Primary cilia are found on nearly every vertebrate cell, and their evolutionary roots run deep. Comparative phylogenetic studies of ciliary genes reveal that the basic blueprint of this organelle has been conserved across vast stretches of life’s history, underscoring how fundamentally important it is to cellular physiology. Structurally, the primary cilium is built around a microtubule-based skeleton called the axoneme, which grows out of a modified centriole known as the basal body. Unlike the beating, motile cilia that line the airways, primary cilia are generally non-motile, and their nine pairs of microtubules are arranged in the classic 9+0 configuration rather than the 9+2 arrangement that powers movement. The entire organelle is sheathed in a specialized extension of the plasma membrane, and its interior is physically separated from the cytoplasm at the base by a selective barrier called the transition zone, which functions as a gatekeeper controlling which proteins can enter.

Building and maintaining a cilium is an energy-intensive logistical operation. The organelle contains no machinery for protein synthesis, so every component must be ferried in from the cell body along the axonemal microtubules. This transport system, called intraflagellar transport, or IFT, uses molecular motors such as KIF3AB and KIF17 to haul cargo building blocks toward the ciliary tip, while dynein motors carry return traffic back to the base. Recent work characterizing the IFT cycle has revealed how trains of these motors are coordinated, and how the number of active motors governs transport velocity under load. A multi-protein complex known as the BBSome, named for its role in Bardet-Biedl syndrome, acts as a cargo adaptor that exports specific signalling proteins from the cilium, adding another layer of control. Disrupting any element of this delivery network can compromise the organelle’s composition and, by extension, its ability to relay signals.

The review emphasizes that cholangiocyte primary cilia are exquisitely positioned to monitor the biliary microenvironment, projecting directly into the lumen of the bile duct. Seminal experiments demonstrated that these cilia detect changes in luminal fluid flow and translate mechanical stimulation into intracellular calcium and cyclic AMP signals, establishing them as bona fide mechanosensors. The polycystin proteins, encoded by genes mutated in polycystic kidney disease, are central to this flow-sensing machinery, a discovery first made in kidney epithelial cells and later extended to the biliary system. On the chemical front, cilia carry an impressive array of receptors: the bitter-taste-like bile acid receptor TGR5 sits in the ciliary membrane, where its precise subcellular localization determines how cholangiocytes respond to bile acid signals, and P2Y12 purinergic receptors allow cilia to detect biliary nucleotides. Osmosensing is handled by ion channels such as TRPV4, which respond to changes in luminal tonicity and trigger bicarbonate secretion, a process critical to maintaining the so-called bicarbonate umbrella that protects cholangiocytes from the detergent-like toxicity of bile acids.

Beyond their sensory role, primary cilia serve as cellular command centres for key developmental and tissue-maintenance signalling pathways. The best studied of these is Hedgehog signalling, which requires the cilium for signal transduction in vertebrate cells. Components of the Hedgehog pathway must traffic through the ciliary compartment for the pathway to be activated, and proteins such as Numb have been shown to positively regulate the signal at the ciliary pocket. In the liver, Hedgehog signalling coordinates the behaviour of hepatic progenitor cells, modulates the response to bile duct injury and even influences hepatic lipid metabolism through GLI transcription factors. Cilia also regulate nutrient and growth pathways including mTORC1, which they control through Lkb1, and they maintain compartmentalized pools of cyclic AMP that differ from those in the rest of the cell, allowing vertebrate cells to interpret ciliary and extracellular cyclic AMP in distinct ways. Critically, cilia are coupled to the cell cycle: most cells resorb their cilium before entering mitosis, and experimental biosensors have shown that cilium persistence after the G1/S transition shapes how cells divide and differentiate.

