A COMPLEX PATTERN OF TRAVELING STRIPES IS PRODUCED BY SWIMMING CELLS OF BACILLUS-SUBTILIS

Motile cells of Bacillus subtilis inadvertently escaped from the surfa ce of an agar disk that was surrounded by a fluid growth medium and fo rmed a migrating population in the fluid. When viewed from above, the population appeared as a cloud advancing unidirectionally into the fre sh medium. The cell population became spontaneously organized into a s eries of stripes in a region behind the advancing cloud front. The num ber of stripes increased progressively until a saturation value of str ipe density per unit area was reached. New stripes arose at a fixed di stance behind the cloud front and also between stripes. The spacing be tween stripes underwent changes with time as stripes migrated towards and away from the cloud front, The global pattern appeared to be stret ched by the advancing cloud front. At a time corresponding to approxim ately two cell doublings after pattern formation, the pattern decayed, suggesting that there is a maximum number of cells that can be mainta ined within the pattern, Stripes appear to consist of high concentrati ons of cells organized in sinking columns that are part of a bioconvec tion system. Their behavior reveals an interplay between bacterial swi mming, bioconvection-driven fluid motion, and cell concentration. A ma thematical model that reproduces the development and dynamics of the s tripe pattern has been developed.