What Are Flagella and How Do They Function?

Flagella are long, whip-like or corkscrew-shaped appendages that cells use to swim through liquid environments. They exist across all three domains of life, but in radically different forms: the spinning propeller of a bacterium, the undulating tail of a sperm cell, and the helical filament of an archaeon are all called “flagella,” yet they evolved independently, run on different fuel sources, and are built from completely unrelated proteins. Understanding how each version works reveals not just a neat piece of biology but also how cells sense their surroundings, cause disease, and keep your own body functioning.

The Bacterial Flagellum

A bacterial flagellum is essentially a tiny outboard motor. It consists of three main structural parts: a long helical filament that sticks out into the surrounding fluid, a short curved hook that acts as a flexible joint, and a basal body embedded in the cell membrane that serves as the engine.¹ The whole apparatus is built from thousands of protein subunits that self-assemble outward from the cell, with each part having distinct mechanical properties suited to its job.

What makes this motor remarkable is how it generates movement. The basal body is a genuine rotary engine, spinning the filament like a propeller. It draws energy from the flow of protons (hydrogen ions) across the cell membrane. As protons stream through dedicated channel proteins called stators, they drive the rotation of a ring inside the motor, which turns the hook and filament.² In some species, sodium ions do the job instead of protons, but the principle is the same: an electrochemical gradient across the membrane is converted into mechanical spinning.³ Measurements in Streptococcus showed that proton flow is directly proportional to motor speed, meaning the motor is tightly coupled rather than wasteful.²

The hook deserves special mention. It works as a universal joint, transmitting the torque produced by the spinning motor to the helical filament even when the filament points in a different direction from the motor axis.⁴ Without this flexible coupling, a rigid connection would jam or snap every time the cell body wobbled.

How Bacteria Steer

Having a motor is only useful if you can point it somewhere helpful. Bacteria navigate through a process called chemotaxis, and the flagellar motor is at the center of it. When bacteria like Escherichia coli swim, they alternate between two behaviors: a smooth “run,” where all flagella spin counterclockwise and bundle together to push the cell forward, and a brief “tumble,” where one or more motors reverse direction, the bundle flies apart, and the cell randomly reorients. This run-and-tumble pattern is the bacterium’s version of steering.

The switch between running and tumbling is controlled by a signaling molecule called CheY-P, which is produced inside the cell in response to chemicals the cell detects in its environment. When CheY-P levels rise, the motor is more likely to reverse its rotation, causing a tumble. When CheY-P levels drop, the cell runs longer in a favorable direction.⁵ Research has confirmed that all of the FliM molecules in the motor’s switching ring participate in reading this CheY-P signal, giving the motor fine-grained sensitivity to the cell’s chemical surroundings.⁶

The result is surprisingly effective navigation. A bacterium cannot see or think, but by biasing its random walk toward higher concentrations of food or away from toxins, it can cover ground quickly. In the ocean, this ability lets bacteria attach to sinking nutrient particles, hover near photosynthetic algae, and colonize animal hosts, with consequences that ripple up to global-scale biogeochemistry.⁷

Eukaryotic Flagella Are a Completely Different Machine

The flagella found on eukaryotic cells, like the tail of a human sperm cell or the surface appendages of the single-celled alga Chlamydomonas, share a name with bacterial flagella but almost nothing else. Where a bacterial flagellum is a rigid helix that rotates, a eukaryotic flagellum is a flexible, bending structure that generates waves along its length to push the cell through fluid.

The core of a eukaryotic flagellum is a structure called the axoneme, built from microtubules, which are hollow tubes made of the protein tubulin. The standard arrangement is nine pairs of microtubules arranged in a ring around two central single microtubules, a layout biologists call the “9+2” pattern.⁸ Molecular motors called dyneins sit along the outer doublets and use ATP as fuel to slide neighboring microtubule pairs against each other. Because the microtubules are anchored at the base, this sliding is converted into bending, producing the wave-like motion you see in a swimming sperm cell.

Although all eukaryotic flagella share this 9+2 blueprint, the actual waveforms, speeds, and amplitudes of their beating are extremely diverse across species.⁹ Some organisms beat their flagella in nearly planar waves, while others produce a three-dimensional corkscrew motion. Mutations that disrupt the 9+2 architecture lead to disorganized axonemes and impaired movement, confirming that the precise geometry is critical for normal function.¹⁰

How Eukaryotic Flagella Build and Maintain Themselves

Because a eukaryotic flagellum extends far from the cell body and cannot make its own proteins, it relies on a dedicated supply chain called intraflagellar transport, or IFT. Protein cargo is loaded onto molecular “trains” that ride along the microtubule tracks inside the flagellum, traveling outward to deliver new building materials to the growing tip and returning inward with old or signaling components.¹¹ Without IFT, the flagellum cannot assemble or even maintain its length; it slowly shortens and disappears.

