Three Dimensional Analysis of the Locomotion of Bacteria near a Surface

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To find the mechanism of formation of bacterial biofilm, we observed 3 dimensional swimming motions of bacteria. We constructed the biplane microscopy that has two orthogonal optical axes. By analysis of the obtained images, 3D trajectories of bacterial swimming are measured. We also conducted measurement of the collective motion of bacteria. Bacillus subtilis cells swim collectively when the culture medium becomes crowded with the average distance less than 5 micrometers. To quantify the degree of collective motion, we compared the velocity correlation of swarmer cells with that of randomly swimming cells. Large difference is found at the distance of 10 micrometers, which is several times as long as the size of a cell.