When the structure or function of cholangiocyte cilia goes awry, the result is a biliary ciliopathy. The most prominent examples are the fibrocystic liver diseases that accompany polycystic kidney disorders. In autosomal recessive polycystic kidney disease, mutations in genes such as PKHD1 and DZIP1L, the latter encoding a transition-zone protein, produce dilated, malformed bile ducts through defective remodelling of the embryonic ductal plate. In autosomal dominant polycystic kidney and liver disease, mutations in genes encoding polycystin-1 and its functional effectors, including the glucosidase II alpha subunit GANAB, drive progressive hepatic cystogenesis. Recent mechanistic work has shown that a transforming growth factor beta axis linking the extracellular matrix to integrin signalling physically reconfigures the bile duct wall to promote cyst expansion, while abnormal positioning of ion transporters and water channels in the cyst-lining epithelium fuels fluid secretion into the cyst lumen. Syndromic ciliopathies add to this spectrum: Meckel-Gruber syndrome, frequently lethal in utero, features absent or defective cilia and gross liver malformations; Bardet-Biedl syndrome and Alström syndrome produce hepatic cystic and fibrotic changes alongside their multisystem features, with the ALMS1 protein shown to regulate TGF-beta signalling and cilium morphology; Joubert syndrome commonly involves congenital hepatic fibrosis; and Sensenbrenner syndrome, a cranioectodermal dysplasia, can progress to end-stage liver disease requiring transplantation.

The reach of ciliary dysfunction extends to acquired and neonatal biliary disease as well. Biliary atresia, the leading indication for paediatric liver transplantation, has long been considered an acquired inflammatory obstruction of the bile ducts, yet genome-wide sequencing of patients has uncovered a broad array of ciliary gene mutations in both syndromic and non-syndromic cases. Morphological studies have repeatedly documented abnormal cholangiocyte cilia in affected infants, and organoid models derived from biliary atresia tissue reveal delayed epithelial development and impaired barrier function. Neonatal sclerosing cholangitis caused by mutations in DCDC2, which encodes a protein that disrupts Wnt signalling when lost, further cements the link between ciliary and biliary pathology. Even in adult medicine, the review points to tantalizing connections: ciliary defects in cholangiocytes trigger sustained epidermal growth factor receptor signalling that drives proliferation and cystic lesions in experimental models, andXIAP-mediated degradation of the IFT protein IFT88 in hepatic stellate cells promotes fibrosis, suggesting that cilium loss in different liver cell populations converges on shared fibrogenic pathways.

Perhaps the most provocative frontier involves liver regeneration and cancer. Cholangiocytes have been identified as facultative liver stem cells capable of replenishing hepatocytes when hepatocyte regeneration fails, and the review argues that primary cilia are central regulators of the biliary regenerative response, including in the context of liver transplantation, where biliary injury, senescence and repair intersect. In cholangiocarcinoma, the outlook is strikingly different: ciliary loss appears to promote tumour growth, driven in part by dysregulated microRNAs that induce overexpression of the deacetylase HDAC6 and a cilia-degrading autophagy process called ciliophagy. Encouragingly, pharmacological HDAC6 inhibition restores ciliary expression and reduces tumour growth in preclinical models, providing proof of principle that the cilium itself can be a drug target. The organelle also participates in DNA damage response and repair, meaning that ciliary status may influence how biliary epithelial cells tolerate the genotoxic stress of chronic inflammation.

The methodological toolbox for studying cilia has expanded dramatically, and the review highlights resources that are accelerating discovery. Curated gene compendia such as CiliaCarta, the SYSCILIA gold standard and the predictive database CilioGenics now catalogue well over a thousand ciliary genes, while spatial proteomics has revealed intrinsic heterogeneity among primary cilia that was previously invisible. Advanced experimental platforms range from conditional mouse models with fluorescently labelled cilia to cholangiocyte organoids and functional ciliated cholangiocytes generated from human pluripotent stem cells, which together allow disease mechanisms to be dissected in human-relevant contexts. Computational simulations of ciliary dynamics in oscillatory flow are refining our understanding of how mechanical signals are encoded, and better fixation protocols are improving the reliability of ciliary immunostaining, an issue that has historically complicated comparisons between studies.

