Figure 1

(a) Schematic representation of the oxygen dye. The fluorescence of the ruthenium complex is quenched by oxygen. The encapsulation of the ruthenium complex in phospholipid micelles, “Ru-micelles,” makes it biologically inert while conserving its sensitivity to oxygen. (b) Bacteria expressing yellow fluorescent protein were suspended in a minimal buffer and deposited in a square adhesive-sealed chamber. Samples were quickly prepared under argon and sealed with epoxy glue. Fluorescent bacteria were then tracked in the focal plane of the microscope, revealing a few thousand 2-dimensional random walks [27]. Movies of 10–20 s were acquired every 2–3 min at a $40\times$ magnification [15].Reuse & Permissions

Figure 2

Motility of bacteria in a sealed sample as a function of time. (a) Oxygen partial pressure $p{\mathrm{O}}_2$ (blue open circles) decreases during the first $\sim 30\pm 10\min$ to reach a negligible and stable value. Mean diffusion coefficients $D$ (black solid circles) are obtained from a linear fit of the mean square displacement (MSD). Inset: MSD of both motile ($M$) and nonmotile ($NM$) bacteria. The dashed line with a slope $\alpha =1$ is a guide to the eyes. (b) Calculated average speed $V$ (purple triangles) and deduced tumbling time ${\tau }_{\mathrm{tumb}}$ (cian circles). ${\tau }_{\mathrm{tumb}}$ is only represented during the first $\sim 35\min$ while bacteria were actively diffusive. (c) Diffusion coefficient $D$ measured in a buffer supplemented with: 250 mM threonine (red squares) and 250 mM serine (black triangles).Reuse & Permissions

Figure 3

(a) Open samples containing fluorescent bacteria and Ru-micelles were prepared under argon and kept under the glove box until bacteria have transited to a nonmotile state. Samples were then disposed onto a transilluminator and exposed to air to let oxygen penetrate through the frame into the anaerobic bacterial solution. Time-lapse images of the whole sample were acquired every 15 min. (b) Picture of a $15\times 15\mathrm{mm}$ sample: in yellow (or light gray) the bacterial fluorescence at $\sim 527\mathrm{nm}$ and in red (or gray) the Ru-micelle fluorescence at $\sim 615\mathrm{nm}$. Ru-micelle fluorescence is quenched by oxygen, leading to a gradient of fluorescence intensity in the sample [15].Reuse & Permissions

Figure 4

Each curve corresponds to a measurement every 4 h from the time oxygen started to diffuse into the bacterial solution. Solid lines represent the oxygen partial pressure $p{\mathrm{O}}_2$ given with an error of $\pm 10\%$ and dashed lines represent the bacterial concentration [$B$] given with an error of $\pm 10\%$ (see 15). Inset: Position $l$ of a 1-dimensional front of bacteria as a function of time. 3 of the 4 edges of the sample were sealed. Oxygen was then allowed to penetrate only from one side leading to the formation of a 1-dimensional front, preventing any dimensional effects of the diffusion [15]. A linear fit yields a velocity of $\sim 70\pm 30\mathrm{nm}/\mathrm{s}$.Reuse & Permissions