The flagellum is a whip-like structure that cells use primarily for movement. In bacteria, it works as a tiny rotary motor that spins to propel the cell through liquid environments. In eukaryotic cells, including human sperm, it beats in a wave-like motion to push the cell forward. But locomotion is only part of the story. Flagella also help cells navigate toward food, sense surfaces, stick to tissues, and form protective communities.
How Bacterial Flagella Power Movement
A bacterial flagellum is essentially a biological outboard motor. It has three main parts: a basal body embedded in the cell membrane that acts as a rotary engine, a flexible hook that works as a universal joint, and a long helical filament that functions like a propeller. The basal body spins the rigid rod (a drive shaft), which transmits torque through the hook to the filament. The whole assembly rotates to push the bacterium forward.
The motor can spin in two directions, and this switching ability gives bacteria a surprisingly effective way to navigate. When all flagella on a cell rotate counterclockwise, the filaments bundle together behind the cell and drive it in a straight line. This is called a “run.” When one or more motors flip to clockwise rotation, the bundle falls apart and the cell tumbles randomly, reorienting itself. By alternating between running and tumbling, bacteria can steer toward nutrients or away from harmful chemicals.
What powers this motor? Not the same energy currency (ATP) that fuels most cellular work. Instead, bacterial flagella run on a flow of charged particles, typically protons, across the cell membrane. Experiments with Streptococcus bacteria showed that starved cells stopped swimming when deprived of glucose, but became motile again when researchers artificially created an electrical or pH gradient across the membrane, even without any significant new ATP production. The motor converts this proton flow into mechanical rotation.
Navigating With Chemotaxis
Flagella do more than just move a cell from point A to point B. They are central to chemotaxis, the ability of bacteria to sense chemical gradients and swim toward favorable conditions (like a food source) or away from threats (like a toxin). Receptor proteins on the cell surface detect chemical signals and relay that information through a signaling cascade to the flagellar motor. This cascade controls whether the motor spins counterclockwise (swim straight) or clockwise (tumble and change direction).
The result is biased movement. A bacterium swimming toward increasing concentrations of a nutrient will tumble less often, extending its runs in the right direction. Swimming into worsening conditions triggers more frequent tumbles, sending the cell off on a new random heading. Over time, the cell drifts toward the good stuff. This system is remarkably sensitive, allowing bacteria to detect tiny changes in chemical concentration over distances many times their own body length.
Sensing Surfaces and Forming Biofilms
Flagella also function as mechanical sensors. When a bacterium encounters a surface, the increased resistance on the flagellar motor changes the flow of ions through the motor’s stationary components (stator units). In high-load conditions, like pushing against a viscous environment or a solid surface, stator units stay attached to the motor longer, allowing the motor to recruit more of them and generate higher torque. In low-load conditions, stator units detach more quickly.
This load-sensing ability is one way bacteria “know” they’ve hit a surface, triggering a transition from free-swimming to surface-attached behavior. Once settled, bacteria can form biofilms, the slimy, structured communities that protect them from antibiotics and the immune system. The flagellum plays a role in the early stages of this process, both by delivering the cell to the surface and by helping it adhere. In Salmonella, for example, flagella can mediate attachment to human intestinal cells either indirectly through motility (getting close enough to stick) or by binding directly to the cell surface.
Flagella in Eukaryotic Cells
Eukaryotic flagella, found in organisms from single-celled protists to human sperm, serve the same basic purpose of movement but work in a completely different way. Instead of rotating like a propeller, they bend in coordinated waves. The internal structure is built around a ring of nine pairs of tube-like proteins surrounding two central tubes. Thousands of tiny molecular motors line the paired tubes and use ATP to slide them past one another, producing the bending motion that drives the cell forward.
The most familiar example is the human sperm cell, which has a single flagellum (its tail) that propels it through the female reproductive tract to reach and fertilize an egg. Effective flagellar beating depends on careful regulation of the chemical environment inside the tail, particularly the balance of ions like calcium and hydrogen. As sperm mature in the reproductive tract, a process called capacitation alters the cell membrane and internal chemistry, eventually triggering “hyperactivated” motility: a powerful, asymmetrical beating pattern that generates enough force to penetrate the egg’s protective layers.
Archaeal Flagella Are a Separate Invention
Archaea, the third domain of life alongside bacteria and eukaryotes, have their own version of the flagellum called the archaellum. It looks and functions much like a bacterial flagellum at first glance: a helical filament rotated by a membrane-embedded motor to generate thrust. But the resemblance is superficial. Archaella evolved independently, descending from a completely different molecular ancestor related to a family of structures involved in secretion and surface attachment.
The archaellum is considerably simpler. The best-studied version, from a heat-loving microbe called Sulfolobus acidocaldarius, is built from just seven proteins. Bacterial flagella, by comparison, require around 25 proteins, and their motors are over 50 nanometers wide. Despite this simplicity, archaella accomplish the same core task: spinning an extracellular filament to push the cell through its environment.
Role in Infection and Disease
For disease-causing bacteria, flagella are tools of invasion. Motility lets pathogens swim through mucus layers, penetrate tissues, and reach specific sites in the body where they can establish infections. Chemotaxis guides them toward the chemical signals released by host cells. And the flagellar filament itself, made of a protein called flagellin, can directly bind to host tissue, giving the bacterium a physical grip on the cells it’s trying to infect.
Research on Salmonella illustrates how these roles overlap. In one strain (serovar Dublin), the flagellar filament was required for both adhesion and invasion of human intestinal cells, but chemotaxis-related genes were dispensable, suggesting that simply being near the cell with a sticky flagellum was enough. In a different strain (serovar Typhimurium), deleting both flagellin and chemotaxis genes reduced adhesion, indicating that directed swimming also mattered for that particular pathogen’s strategy. The immune system recognizes flagellin as a danger signal, which is why some pathogens have evolved ways to modify or hide their flagella to avoid detection.