Abstract
To elucidate the reason why bacteria exhibit collective motion, we performed microscopic observation of a colony of Bacillus subtilis cells. Bacteria were inoculated on a semi-solid medium. They swim collectively when the average distance between cells is less than 5 μm. PIV measurement instead of measuring individual cells revealed the flow field, from which the velocity correlation was deduced. As the colony becomes crowded with cells, the velocity correlation becomes stronger within the distance of 10 μm. Furthermore, we limited a region of the correlation analysis to the tangential direction and the normal direction of the reference vector. Different characteristic lengths are found between two directions; The width and length of the swarm turned to be 13 times Bacillus subtilis cell body's average width and 10 times cell body's length, respectively. In addition, we estimated vorticity. The diameter of vortex was approximately 10-25 μm.
