Organized cell swimming motions in Bacillus subtilis colonies: Patterns ofshort-lived whirls and jets

The swimming motions of cells within Bacillus subtilis colonies, as well as the associated fluid flows, were analyzed from video films produced during colony growth and expansion on wet agar surfaces. Individual cells in very wet dense populations moved at rates between 76 and 116 mu m/s. Swimming c ells were organized into patterns of whirls, each approximately 1,000 mu m( 2), and jets of about 95 by 12 mu m. Whirls and jets were short-lived, last ing only about 0.25 s. Patterns within given areas constantly repeated with a periodicity of approximately 1 s. Whirls of a given direction became dis organized and then re-formed, usually into whirls moving in the opposite di rection. Pattern elements were also organized with respect to one another i n the colony. Neighboring whirls usually turned in opposite directions. Thi s correlation decreased as a function of distance between whirls. Fluid flo ws associated with whirls and jets were measured by observing the movement of marker latex spheres added to colonies. The average velocity of markers traveling in whirls was 19 mu m/s, whereas those traveling in jets moved at 27 mu m/s. The paths followed by markers were aligned with the direction o f cell motion, suggesting that cells create flows moving with them into whi rls and along jets. When colonies became dry, swimming motions ceased excep t in regions close to the periphery and in isolated islands where cells tra veled in slow whirls at about 4 mu m/s. The addition of water resulted in i mmediate though transient rapid swimming (> 80 mu m/s) in characteristic wh irl and jet patterns. The rate of swimming decreased to 13 mu m/s within 2 min, however, as the water diffused into the agar. Organized swimming patte rns were nevertheless preserved throughout this period. These findings show that cell swimming in colonies is highly organized.