Thanks for publishing this, those experiments really are a lot of fun!

Thanks for reading! Here's the link again. (9/9)
biorxiv.org/cgi/content/...

This raises some interesting questions: what sets the value of this limiting torque and how can we use this in our models of the flagellar motor? In which regime (saturated? Linear?) do cells typically swim in their typical ecological niche? (8/9)

As expected, the saturation at high PMF/torque became visible when we increased the viscosity of the medium 3.5 fold to mimic biological fluids: the swimming load was now equivalent to that of a 1 um bead and the swimming speed no longer increased with PMF at the highest PMF. (7/9)

This load is close to the threshold above which the PMF-speed relationship saturates at high PMF/high torque, meaning that cells swimming in low-viscosity buffer may experience this saturation, but only when fully energized. (6/9)

The first direct result is that our protocols allow us to estimate experimentally the mechanical load under which the flagellar motors operate in free-swimming E. coli, which is hard to compute theoretically. We find the load to be equivalent to that of a 0.6 um diameter bead.

We then wondered if this matters only to us flagellar motor aficionados who like to stick beads to bacteria, or if this has implications for swimming bacteria too. Short answer: it has! For this, we measured the average swimming speed of E. coli while changing PMF. (4/9)

To confirm our findings, we studied cells that had both a small bead and a large bead. This way, both motors were driven by the exact same PMF. Again, small bead (low torque) motors remained sensitive to PMF in all conditions, while large bead speeds saturated at high PMF. (3/9)

We attached beads of various sizes to FM of single cells, and measured their rotation speed while modifying the PMF. Motors attached to larger beads were much less sensitive to those changes, suggesting that the well-established PMF-speed linearity saturates at high torque. (2/9)