This transport system also shuttles signaling molecules between the flagellum and the cell body, connecting the flagellum to the cell’s internal decision-making circuits.¹² The flagellum, in other words, is not just a paddle. It is a dynamic compartment with its own traffic flow.

Three Independent Inventions

One of the most striking facts about flagella is that motile swimming appendages evolved independently at least three times: once in bacteria, once in archaea, and once in eukaryotes.¹³ The three systems share no common ancestor and differ in virtually every detail.

Bacterial flagella are protein filaments rotated by a proton-driven motor. Archaeal flagella (often called “archaella” to emphasize their distinctness) are thinner filaments assembled and rotated by a motor powered by ATP hydrolysis rather than ion flow.¹⁴ Their subunit proteins do not share ancestry with bacterial flagellin. Eukaryotic flagella, meanwhile, use an entirely different engine: dynein motors walking along microtubule tracks, fueled by ATP, producing bending waves instead of rotation. Despite these fundamental differences, all three systems converge on helical or wave-like geometries that work well in the viscous, low-speed world that microbes inhabit.¹⁵

In a recent surprise, functional archaellum-like appendages were found in certain bacteria belonging to the Chloroflexota phylum, suggesting that the archaeal-type system has crossed domain boundaries at some point in evolutionary history. The bacterial versions retain the ATPase-driven assembly and rotation mechanism characteristic of archaella rather than the proton-driven mechanism of typical bacterial flagella.¹⁶

The Bacterial Flagellum’s Evolutionary Legacy

The bacterial flagellar motor shares its core export machinery, called a type III secretion system, with a completely different structure: the injectisome, a needle-like device that pathogenic bacteria use to inject toxins directly into host cells.¹⁷ Evolutionary analysis strongly supports the idea that the flagellum came first and the injectisome evolved from it later, rather than the other way around. Phylogenetic trees place the root of the type III secretion family squarely within flagellar sequences, and the large majority of bootstrap trees favor the flagellum-first scenario.¹⁸ This means a structure that originally evolved for swimming was repurposed into a weapon for attacking host cells.

Spirochetes and Hidden Flagella

Not all bacterial flagella wave freely in the surrounding fluid. Spirochetes, the corkscrew-shaped bacteria responsible for Lyme disease, syphilis, and leptospirosis, keep their flagella tucked inside the cell, in the narrow space between the inner and outer membranes. These periplasmic flagella rotate within this confined corridor, and their movement forces the entire cell body into a twisting, drilling motion that is remarkably effective at boring through thick, viscous environments like mucus and connective tissue.¹⁹

In the Lyme disease agent Borrelia burgdorferi, the periplasmic flagella do not bundle into a single cable as once thought. Instead, they form a flat ribbon that wraps tightly around the cell cylinder. This ribbon arrangement turns out to be mechanically superior for generating the backward-moving waves that propel the cell, since every filament contacts the cell wall directly.²⁰ The connection between spirochete motility and disease is direct: mutants that lose their periplasmic flagella become non-motile and typically lose their ability to infect.

Beyond Swimming: Surface Sensing and Biofilm Formation

Flagella do more than propel a cell through open water. They also help bacteria decide when to stop swimming and settle down. When a free-swimming bacterium encounters a solid surface, its flagellar motor is among the first structures to “feel” the difference. The stalling or slowing of the motor alters ion flow through the stator channels, and this change acts as a signal. It feeds into internal signaling circuits that ultimately trigger the production of sticky substances called exopolysaccharides, which glue cells to the surface and to each other in a biofilm.²¹

Research on Pseudomonas aeruginosa, a bacterium notorious for causing chronic lung infections, has shown that mutations disrupting flagellar components like the hook or the flagellin protein lead to overproduction of the Pel exopolysaccharide and elevated levels of the signaling molecule c-di-GMP, both hallmarks of the biofilm program.²² In effect, when the cell detects that its flagellar system is impaired or physically obstructed, it interprets this as being on a surface and switches into colonization mode. The flagellum is functioning as a touch sensor, not just a motor.

