# Cell$-$cell and Cell$-$noise Interactions of Bacterial Cells in a Shallow Circular Pool and Transitions of Collective Motions

## TL;DR

This study experimentally investigates how bacterial cells transition between different collective motion phases in a shallow pool, highlighting the role of cell-cell interactions in sharp phase transitions at specific cell length ratios.

## Contribution

It provides detailed analysis of the transition mechanisms between collective motion phases, emphasizing the impact of cell-cell interactions at a critical cell length ratio.

## Key findings

- Sharp transition at λ ≈ 0.1 between random and rotational motion phases.
- Cell-cell interactions drive phase transitions even with significant noise.
- Order parameter Q effectively characterizes the transition.

## Abstract

We have experimentally investigated the transitions of collective motions of bacterial cells in a shallow circular pool using the bacterial species $Bacillus$ $subtilis$. In our previous paper, we reported that the collective motions were classified into six phases on a phase diagram with two parameters, namely, the reduced cell length $\lambda$ and the cell density $\rho$. In this study, we focused on the sharp transitions at $\lambda\cong0.1$ ($\equiv \lambda_{\rm C1}$) with low values of $\rho$ between the $random$ $motion$ phase and the $one$-$way$ $rotational$ $motion$ phase. By introducing the order parameter $Q$ which measures the aligned cell motion along the circumferential direction of a pool, the transitions at $\lambda=\lambda_{\rm C1}$ were clearly characterized. On the basis of the detailed observations of single-cell trajectories in a pool, we verified that the effect of cell$-$noise interactions was widely distributed. We conclude that, even in such random environment, the sharp transitions of collective motions were caused by the cell$-$cell interactions at $\lambda=\lambda_{\rm C1}$.