抄録
To elucidate the reason why bacteria exhibit collective motion, we first performed microscopic observation of a colony of Bacillus subtilis cells. Bacteria were inoculated on a semi-solid media, and 200 minutes after they began swarming. They swim collectively when the average distance between cells is less than 5 μm. PIV measurement instead of measuring indivisual 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 pm 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, as for the maximum velocity of swarming, number density showed a peak 25 pm/s in about 1.0 X 10^6 - 1.0 X10^7 cells/mm^2.