How Your Immune System Spots Flagella

Your body has evolved to recognize bacterial flagella as a danger signal. The protein flagellin, which makes up the filament of a bacterial flagellum, is detected by a receptor called Toll-like receptor 5 (TLR5), part of the innate immune system’s early-warning network. TLR5 recognizes a region of flagellin that is essential for the filament to assemble and function, making it hard for bacteria to mutate around detection without losing their ability to swim.²³

When TLR5 binds flagellin, it triggers an inflammatory response aimed at clearing the invader. Studies in mice have confirmed that TLR5 is essential for recognizing flagellated bacteria both at the whole-animal level and in individual immune cells.²⁴ Human and mouse TLR5 do differ somewhat in which flagellins they respond to most strongly, which matters for interpreting animal research on flagellin-based vaccines and therapies.²⁵ Some pathogenic bacteria have evolved flagellins with altered recognition sites that partially evade TLR5 detection, an ongoing molecular arms race between microbial motility and host surveillance.

Primary Cilia and the Non-Motile Relatives

Most cells in your body carry a structure that is structurally related to a eukaryotic flagellum but does not move at all. These are primary cilia: short, solitary projections that protrude from the surface of nearly all vertebrate cells. Their axoneme follows a “9+0” pattern, meaning the nine outer microtubule doublets are present but the central pair and dynein motors are absent, so they cannot beat.²⁶

Instead of generating movement, primary cilia function as antennae. They display specific receptors on their surface and relay incoming signals to the rest of the cell. Kidney primary cilia, for instance, display a calcium channel that senses fluid flow in the kidney tubules. When the cilium bends under flow, it opens the channel and triggers a calcium signal that spreads through the cell and its neighbors, helping regulate cell growth and differentiation.²⁷ Other cell types use their primary cilia to detect different signals, including hormones like somatostatin and neurotransmitters like serotonin. The list of signaling pathways that run through primary cilia is long and still growing, including Hedgehog, Wnt, Notch, and others that are fundamental to embryonic development and tissue maintenance.²⁶

When Flagella and Cilia Go Wrong

Because motile cilia and flagella rely on the same axonemal machinery, genetic defects that disrupt this machinery tend to cause problems across multiple organ systems at once. Primary ciliary dyskinesia (PCD) is a genetic condition in which motile cilia and sperm flagella do not beat normally, often due to mutations in genes encoding dynein motor components.²⁸

The consequences are wide-ranging. In the airways, immotile cilia cannot clear mucus, leading to chronic respiratory infections and bronchiectasis. In about half of PCD patients, the internal organs are mirror-reversed (a condition called situs inversus), because the left-right patterning of the body during embryonic development depends on the directional flow created by motile cilia in the embryonic node. PCD also affects fertility. In men, immotile sperm flagella frequently cause infertility.²⁹ In women, impaired cilia in the fallopian tubes can reduce the ability to transport eggs, and the condition affects cilia throughout the reproductive tract.³⁰

How Flagella Are Built Step by Step

Assembling a bacterial flagellum is not a haphazard process. The cell builds it from the inside out, starting with the components closest to the membrane and finishing with the filament tip. The genes encoding flagellar parts are organized into a transcriptional hierarchy, often divided into classes. A master regulator gene is expressed first and switches on a set of structural and regulatory genes, which in turn activate later genes as their products are needed for the next stage of assembly.³¹ Checkpoints at each stage ensure that the cell does not waste resources building a filament if the motor has not been properly assembled first. This regulated cascade is one reason the flagellum has attracted so much attention from biologists: it is a case study in how complex structures can be built with precision using gene regulatory logic.

Flagella-Inspired Technology

The physics of flagellar swimming operates in a regime where viscosity dominates and inertia is irrelevant. At the scale of a bacterium, water feels as thick as honey does to us. In this world, a rotating helix naturally translates forward; the geometry of the helix couples rotation to thrust as a consequence of the underlying fluid physics.³² Engineers working on microscale robots have taken direct inspiration from this. Biohybrid swimmers, for example, use living muscle cells cultured onto flexible polymer tails to produce flagellum-like undulations, creating self-propelled devices that swim without any external engine.³³ The spirochete ribbon design mentioned earlier has also been flagged as a potential blueprint for micro-robots that need to move through thick biological fluids.¹⁹

Meanwhile, cryo-electron tomography has given researchers the ability to reconstruct the three-dimensional arrangement of proteins inside cilia and flagella at close to three-nanometer resolution, revealing the exact positions and shapes of dynein motors and their regulatory partners.³⁴ These structural maps are feeding back into both medical research on ciliopathies and engineering efforts to design synthetic molecular motors. The flagellum, whether bacterial or eukaryotic, remains one of the most productive sources of design inspiration at the intersection of biology and nanotechnology.