Average swimming speed of cells as a function of viscosity, varied by addition of glycerol or PEG 35000. The speed data were generated by MATLAB analysis of videos taken at 5 minutes after mixing bacteria with the viscous solution and within 15 minutes of synchronizing the swarmer cells. The solid lines are power law fits to collective data of all three strains, yielding exponents of −1.06 in viscous (glycerol) and −0.45 in viscoelastic (PEG 35000) solutions, respectively. The numbers of cells with their speeds measured were, for wild-type, 80 in PYE (control), 2084 in glycerol, and 773 in PEG 35000; for Δpilin, 23 in PYE, 270 in glycerol, and 170 in PEG 35000; and for SB3860, 181 in PYE, 1033 in glycerol, and 868 in PEG 35000. The error bars represent standard deviations, to convey the fact that there was a natural spread of swimming speed under each condition.