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. 2020 Jan 28;9:e47551. doi: 10.7554/eLife.47551

Figure 1. 3D tracking in physiochemically controlled conditions.

(A) MC-1 cells were transitioned into agar-free medium, the solution was degassed to remove the oxygen and inserted into a microcapillary. Due to one open and one closed end, an oxygen gradient formed and the cells accumulated near their preferred microoxic conditions (the oxic-anoxic-interface, OAI). (B) The cells were observed near the band using 400 fps phase contrast video microscopy with a spherical aberration, which causes interference patterns around the spherical swimmers. These patterns can be correlated with patterns from silica beads of known height relative to the focal plane. (C) A tracking algorithm enables high-throughput 3D tracking of the microswimmers (Taute et al., 2015). Colors indicate different cells. (D) Individual tracks were analyzed and a clockwise helical travel path with a radius close to the cell diameter was found as well as instantaneous traveling speeds between 100 µm s−1 and 500 µm s−1. Tracks can be interrupted by rapid reorientation events that last only 2.5–5 ms. The helix parameters like pitch and period time do not change before and after an event, but apparently do so in the projected 2D tracks (see projected shadow in D).

Figure 1.

Figure 1—figure supplement 1. Validation of event identification algorithm for 3D tracking on simulated tracks with known parameters.

Figure 1—figure supplement 1.

Top simulation parameters: Theta (reorientation angle) was 90 °, velocity was 100 µm/s, and runtime was 0.86 s. Bottom parameters: Theta was 170 °, velocity was 14 µm/s and runtime was 0.86 s. The mean time each reorientation lasted was 0.14 s for both cases.

Figure 1—figure supplement 2. MC-1 swimming statistics.

Figure 1—figure supplement 2.

(A) Turning angle θ after a reorientation event, (B) effective traveling speed, (C) runtimes and D) runlengths in-between events.

Figure 1—figure supplement 3. Oblique and top view of a typical event in an MC-1 swimming track.

Figure 1—figure supplement 3.

The radius, pitch and period time of the helix remain constant before and after the event, which is identified as a drop in velocity to 0 and which lasted 3.75 ms ± 1.25 ms (error due to frame rate). While the top view indicates a change in helix parameters, the 3D view reveals no apparent changes.