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.