The overarching message of the review is one of therapeutic optimism tempered by unmet need. Most biliary ciliopathies currently lack treatments that address their underlying mechanisms; management relies on symptomatic interventions such as drainage of cysts, transplantation or supportive care. By defining the cilium as a signalling hub where mechanical, chemical and osmotic information converges on pathways such as calcium handling, cyclic AMP signalling, Hedgehog signalling, mTORC1 and TGF-beta, the authors lay the groundwork for interventions that could restore ciliary function or correct the downstream signalling errors it produces. With existing drugs already capable of modulating some of these nodes, and with ciliary biology increasingly understood as a common thread linking inherited fibrocystic disease to acquired conditions like sclerosing cholangitis, biliary atresia and post-transplant strictures, these cellular antennae may soon graduate from anatomical curiosities to clinically actionable targets in hepatology.

Subject of Research: The role of cholangiocyte primary cilia in biliary homeostasis and the pathogenesis of biliary ciliopathies

Article Title: Primary cilia: sensors of biliary homeostasis and disease

Article References: Esser, H., & Ferreira-Gonzalez, S. (2026). Primary cilia: sensors of biliary homeostasis and disease. Nature Reviews Gastroenterology & Hepatology. https://doi.org/10.1038/s41575-026-01262-0

Image Credits: AI Generated

DOI: 10.1038/s41575-026-01262-0

Keywords: primary cilia, cholangiocytes, bile ducts, biliary ciliopathies, intraflagellar transport, polycystic liver disease, Hedgehog signalling, biliary atresia, liver fibrosis, mechanosensing, TGR5, liver regeneration

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

The study, published in Nature Microbiology by Alexandre Persat’s Microbial Mechanics Lab in collaboration with Sangwoo Kim’s Mechanics of Soft and Biological Matter Laboratory, both at EPFL, reveals that the opportunistic pathogen Pseudomonas aeruginosa actively senses when it collides with neighboring cells and responds by reversing its direction of travel within seconds. This touch-based feedback loop allows the bacterial population to do something remarkable: stay highly organized where organization matters, such as at the advancing edge of a growing colony, while deliberately dissolving that order in crowded regions where it would otherwise trap individual cells. According to the authors, this shows that collective order in bacterial groups is not merely a passive consequence of physics but an actively regulated state that the cells themselves control.

The problem the bacteria face is one that anyone who has ever been stuck in a dense crowd will recognize. P. aeruginosa moves across surfaces using a form of motility that depends on physical contact with the substrate, and in dense communities the cells frequently line up and stream together in coordinated groups. These physical interactions between neighboring cells create alignment, and alignment creates order. But order has a downside. Individual cells embedded within a coordinated stream can become trapped inside the group, unable to turn away and strike out into new territory. For a bacterium, that inability to disperse is a serious handicap: it prevents the cell from finding fresh nutrients, and it hinders escape from crowded, structurally complex environments such as soil pores or the tissues of a host during infection.

Laure Le Blanc, the paper’s first author, explains the mechanism in simple terms. As the bacteria move across surfaces, they sense when they bump into neighbors, and in response they reverse direction within seconds. That rapid reversal is the key. Each collision acts as a piece of sensory information, telling the cell that its local environment is congested and that continuing straight ahead would be counterproductive. By reversing, the cell injects disorder into the collective motion, loosening the alignment that physical contact tends to create. Where collisions are rare, for example at the open frontier of a colony, the cells align strongly and move together toward unexplored space. Where collisions are frequent, the reversals accumulate and the group remains disordered and mobile. The population effectively uses touch to tune its own degree of collective organization from moment to moment.

To disentangle how individual collisions translate into group-level behavior, the team combined three complementary approaches: live microscopy to watch the bacteria in action, single-cell tracking to follow the trajectories of individual cells, and computer simulations to test how collision-driven reversals shape population dynamics. Crucially, they compared wild-type bacteria, which possess the full sensory machinery, with mutant strains that lack it. This side-by-side comparison allowed the researchers to isolate the specific contribution of collision sensing, rather than inferring it indirectly from the behavior of intact populations alone. By tracking individuals and analyzing overall group movement simultaneously, they could observe in real time how a microscopic bump between two cells rippled outward to reshape the behavior of the entire community.

