Bacterial flagella are rotating, whip-like appendages that propel bacteria through liquid environments, and they rank among the most sophisticated molecular machines found in nature. Each flagellum consists of three major parts: a long helical filament that acts as a propeller, a short curved hook that works as a universal joint, and a basal body embedded in the cell envelope that houses the rotary motor. What makes these structures remarkable is not just their complexity but how efficiently they self-assemble, how they switch direction on a millisecond timescale, and how their arrangement on the cell surface varies dramatically between species.
The Filament, Hook, and Basal Body
The filament is the visible part of the flagellum, the long helical tube extending from the cell surface. A single filament is typically built from roughly 30,000 copies of a protein called flagellin, stacked into a tubular structure containing 11 near-longitudinal rows of subunits called protofilaments.1PubMed Central. A “mechanistic” explanation of the multiple helical forms adopted by bacterial flagellar filaments This filament is not rigid. Flagellin subunits can adopt two slightly different packing arrangements, and depending on how many protofilaments sit in each state, the filament takes on different helical shapes. Theoretical models predict a family of 12 distinct helical forms, ranging from left-handed to right-handed, and experimental measurements confirm that real filaments closely match these predictions. The ability to shift between helical forms is central to how bacteria change direction, a point we will return to.
The double-tubular architecture of the filament’s core is what gives it this shape-shifting ability. The inner tube is formed by the folded terminal regions of each flagellin molecule, and proper interactions between those terminal regions are what hold the whole thing together.2PubMed. Direct interaction of flagellin termini essential for polymorphic ability of flagellar filament Without this architecture, flagella would be stuck in a single shape and unable to support the tumbling motions that bacteria use to reorient themselves.
Connecting the filament to the motor is a short, curved segment called the hook. The hook functions as a universal joint, transmitting torque from the spinning motor to the helical filament propeller regardless of the angle between them. Cryo-electron microscopy at 3.6 Å resolution has revealed how the hook accomplishes this: its 11 protofilaments undergo continuous cycles of compression and extension, rotating in a pattern resembling a smoke ring, which allows the hook to bend freely while remaining rigid against twisting.3Nature Communications. Structure of the native supercoiled flagellar hook as a universal joint The coiled-coil arrangement of the hook protein’s helices is specifically oriented to permit this axial compression and extension, a design quite different from the filament’s more rigid packing.4PubMed. Specific arrangement of alpha-helical coiled coils in the core domain of the bacterial flagellar hook for the universal joint function
The basal body is the motor and anchor, sitting within the cell envelope. Its detailed architecture was recently resolved at near-atomic resolution, revealing that 173 protein molecules of 13 different types assemble into a complex that spans both the inner and outer membranes and the cell wall in between.5PubMed Central. Molecular structure of the intact bacterial flagellar basal body At the heart of the basal body is the MS ring, an annular structure embedded in the inner membrane. For decades, researchers believed the MS ring had 26-fold symmetry, but high-resolution cryo-EM analyses have now established that it is formed by 34 copies of the protein FliF.6PubMed Central. Structure, Assembly, and Function of Flagella Responsible for Bacterial Locomotion Sitting above the MS ring in the periplasm is the rod, a drive shaft that passes through an LP-ring bushing complex anchored in the outer membrane. This bushing acts as a molecular bearing, allowing the rod to spin freely while holding it in place.
How the Motor Spins
The bacterial flagellar motor is not powered by ATP directly. Instead, it runs on the proton motive force, the electrochemical gradient of hydrogen ions across the inner membrane. This was demonstrated in the 1970s when researchers showed that cells with depleted ATP reserves became motile the moment an electrical potential or pH gradient was artificially imposed across their membranes.7PubMed Central. A protonmotive force drives bacterial flagella In bacteria, the proton motive force also drives ATP synthesis and other essential processes, so the motor is tapping into the cell’s most fundamental energy currency.8PubMed Central. Spatiotemporal dynamics of the proton motive force on single bacterial cells
Torque comes from stator units arranged around the rotor. Each stator unit is built from a ring of five MotA proteins surrounding a central pair of MotB proteins. The MotB dimer anchors to the cell wall, while the MotA pentamer conducts protons across the membrane and transmits mechanical force to the cytoplasmic C-ring of the rotor. Recent work has tested the idea that each stator unit is itself a tiny rotary motor, with the MotA pentamer spinning around the MotB dimer like intermeshed gearwheels driving the larger rotor ring.9PubMed Central. Torque-generating units of the bacterial flagellar motor are rotary motors Up to 11 or more stator units can engage the rotor simultaneously, and each stator has two proton channels passing about 50 protons per revolution, meaning a fully loaded motor consumes roughly 1,100 protons for every turn of the flagellum.10PubMed Central. Flagellar energy costs across the tree of life The maximum spin rate in species such as E. coli reaches about 380 revolutions per second, though normal swimming speeds correspond to lower rates.
