Actin-like cytoskeleton filaments contribute to cell mechanics in bacteria

Abstract

A filamentous cytoskeleton largely governs the physical shape and mechanical properties of eukaryotic cells. In bacteria, proteins homologous to all three classes of eukaryotic cytoskeletal filaments have recently been discovered. These proteins are essential for the maintenance of bacterial cell shape and have been shown to guide the localization of key cell-wall-modifying enzymes. However, whether the bacterial cytoskeleton is stiff enough to affect the overall mechanical rigidity of a cell has not been probed. Here, we used an optical trap to measure the bending rigidity of live Escherichia coli cells. We find that the actin-homolog MreB contributes nearly as much to the stiffness of a cell as the peptidoglycan cell wall. By quantitatively modeling these measurements, our data indicate that the MreB is rigidly linked to the cell wall, increasing the mechanical stiffness of the overall system. These data are the first evidence that the bacterial cytoskeleton contributes to the mechanical integrity of a cell in much the same way as it does in eukaryotes.

Fig. 1.

Experimental setup. (A) Schematic representation of the experimental setup. The free part and stuck part of the cell are shown in yellow and blue respectively. The optically trapped bead is shown in red. (B) Typical DIC image of an E. coli filament with bound bead.

Fig. 2.

The force-displacement curves and bending stiffness of single E. coli cells. (A) The force-displacement curves of the tip of a cell before and after A22 treatment. (B) The bending stiffness of a cell upon repeated addition and removal of A22. (C) The stiffness of single cells over time after the addition of A22 (Red) or ampicillin (Blue). Strains: BW25113 motA <  > Amp^R/ pWR20 for A and B; BW25113 motA <  > Kan^R (ampicillin sensitive) for C. Error bars indicate 95% confidence intervals.

Fig. 3.

Statistics on the A22-induced decrease in flexural rigidity. (A) The mean and standard error of the flexural rigidity before and after A22 treatment for 10 wild-type WA220 cells. (B) The distribution of the flexural rigidity of the 10 cells in A. (C) The flexural rigidity drop after A22 treatment versus the original flexural rigidity for the 10 wild-type cells and 10 A22 resistant cells. (D) The mean and standard error of the relative flexural rigidity after A22 treatment. (The values are normalized to the flexural rigidity prior to A22 treatment.) Wild type: WA220 (W3110 zhc-12::Tn10 mreB+). A22 resistant strain: WA221 (W3110 zhc-12::Tn10 mreB221). Control: WA220 treated with methanol. **: p < 0.01.