new perspective on the role of cell aspect ratio and alignment of cells with regards to collective motion in nature.
Highlights
Motile bacteria exhibit a large variety of motility mechanisms, among which some require cooperation between thousands of cells
Observation of collective dynamics A 5-ml drop of an overnight (18 h) wild type (WT) S. marcescens 274 culture was placed on a glass slide and observed in upright light microscopy
For quantitative measurements the drop was constrained by a super-hydrophobic ring printed on the glass (polytetrafluoroethylene (PTFE) printed glass slides (63429-04) Electron Microscopy Sciences, Hatfield, PA) in order to prevent wetting and spreading, which may affect the dynamics of the bacteria or cause drifting
Summary
Motile bacteria exhibit a large variety of motility mechanisms, among which some require cooperation between thousands of cells. Swarming generally involves an organized, hyperflagellated-based, cell motion and a collective secretion of surfactants that decrease surface tension, enabling fast expansion This motion has been studied extensively for different species [9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28] where some colonies can cover an entire Petri dish (8.8 cm in diameter) within a few hours. Since swarming was never observed in sphere-like bacteria, collective motion of such cells was questioned
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