Phase Diagram of Collective Motion of Bacterial Cells in a Shallow Circular Pool

TL;DR

This study systematically explores how bacterial cells of Bacillus subtilis exhibit six distinct collective motion patterns in a shallow circular pool, depending on cell density and reduced cell length, and presents a phase diagram of these behaviors.

Contribution

It provides the first comprehensive phase diagram categorizing bacterial collective motions based on cell density and reduced cell length.

Findings

Six types of collective motion identified

Two critical reduced cell lengths induce phase transitions

A phase diagram illustrating motion types based on parameters

Abstract

The collective motion of bacterial cells in a shallow circular pool is systematically studied using the bacterial species $Bacillus$ $subtilis$. The ratio of cell length to pool diameter (i.e., the reduced cell length) ranges from 0.06 to 0.43 in our experiments. Bacterial cells in a circular pool show various types of collective motion depending on the cell density in the pool and the reduced cell length. The motion is classified into six types, which we call random motion, turbulent motion, one-way rotational motion, two-way rotational motion, random oscillatory motion, and ordered oscillatory motion. Two critical values of reduced cell lengths are evaluated, at which drastic changes in collective motion are induced. A phase diagram is proposed in which the six phases are arranged.