Switching Direction and Chemotaxis
The flagellar motor is not a one-way device. It can abruptly reverse its rotation, and this switch is the physical basis of bacterial chemotaxis, the ability to navigate toward nutrients and away from toxins. In species like E. coli, the default rotation is counterclockwise. When all flagella on a cell spin counterclockwise, the helical filaments bundle together into a coherent propeller and the cell swims smoothly forward. When the motor switches to clockwise rotation, the filament bundle flies apart because the helical sense of the filaments no longer cooperates, and the cell tumbles randomly, reorienting itself.
The molecular trigger for this switch is a small signaling protein called CheY. When CheY is phosphorylated by the chemotaxis signaling network, it binds to components of the rotor’s C-ring, promoting clockwise rotation and causing the filament bundle to dissociate.11PubMed Central. Chemotaxis signaling protein CheY binds to the rotor protein FliN to control the direction of flagellar rotation in Escherichia coli The concentration of phosphorylated CheY rises when the cell detects worsening conditions and drops when conditions improve, so the bacterium effectively suppresses tumbling when it is heading in a favorable direction and tumbles more often when things are getting worse. This biased random walk, alternating between smooth runs and reorienting tumbles, allows bacteria to migrate toward attractants over time despite having no brain or nervous system.12PubMed. Control of direction of flagellar rotation in bacterial chemotaxis
Arrangements on the Cell Surface
Not all bacteria wear their flagella the same way, and the arrangement pattern has real consequences for how the organism moves and behaves.
- Peritrichous: Flagella distributed across the entire cell surface, as in E. coli and Salmonella. These cells form the classic run-and-tumble swimming pattern described above.
- Polar: A single flagellum (monotrichous) or a tight tuft of flagella (lophotrichous) at one or both ends of the cell. Many aquatic and soil bacteria use this arrangement for fast, directed swimming.
- Amphitrichous: Flagella at both cell poles. Some species use this to reverse swimming direction without tumbling.
- Periplasmic: Spirochetes like Borrelia and Treponema house their flagella inside the periplasmic space, between the inner and outer membranes, rather than extending them into the surrounding fluid.13PubMed Central. Spirochete Flagella and Motility
Spirochete periplasmic flagella are worth special attention. Because these flagella rotate within the narrow periplasmic space, they generate a corkscrew-like motion of the entire cell body. This form of motility is exceptionally effective in viscous environments, such as connective tissue or the mucous membranes of a host organism. The design has even attracted engineering interest as a model for micro-robots that could navigate thick biological fluids.
Genomic studies have revealed that flagellar diversity runs deeper than the physical arrangement suggests. Among the Enterobacterales alone, comparative analysis found five distinct types of flagellar gene clusters. Roughly a fifth of the species examined carry dual flagellar systems encoded by separate genetic loci, and a handful encode three distinct systems.14PubMed Central. Flagella by numbers: comparative genomic analysis of the supernumerary flagellar systems among the Enterobacterales Some of these secondary systems produce lateral flagella that are deployed only under specific environmental conditions, such as growth on solid surfaces or in thick fluids, while the primary system handles routine swimming.
