The team focused on twitching motility, the form of surface movement that P. aeruginosa powers by extending, attaching and retracting type IV pili—slender protein filaments at one pole of the cell. Previous work from the same group had established that this bacterium practices mechanotaxis: when its pili fail to attach to an obstacle, such as another cell, a molecular switch flips the cell’s polarity and it reverses direction within seconds. The Pil–Chp mechanosensory system lies at the heart of this response. When pili grip the surface successfully, the response regulator PilG promotes forward runs; when attachment is blocked, PilH triggers the polarity switch that produces a contact-induced reversal. By engineering mutants that either reverse constitutively or almost never reverse, the researchers could isolate the role of these collision responses with unusual precision.

The paradox that motivated the study was already visible in earlier experiments. The non-reversing mutant, which lacks the PilH regulator, moves faster than wild-type cells at the single-cell level, yet it spreads poorly across surfaces at the colony scale. Something about the ability to reverse direction must matter for the group, even though it seems to slow individuals down. To resolve this contradiction, the team combined live microscopy, single-cell tracking of fluorescently labelled cells, and agent-based simulations in which bacteria were modelled as self-propelled spherocylinders that reverse upon collision with a defined probability.

The first surprise came from the spatial organization of dense populations. At high surface coverage, the non-reversing mutant self-organized into dense clusters with long-range nematic alignment, exactly as classical models of self-propelled rods predict. Wild-type cells, by contrast, remained uniformly distributed and largely misaligned, even at the same density. Using Voronoi tessellation to quantify spatial spreading and nematic correlation functions to measure alignment, the researchers found that wild-type populations stayed disordered across the entire density range, while the non-reversing mutant’s order grew dramatically as cells crowded together. The constitutively reversing mutant behaved like the wild type, confirming that reversals—not some other difference—were suppressing the ordering.

Simulations then mapped the full parameter space of cell shape and reversal probability, revealing that increasing the collision-reversal probability was sufficient to disrupt clustering and dissipate nematic order at any aspect ratio. Crucially, experimental validation with carefully chosen mutants separated the contributions of shape and behaviour. An elongated mutant that still reversed direction failed to form clusters, while a short, non-reversing mutant formed groups but could not achieve strong nematic alignment. The conclusion was unambiguous: collective order is not dictated solely by physical properties such as cell length and density. It can be actively regulated through mechanosensory control of motility, a biological feedback loop layered on top of the physics.

Single-cell trajectories revealed how this regulation works in practice. The non-reversing mutant followed persistent, near-ballistic paths, streaming cohesively with neighbours and forming vortices. The constitutively reversing mutant jiggled diffusively in place at all densities. Wild-type cells did something more sophisticated: they adapted their movement to local crowding. At low density they moved persistently, with mean squared displacement scaling close to ballistic, but in crowded regions their paths became increasingly tortuous and weakly superdiffusive. Simulations confirmed that collision-induced reversals alone were enough to drive this transition from linear to diffusive motility as density increased.

The most striking behaviour appeared at the expanding edge of the colony. There, wild-type cells switched back to persistent, quasi-ballistic motion and self-organized into ordered rafts that moved coherently into unoccupied space. Imaging of a fluorescent PilG fusion protein provided direct evidence that mechanosensing drives this spatial adaptation. In the dense colony core, the polarity of PilG was randomly oriented, but at the leading edge it was strongly polarized toward open, unexplored space. Because the mechanosensitive pili sit at the cell poles, lateral contacts with neighbours fail to trigger reversals, allowing cells at the front to maintain alignment while the crowded interior disperses. The population effectively reads its own density through collisions and reorganizes accordingly.

This adaptive switching translated into a clear competitive advantage. In head-to-head competition assays, wild-type cells systematically outcompeted both mechanosensing mutants at the colony front. The non-reversing mutant, despite its individual speed, became trapped in ordered groups and vortices, while wild-type cells navigated away from the crowd to colonize new territory. The advantage could not be explained by growth-rate differences or quorum sensing, and it persisted against a short, non-reversing mutant with wild-type-like ordering, demonstrating that the benefit comes specifically from the ability to tune collective order in response to collisions—not merely from being less ordered.

The same sensory logic proved powerful in structured environments. The researchers fabricated micrometre-scale glass mazes by photolithography and released bacteria at the entrance. Non-reversing mutants rapidly accumulated along walls and jammed at corners, clogging the maze. Wild-type cells, by contrast, reversed upon contact with boundaries, using the physical obstacles as navigational cues. Within four hours, wild-type cells explored roughly 92 percent of the maze area, compared with only 65 percent for the mutant, and about 20 percent of wild-type cells successfully reached the exit versus a mere 1 percent of mutants. Competition experiments inside a second maze geometry confirmed the same outcome, mirroring the population-level assays on open surfaces.

The findings reframe how scientists think about bacterial collectives. Classical active-matter models treat order as an emergent property of density, shape and random reorientations; this study shows that a sensory system can actively regulate that order, acting as a rapid, density-dependent behavioural switch that complements slower chemical signalling such as quorum sensing. The principle echoes contact inhibition of locomotion in animal cells, where collisions also reverse polarity and disperse migrating cells, suggesting that mechanosensory feedback is a convergent solution for controlling collective organization across kingdoms of life. For P. aeruginosa, a pathogen that spreads across host tissues and forms biofilms on mucus, the ability to escape crowds and navigate complex terrain may directly contribute to virulence. Beyond biology, the work offers a design principle for active matter and robotic swarms: interactions within a collective can encode information that feeds back to regulate the collective itself.

Subject of Research: Mechanosensory regulation of collective motility in Pseudomonas aeruginosa through contact-induced reversals

Article Title: Bacteria regulate collective behaviour by mechanosensing cell–cell collisions

Article References: Le Blanc, L., Cattaneo, G., Heraud, N., Sarikhani, E., Meganathan, D. P., Tsai, C.-N., Tahir, A., Kühn, M. J., Jahed, Z., Kim, S., & Persat, A. (2026). Bacteria regulate collective behaviour by mechanosensing cell–cell collisions. Nature Microbiology. https://doi.org/10.1038/s41564-026-02505-1

Image Credits: AI Generated

DOI: 10.1038/s41564-026-02505-1

Keywords: Pseudomonas aeruginosa, mechanosensing, twitching motility, type IV pili, collective behaviour, contact-induced reversals, Pil-Chp system, active matter, bacterial colonies, micromazes, mechanotaxis, self-propelled particles