The contrast between the two strains was striking. The researchers worked with a mutant lacking a gene critical for the bacterium’s normal motility pattern; the mutant always moves forward, even after colliding with other cells or with boundaries. When traveling alone, the mutants actually performed well, covering greater distances than their wild-type counterparts, since relentless forward motion pays off in empty space. But when moving together, the advantage evaporated. Without the ability to respond to collisions, the cells quickly became trapped in dense clusters, jamming against one another much like a crowd pressing into a blocked exit. The wild-type cells, by contrast, used each bump as information, reversing in crowded areas to keep the group fluid and exploratory, while maintaining tight alignment at the colony edge where open space invited coordinated advance.

The most dramatic demonstration came from an experiment that sounds almost whimsical: the researchers built micro-mazes, tiny obstacle-filled labyrinths, and released the two strains inside to see how they would navigate the confined, structured environment. The results were decisive. Only one percent of the collision-blind mutant bacteria managed to reach the maze exit within four hours. The wild-type cells, equipped with their touch-based navigation system, explored an average of ninety-two percent of the maze, and twenty percent of the cells eventually found the exit. In other words, the ability to feel a collision and reverse course transformed a population of microscopic wanderers into efficient problem-solvers capable of threading their way through a physical puzzle that stumped the mutants almost completely.

At the mechanistic level, the findings reframe what bacterial collectives are doing on surfaces. The classical picture treats alignment and streaming in dense bacterial groups as emergent physics, the inevitable result of cells pushing against one another as they crawl. The new work shows that P. aeruginosa actively modulates that physics, converting mechanical interactions into sensory input that feeds back onto individual behavior and, from there, onto the architecture of the whole population. The cells are, in effect, performing a computation: the frequency of collisions in their immediate surroundings encodes the local density of the crowd, and the reversal response reads that signal out as a behavioral decision. This allows the same population to display two very different collective states, ordered at the frontier and disordered in the interior, without any central coordination or chemical signaling between distant cells.

The implications extend well beyond basic microbiology. P. aeruginosa is a notorious opportunistic pathogen, and the researchers suggest that this collision-sensing mechanism may help the bacterium spread through host tissues and contribute to biofilm formation during human infections. Understanding how the cells regulate their collective organization could therefore inform new strategies for interfering with bacterial dispersal and biofilm development, both of which are major challenges in clinical settings where antibiotic-resistant infections are a growing concern. The discovery also carries a lesson for engineers. Robot swarms deployed in crowded or unknown environments face exactly the same trade-off between cohesion and exploration that bacteria face, and they often get stuck for precisely the reasons the mutant cells do. A simple rule, reverse when you bump into something, turns out to be a remarkably effective decentralized algorithm for keeping a swarm mobile, and it requires no central controller, no map, and no communication beyond the moment of contact itself.

What makes the study especially compelling is the way it connects scales. A single molecular event, a cell brushing against a neighbor, triggers a reversal within seconds; thousands of such events sculpt the shape and exploratory capacity of an entire colony; and in a maze, that collective behavior determines whether the population escapes or perishes. Persat’s framing of the question, drawn from fish schools, bird flocks, and human crowds, finds its answer in one of the simplest organisms on Earth: individuals in a group can balance coordination with exploration by using their own bodies as sensors. For bacteria, the crowded environment is not an obstacle to navigation but the very source of the information needed to navigate it. The bump in the crowd, it turns out, is the message.

Subject of Research: Mechanosensing of cell-cell collisions regulating collective navigation in Pseudomonas aeruginosa

Article Title: Bacteria use collisions to navigate crowded environments

Article References: Bacteria use collisions to navigate crowded environments. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Pseudomonas aeruginosa, bacteria, mechanosensing, collective behavior, cell-cell collisions, twitching motility, biofilms, micro-mazes, EPFL, Nature Microbiology, robot swarms, bacterial navigation