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 the six phases on a phase diagram with the two parameters,  the reduced cell length \lambda \ and the cell density \rho. In the present study, we focused on the sharp transitions at \lambda\cong0.1 (\equiv \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=\rm C1 were clearly characterized. Based on 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,  sharp transitions of collective motions were caused by the cell-cell interactions at \lambda=\rm C1.
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 \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=\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=\rm C1.
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