The slow one wins upstream
Put three kinds of bacteria at the bottom of a channel with water running down it. Vibrio cholerae swims at sixty-four microns a second. E. coli manages twenty. Two hours later the upstream end holds about a hundred thousand E. coli for every Vibrio.
Speed barely matters. In a pipe the flow is zero at the wall and fastest in the middle, so a cell one micron off the glass feels a trickle and a cell ten microns out gets carried off. What wins is staying low and pointing the right way. E. coli has four flagella around its body. Vibrio has one, at the back.
The mechanism has a nice name: the weathervane. Close to the wall the body drags on the glass and works as a pivot. The shear pushes the flagella downstream, the way wind pushes a vane’s tail, so the head swings round to face upstream. The Penn group simulated the whole cell, body and helical filaments in the fluid, and found the count of flagella decides the pitch. With one or two the nose tips up, off the wall, and the cell drifts out into fast water and turns downstream. With more than two the nose points down, the cell stays in the slow layer, and it turns into the current.
The best experiment in the paper didn’t come from physics. They spread E. coli on soft agar, took the cells from the fast-spreading edge, and replated them every day for thirty days. Then they sequenced the winners. The line had duplicated fourteen genes, including the master switch for building flagella, and now carried seven instead of four. It also swam upstream better. Nobody set up the plates to select for hydrodynamics. Spreading through agar happened to reward the same extra tails, and the tails turned out to be what beats a current.
I checked the numbers, because that’s what I do with a paper, and one of them doesn’t hold. The authors link their holography (how high cells ride above the glass) to their flow data (how fast the cells lose ground as shear goes up) and call the two consistent. Their ranking does match. The values don’t. For every E. coli strain the measured height is at least as large as the most that could be true at the highest shear even if every cell faced straight upstream, and their own histograms show only a third do. So in flow, the many-tailed cells have to be riding lower than the holography says.
I don’t think this hurts them. My best guess is that the heights were measured in still water and that peritrichous cells press closer to the wall as the shear rises. That is the nose-down pinning their simulation predicts, only stronger than they claimed. It would take one more experiment to check. I sent the corresponding author the arithmetic.
The second number is smaller but closer to the clinic. In their narrow channels E. coli can make headway up to a shear rate of about 80 per second. On their own flat surface the limit is about 6, which matches older work. Corners explain the gap: where two walls meet, the flow almost stops. The discussion then compares 80 with blood flow. A blood vessel is flat to a two-micron cell, so the 6 is the number that belongs there.
The picture I’m keeping is the race itself. The fastest swimmer in the channel goes nowhere because it can’t stay down and can’t turn around. The slow one has more tails than it strictly needs for thrust, and the extras are what keep it pointed the right way.
