Bacterial thermotaxis by speed modulation

Abstract

Naturally occurring gradients often extend over relatively long distances such that their steepness is too small for bacteria to detect. We studied the bacterial behavior in such thermal gradients. We find that bacteria migrate along shallow thermal gradients due to a change in their swimming speed resulting from the effect of temperature on the intracellular pH, which also depends on the chemical environment. When nutrients are scarce in the environment the bacteria's intracellular pH decreases with temperature. As a result, the swimming speed of the bacteria decreases with temperature, which causes them to slowly drift toward the warm end of the thermal gradient. However, when serine is added to the medium at concentrations >300 μM, the intracellular pH increases causing the swimming speed to increase continuously with temperature, and the bacteria to drift toward the cold end of the temperature gradient. This directional migration is not a result of bacterial thermotaxis in the classical sense, because the steepness of the gradients applied is below the sensing threshold of bacteria. Nevertheless, our results show that the directional switch requires the presence of the bacterial sensing receptors. This seems to be due to the involvement of the receptors in regulating the intracellular pH.

Figure 1

Experimental setup. Two Peltier devices, 10 mm apart, were attached to a microscope slide via thin copper plates on one side and to a heat sink on the other side. The heat sink was simply composed of an aluminum plate through which water was circulated by a water bath to maintain its temperature. The polydimethylsiloxane channels were attached to the other side of the microscope slide and all the exposed spaces of the slide were then covered with Plexiglas for better thermal isolation. All contacts were made using thermal grease.

Figure 2

The effect of serine concentration on the direction of bacterial migration in a shallow temperature gradient. Examples of concentration profiles of the bacteria, without serine added to the medium (A) and with 600 μM serine (B) measured at different times after applying the temperature gradient. The concentrations were measured as explained in the Materials and Methods. All measurements were normalized by the initial concentration at each location to allow better comparison of different experiments. (C) The shift of the bacterial population's center of mass as a function of serine concentration calculated after ∼45 min from applying the temperature gradient. A positive shift indicates migration to the right (higher temperature), whereas a negative shift indicates migration to the left (lower temperature). Each point on the graph is the average of at least three different experiments, and the error bars represent the standard deviation between experiments. Note that the error bar of the 300 μM measurement is very large. That is because some of the experiments exhibited a shift toward higher temperature, whereas in other experiments the bacteria migrated toward a lower temperature.

Figure 3

The effects of serine concentration and temperature on the bacterial swimming speed. (A) The swimming speed of bacteria as a function of serine concentration for different temperatures as indicated in the legend. Note that the increase in the speed occurs around the same serine concentration for all temperatures. The lines depict the function: v(T, S) = v_min(T) + [v_max(T) − v_min(T)] / [1 + exp[ − (S − S_H) / S₀]], with S_H ≈ 250 mM, S₀ ≈ 30 mM, and v_min(T) and v_max(T) are temperature-dependent functions that represent the speed of the bacteria at low and high serine concentration, respectively. Here, v_min(T) was calculated by averaging the speed measured at 0 and 100 μM serine, whereas v_max(T) was calculated by averaging the speed measured at 400, 500, and 600 μM serine. To get a better sense of how v_min(T) and v_max(T) depend on temperature, the speed of bacteria as a function of temperature (in MB without serine and with 600 μM serine) is presented in (B). Each point in these graphs was calculated from a few thousand trajectories acquired in at least two different experiments.

Figure 4

The effects of serine and temperature on the membrane potential. (A) The membrane potential of bacteria as a function of serine concentration for different temperatures as indicated in the figure legend. Note that the membrane potential is not affected by an increase in the serine concentration at different temperatures. (B) The membrane potential of bacteria as a function of temperature at different serine concentrations as indicated in the figure legend. The membrane potential increases up to 35°C and decreases above that temperature for all serine concentrations. All measurements were carried out as described in the Materials and Methods.

Figure 5

The effect of serine on the intracellular pH. (A) The intracellular pH at different temperatures as a function of serine concentration in the medium, measured as described in the Materials and Methods. (B) The difference between the intracellular and extracellular pH as a function of serine at different temperatures calculated using the measurements in (A) and Fig. 6A inset. The lines in the graphs are to guide the reader.

Figure 6

The effect of temperature on the pH difference and the PMF. (A) The intracellular pH as a function of temperature with and without 600 μM serine. The pH in both cases is almost the same up to 30°C, after which it starts increasing with temperature when serine is present in the medium and decreasing when no nutrients are added. The extracellular pH on the other hand decreases with temperature exactly the same with and without serine as depicted in the inset. (B) The PMF calculated as defined in Eq. 3 using the measurements of the membrane potential and the pH described previously in Fig. 4B and Fig. 6A, respectively.