How a Flagellum Gets Built
Assembling a structure 20 micrometers long from a cell that is only about 2 micrometers wide is no small feat. The flagellum is built from the inside out: first the basal body and export machinery, then the rod, then the hook, and finally the long filament. Structural subunits are pumped through the base by a type III export apparatus that uses the proton motive force as its primary energy source, supplemented by ATP hydrolysis.15PubMed. Protein export through the bacterial flagellar type III export pathway Once exported, subunits travel through a narrow channel only about 2 nanometers wide that runs through the center of the growing structure, all the way to the tip, where they self-assemble onto the end.16PubMed Central. Building a flagellum outside the bacterial cell
The growth mechanism is often described as injection-diffusion: the export apparatus injects unfolded subunits into the channel, and they diffuse along it to the growing tip.17eLife. Bacterial flagella grow through an injection-diffusion mechanism As the filament gets longer, it takes subunits more time to reach the tip by diffusion, so the growth rate slows down. This explains why flagella reach a characteristic length and then effectively stop growing rather than extending indefinitely.
Gram-Positive Versus Gram-Negative Basal Bodies
The basal body is not the same in all bacteria, and the differences trace directly to cell-wall architecture. In Gram-negative species like E. coli, which have both an inner and an outer membrane, the basal body contains four rings arranged in two pairs mounted on a rod. The upper pair forms a cylindrical bushing that sits in the outer membrane and peptidoglycan layer. In Gram-positive species like Bacillus subtilis, which lack an outer membrane, that upper pair of rings is simply absent.18PubMed Central. Fine structure and isolation of the hook-basal body complex of flagella from Escherichia coli and Bacillus subtilis The motor still works, the rotor and stator components are conserved, but the outer bushing has no membrane to anchor to, so evolution discarded it. This is a clean example of how molecular structures reflect the physical constraints of the cell that houses them.
The Energy Budget of a Flagellum
Building and running flagella is expensive. A detailed accounting for E. coli found that constructing a single flagellum about 7.5 micrometers long costs roughly 232 million ATP molecules, with 99% of that cost going into manufacturing the tens of thousands of flagellin subunits that form the filament and hook. An average E. coli cell carries about 3.4 flagella, bringing total construction costs to around 788 million ATP per cell.10PubMed Central. Flagellar energy costs across the tree of life Operating them continuously adds a similar bill: roughly 808 million ATP per cell division cycle at a moderate spin rate, once you convert the proton flow through the motor into ATP equivalents. Together, building and spinning flagella consume about 10% of the cell’s total energy budget.
This is a substantial investment, and bacteria don’t spend it carelessly. Many species shut down flagellar gene expression when they transition to a sessile lifestyle, such as forming a biofilm. Stopping motility frees up energy and metabolic resources that can be redirected toward producing the extracellular matrix that holds a biofilm together, or toward stress tolerance.19PubMed Central. To Move or Not to Move: When and How Bacteria Suppress Flagellar Motility In the soil bacterium Pseudomonas putida, deleting the entire flagellar gene region, about 1.1% of the genome, measurably relieved the cell of a metabolic burden, underscoring how real these costs are.20PubMed. The metabolic cost of flagellar motion in Pseudomonas putida KT2440
Moving Through Thick Fluids
Bacteria do not always swim through thin, watery environments. Many inhabit mucus layers, gut contents, waterlogged soil, or biofilm margins where the surrounding fluid is far more viscous than water. How well flagella perform under these conditions depends on their arrangement and density. Classic experiments found that all flagellated bacteria initially swim faster as viscosity increases modestly, reaching a peak velocity at a characteristic viscosity, and then slow down as the fluid gets thicker still. Peritrichously flagellated bacteria reach their maximum velocity at higher viscosities than polarly flagellated ones.21PubMed Central. Effect of viscosity on bacterial motility
Some species take this adaptation much further. Certain bacteria isolated from nature based on their ability to migrate through agar gels remain motile at viscosities above 1,000 centipoise, far beyond what stops most flagellated cells.22PubMed Central. Motility of flagellated bacteria in viscous environments Proteus mirabilis, a pathogen of the urinary tract, provides a well-studied example. When this bacterium transitions from a typical vegetative cell to a swarmer cell, it dramatically increases the surface density of its flagella, roughly five-fold. This denser coat of flagella, more than the increase in cell length, is what allows swarmer cells to push through fluids viscous enough to stop normal cells in their tracks.23PubMed Central. Flagellum density regulates Proteus mirabilis swarmer cell motility in viscous environments The adaptation may be directly relevant to how the bacterium moves across catheter surfaces and through the urinary tract during infection.
