The primary extracellular structure that helps prokaryotes move is the flagellum, a long, whip-like filament that rotates like a propeller to push the cell through liquid environments. Prokaryotes also use other structures for movement, including pili that pull cells across surfaces, but the flagellum is the most widespread and well-studied motility apparatus in both bacteria and archaea.
How the Bacterial Flagellum Works
A bacterial flagellum has three main parts: a basal body embedded in the cell membrane, a flexible hook, and a long filament that extends into the surrounding environment. The basal body acts as a rotary motor, spinning the entire structure. The hook works like a universal joint, transferring that rotational force to the filament. The filament itself is the largest component, built from roughly 30,000 copies of a protein called flagellin and growing up to about 15 micrometers long.
What makes this motor remarkable is its energy source. Rather than burning ATP directly, the flagellar motor is powered by a flow of ions (usually protons or sodium ions) across the cell membrane. This electrochemical gradient spins the motor the way water flowing over a waterwheel generates rotation. Under low-resistance conditions, a single proton-powered motor unit can spin at about 30 revolutions per second. With multiple motor units working together, speeds can reach 250 to 300 revolutions per second, and sodium-driven motors have been clocked at 400 revolutions per second.
Flagellar Arrangement Patterns
Not every bacterium positions its flagella the same way, and the arrangement affects how the cell moves. There are four main patterns:
- Monotrichous: a single flagellum at one end of the cell
- Amphitrichous: a single flagellum (or tuft) at each end
- Lophotrichous: a tuft of several flagella extending from one or both ends
- Peritrichous: multiple flagella distributed randomly across the entire cell surface
Peritrichous bacteria like E. coli bundle their flagella together during forward swimming. When the motors reverse direction, the bundle flies apart and the cell tumbles randomly before reorienting and swimming in a new direction.
Run and Tumble: How Prokaryotes Steer
Flagella don’t just push cells forward blindly. Bacteria use a signaling system called chemotaxis to detect chemicals in their environment and adjust their swimming direction accordingly. Receptor proteins clustered at the cell’s poles sense whether conditions are getting better or worse. They relay that information through a messenger protein that travels to the flagellar motor and changes the probability of the motor switching its rotation direction.
When the motor spins counterclockwise (the default in E. coli), the cell swims smoothly forward in what’s called a “run.” When the motor briefly switches to clockwise rotation, the cell tumbles and reorients. If a bacterium is swimming toward a food source, it tumbles less often and runs longer. If it’s heading away, tumbling frequency increases, giving it more chances to find a better direction. A typical E. coli cell has five to ten flagellar motors scattered around its surface, all coordinated through this signaling system.
Archaella: The Archaeal Version
Archaea have their own rotary propellers called archaella. These look superficially similar to bacterial flagella, spinning an extracellular filament to generate thrust, but they are fundamentally different structures. The simplest known archaellum, from the heat-loving archaeon Sulfolobus acidocaldarius, is built from just seven proteins, compared to the roughly 25 proteins in a bacterial flagellum. The motor complex is also much smaller, measuring under 50 nanometers across versus the larger bacterial motor.
The energy source differs too. Bacterial flagella run on ion flow across the membrane, but archaella appear to be powered by ATP hydrolysis. A single protein handles both the assembly of the filament and its rotation. The filament grows to several micrometers long before the machinery switches from building mode to spinning mode. Another notable difference: archaellar filaments maintain a constant right-handed helix shape regardless of which direction they spin, while bacterial flagella can shift between several different helical forms.
Type IV Pili and Twitching Motility
Flagella aren’t the only game in town. Many bacteria also move using thin, hair-like appendages called type IV pili. These work on a completely different principle. Instead of rotating, pili extend outward from one pole of the cell, attach to a surface at their tip, and then retract like a grappling hook, dragging the cell forward. This produces a jerky, crawling movement called twitching motility.
The extension step is driven by assembling pilin protein subunits into a fiber that pushes through a pore in the outer membrane. Retraction depends on a specific protein that uses ATP to pull the fiber back in. Bacteria with mutations that disable this retraction protein still produce pili and can attach to surfaces, but they can’t pull themselves forward. This retraction force is surprisingly powerful for a single-celled organism and plays a role in how certain pathogens like Neisseria gonorrhoeae and Pseudomonas aeruginosa colonize host tissues.
Periplasmic Flagella in Spirochetes
Spirochetes, the corkscrew-shaped bacteria responsible for Lyme disease and syphilis, take a different approach entirely. Their flagella are hidden inside the cell, sandwiched between the inner and outer membranes in a space called the periplasm. The Lyme disease agent Borrelia burgdorferi has 7 to 11 of these periplasmic flagella inserted at each end of the cell. They wrap around the cell body in flat ribbons, and their rotation within the confined periplasmic space causes the entire cell to twist and bore forward like a corkscrew.
This design is exceptionally effective in thick, viscous environments. While conventional flagella work best in watery conditions, spirochetes can power through gel-like tissues and mucus far more efficiently than surface-flagellated bacteria. The periplasmic flagella share the same basic components as external flagella (motor, hook, and filament) but are entirely internal, which also gives the cell its characteristic flat-wave shape.
Gliding Motility
Some bacteria move across solid surfaces without any visible appendage at all, in a process called gliding motility. In Myxococcus xanthus, gliding is powered by motor complexes in the cell membrane that travel along helical tracks at speeds of 3 to 5 micrometers per second. These motors use the same type of proton-driven energy that powers flagella, and the core motor proteins are structurally related to flagellar motor components. A bacterial version of actin forms internal filaments that appear to guide the motor complexes along their helical paths, propelling the cell forward as they move.
Some filamentous cyanobacteria use a modified version of type IV pili to push rather than pull themselves forward, blurring the line between pilus-based and gliding motility. The full molecular details of gliding remain an active area of investigation, with dozens of associated proteins identified across the inner membrane, outer membrane, and the space between them.