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We have experimentally investigated the transitions of collective motions of bacterial cells in a shallow circular pool using the bacterila species Bacillus subtilis. In our previous study, we reported that the collective motions were classified into 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=0.1 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=0.1 were clearly characterized. Based on the detailed observations of a 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=0.1.