Pseudomonas bacteria turn around on contact to keep crowded colonies from jamming

The most useful result in the study is one the mutant wins. Mutant Pseudomonas aeruginosa cells that keep moving forward after every collision travelled farther when they were alone ^[9]. Moving together, the same cells quickly became trapped in dense clusters ^[9]. That jamming is the problem Alexandre Persat, the EPFL professor whose lab ran the study with a second EPFL group, frames in general terms ^[4]. "Whether in fish schools, bird flocks or human crowds, moving as a group can offer safety and efficiency, but it can also lead to congestion and jamming," he said ^[2].

Wild-type cells get out of jams by turning around. "As they move on surfaces, these bacteria sense when they bump into neighbors," said Laure Le Blanc. "In response, they reverse direction within seconds." ^[5] In crowded patches, frequent bumps set off reversals that kept the group disordered and moving ^[10]. Physical contact alone tends to line neighbouring cells up into coordinated streams that can trap individuals ^[3]. The phys.org report describes the resulting collective order as an actively regulated state ^[15].

One limit follows from how the mutant is described. It keeps going forward after hitting boundaries as well as other cells ^[8], so the maze comparison on its own cannot separate sensing a neighbour from sensing a wall. The claim about neighbours rests on the crowd tracking and the simulations, where the bumps are between cells ^[6][10].

The maze figures are large. Only 1% of mutants reached the exit within four hours ^[12], against 20% of wild-type cells ^[14], about twenty times the rate ^[16]. The report ties the four-hour window to the mutant figure and says the wild-type cells reached the exit "eventually" ^[14], so the two percentages may not cover the same period. Four in five wild-type cells did not get out either ^[17].

The report frames the finding as "physical feedback rather than chemical signals" ^[1]. The experiment compared strains that differ in how they respond to collisions ^[6], and the report does not describe any measurement of chemical signalling. Every experiment it describes ran on lab surfaces or in micro-mazes ^[5][7].

On the design described, I think the surface result holds for this species. The report reaches further. It suggests the work could inform robot swarms that must cross crowded or unknown spaces without getting stuck ^[19]. For the bacteria themselves, it names soil and host tissue as crowded settings where trapped cells struggle to find nutrients or escape ^[3], and says that in people with infections the behaviour may help the bacterium move through host tissue and play a part in forming biofilms ^[18].