Beyond Swimming
Flagella are best known for propulsion, but they also serve as environmental sensors. When a flagellum encounters a surface, the mechanical load on the motor changes, and the cell can detect this shift. This surface-sensing function acts as a kind of swim-or-stick switch, helping bacteria decide whether to keep swimming or to settle down and begin forming a biofilm.24Trends in Microbiology. Bacterial Flagella: Structure, Function, and Arrangement Once a biofilm commitment is made, flagellar genes are typically shut off, as described in the section on energy costs.
Flagella also play a role in how the immune system detects bacterial invaders. Flagellin, the structural protein of the filament, is recognized by a receptor in human and animal cells called Toll-like receptor 5 (TLR5). This receptor specifically senses monomeric flagellin, flagellin molecules that have been released from the filament, and triggers an inflammatory immune response.25PubMed Central. Involvement of Toll-like receptor 5 in the recognition of flagellated bacteria Flagellin recognition is so fundamental to innate immunity that some pathogenic bacteria have evolved modified flagellin sequences that evade TLR5 detection, while others downregulate flagellar expression once inside a host to avoid alerting the immune system.
Not Related to Archaeal or Eukaryotic Flagella
The word “flagellum” gets applied to motility structures across all domains of life, but this is a naming convention, not an indicator of shared ancestry. Flagellar motility has arisen independently at least three times during evolution: in bacteria, in archaea, and in eukaryotes.26PubMed Central. The evolution of archaeal flagellar filaments Bacterial flagella and archaeal motility structures, now called archaella to avoid confusion, are built from completely unrelated proteins with no common ancestor. They also use different energy sources: bacterial flagella run on ion gradients across the membrane, while archaella are powered by ATP hydrolysis through a dedicated motor protein.27PubMed. Assembly, Functions and Evolution of Archaella, Flagella and Cilia Even structurally, the archaellum lacks the central hollow channel that bacterial flagella use to transport subunits to the growing tip.28PubMed Central. CryoEM structure of the Methanospirillum hungatei archaellum reveals structural features distinct from the bacterial flagellum and type IV pilus The eukaryotic flagellum, found in sperm cells and many single-celled organisms, is different still, containing hundreds of distinct proteins assembled into a complex internal scaffold.
One evolutionary connection that is genuine, however, is between the bacterial flagellum and a very different piece of molecular machinery: the needle-like injection systems that pathogenic bacteria use to deliver toxins directly into host cells. These injection systems, called non-flagellar type III secretion systems, share a core export apparatus with the flagellum, and phylogenomic analysis of over 1,000 genomes argues that the injection system evolved from an ancestral flagellum through a process of co-option, repurposing part of the flagellar structure for a new protein-delivery function.29PubMed Central. The non-flagellar type III secretion system evolved from the bacterial flagellum and diversified into host-cell adapted systems Both systems still share a recognizable type III secretion system at their core, even though one builds a propeller and the other builds a syringe.30PubMed Central. Type III secretion systems: the bacterial flagellum and the injectisome
Swimming at Low Reynolds Number
Understanding why flagella look and behave the way they do requires appreciating the physics of being very small. At the scale of a bacterium, water behaves nothing like what we experience. Inertia is negligible. When a bacterium stops spinning its flagella, it coasts for less than the diameter of an atom before viscous drag brings it to a halt. In this world, a bacterium pushes against the surrounding fluid and the resulting flow dies off over a distance comparable to the cell’s own size.31PubMed Central. Collective swimming and the dynamics of bacterial turbulence Physics at this scale is governed by low Reynolds number flow, where viscous forces completely dominate over inertial ones.32PubMed. Hydrodynamic analysis of flagellated bacteria swimming near one and between two no-slip plane boundaries
This physical regime is why bacterial flagella are helical rather than paddle-shaped. A simple paddle stroke would push fluid forward on the power stroke and then pull it right back on the recovery stroke, producing zero net movement. A rotating helix, on the other hand, breaks this symmetry by continuously converting rotary motion into a thrust that always points in one direction. It is one of the few geometries that produces net movement when inertia offers no help. The whole architecture of the flagellum, from the rotary motor to the helical filament to the universal-joint hook, represents a remarkably complete engineering solution to the problem of moving through a world where coasting is physically impossible.