A complex pattern of traveling stripes is produced by swimming cells of Bacillus subtilis

Abstract

Motile cells of Bacillus subtilis inadvertently escaped from the surface of an agar disk that was surrounded by a fluid growth medium and formed a migrating population in the fluid. When viewed from above, the population appeared as a cloud advancing unidirectionally into the fresh medium. The cell population became spontaneously organized into a series of stripes in a region behind the advancing cloud front. The number of stripes increased progressively until a saturation value of stripe density per unit area was reached. New stripes arose at a fixed distance behind the cloud front and also between stripes. The spacing between stripes underwent changes with time as stripes migrated towards and away from the cloud front. The global pattern appeared to be stretched by the advancing cloud front. At a time corresponding to approximately two cell doublings after pattern formation, the pattern decayed, suggesting that there is a maximum number of cells that can be maintained within the pattern. Stripes appear to consist of high concentrations of cells organized in sinking columns that are part of a bioconvection system. Their behavior reveals an interplay between bacterial swimming, bioconvection-driven fluid motion, and cell concentration. A mathematical model that reproduces the development and dynamics of the stripe pattern has been developed.

FIG. 1

Images of sequential frames illustrating pattern formation, dynamics, and decay. Panels 1 through 8 depict the following: 1, the state of colony growth on the agar disk prior to cloud appearance; 2, bacterial cloud in the left region; 3, two stripes formed in the left region; 4, 10 stripes in the left region; 5, the start of chaos in the left region; 6, bacterial cloud in the right region; 7, pattern decay in the left region, numerous stripes in the right region; 8, an early stage of pattern decay in the right region.

FIG. 1

Images of sequential frames illustrating pattern formation, dynamics, and decay. Panels 1 through 8 depict the following: 1, the state of colony growth on the agar disk prior to cloud appearance; 2, bacterial cloud in the left region; 3, two stripes formed in the left region; 4, 10 stripes in the left region; 5, the start of chaos in the left region; 6, bacterial cloud in the right region; 7, pattern decay in the left region, numerous stripes in the right region; 8, an early stage of pattern decay in the right region.

FIG. 2

Position of the cloud front and the newest stripe formed behind it as a function of time in the right region. The rate of cloud migration was 1 mm/4.44 min (slope = 0.225). Stripes appeared approximately 24 min after the cloud front passed a given position corresponding to 6.7 mm behind the cloud front. The bacterial doubling time in the same growth medium at the same temperature was 90 min (data not shown). Symbols: □, the cloud front; •, the newest stripe formed behind the cloud front.

FIG. 3

Comparison of stripe patterns in the left region (top) and right region (bottom). The upper video frame was taken at 75 min 29 s and the lower frame was taken at 238 min 46 s after the first stripes appeared in the left region. The inserts give the pattern intensity as a function of position along the line drawn on each frame (1 U on the x axes of the inserts corresponds to 0.155 mm of actual length.)

FIG. 4

(A) Space-time diagram showing the evolution of stripes in the right region. A portion of each video frame showing stripes in the right region was selected and transferred by computer to form an aligned composite figure containing 51 time points that span 80 min in real time. New bands that arose between old ones are shown in red. (B) Space-time diagram showing the evolution of the bacteria concentration as a function of time obtained from the mathematical model. The times range from t = 0 to t ≃ 970 (arbitrary units). The model parameters used to produce this diagram are ρ_c = 0.3, α_r = 2.0 + 0.85μ, α_i = 0.8 + 0.5μ, β_r = 1.0, β_i = −1.3 − 2.0μ, k₀ = 0.5 + 0.5μ + 0.8μ², γ = 0.453, and ν = −0.1, where μ = ρ − ρ_c.

FIG. 5

Examples of bacterial bioconvection stripe patterns produced adjacent to various boundaries. (A) Strain OI2836 patterns produced from cells originating in a colony grown on soft TB agar (0.6%). Panel 1 shows stripes formed both at the periphery of the petri dish and near the agar disk. The white arrows marked M and P indicate the direction of cloud front propagation and new stripe formation behind the front (P) and of stripe migration (M). Panels 2, 3, and 4 show progressively later times (15, 68, and 102 min after panel 1, respectively). Panel 4 illustrates decay of the peripheral stripe pattern and the beginnings of decay in the inner pattern surrounding the agar disk. (B) Stripe patterns produced by strain OI1085 fluid populations grown near linear boundaries. Panels 1 and 2 show patterns along rough and smooth surface plastic boundaries, respectively. Panel 3 shows a pattern produced along a linear agar boundary. Black triangles indicate the points of inoculation of the fluid. White arrows indicate the direction of front migration and stripe migration (as described above for panel A).

FIG. 5

Examples of bacterial bioconvection stripe patterns produced adjacent to various boundaries. (A) Strain OI2836 patterns produced from cells originating in a colony grown on soft TB agar (0.6%). Panel 1 shows stripes formed both at the periphery of the petri dish and near the agar disk. The white arrows marked M and P indicate the direction of cloud front propagation and new stripe formation behind the front (P) and of stripe migration (M). Panels 2, 3, and 4 show progressively later times (15, 68, and 102 min after panel 1, respectively). Panel 4 illustrates decay of the peripheral stripe pattern and the beginnings of decay in the inner pattern surrounding the agar disk. (B) Stripe patterns produced by strain OI1085 fluid populations grown near linear boundaries. Panels 1 and 2 show patterns along rough and smooth surface plastic boundaries, respectively. Panel 3 shows a pattern produced along a linear agar boundary. Black triangles indicate the points of inoculation of the fluid. White arrows indicate the direction of front migration and stripe migration (as described above for panel A).