Bacteria move using a surprisingly diverse toolkit of mechanisms, from spinning tail-like appendages to grappling hooks to hijacking the internal machinery of the cells they infect. The method depends on the species and the environment. Some swim through liquid at speeds of 400 micrometers per second, while others crawl across surfaces or bore through thick, gel-like mucus. Here’s how each system works.
Swimming With Flagella
The most common form of bacterial movement is swimming, powered by one or more whip-like structures called flagella. Each flagellum is a semi-rigid, corkscrew-shaped filament anchored to a tiny rotary motor embedded in the cell membrane. This motor doesn’t run on the chemical fuel most cells use (ATP). Instead, it’s driven by a flow of charged particles across the membrane, essentially a current of protons that spins the motor like water turning a turbine. The flagellum rotates at hundreds of revolutions per second, propelling the bacterium forward.
Different species arrange their flagella in different ways, and the arrangement changes how they swim. Some bacteria have a single flagellum at one end. Others have a tuft of flagella clustered together, or flagella distributed all over the cell surface. E. coli, one of the most studied swimmers, has flagella scattered across its body. When all those motors spin counterclockwise, the flagella bundle together into a single propeller and the cell cruises in a straight line. When one or more motors reverse direction, the bundle flies apart and the cell tumbles randomly, reorienting before the next straight run.
E. coli tops out at about 40 micrometers per second, which sounds slow until you consider the cell is only about 2 micrometers long. That’s roughly 20 body lengths per second, proportionally faster than most fish. Some marine bacteria reach speeds of 400 micrometers per second.
How Bacteria Steer
Swimming in a straight line isn’t useful without the ability to steer. Bacteria navigate through a process called chemotaxis, alternating between straight runs and random tumbles to gradually move toward food or away from toxins. The system works like a simple memory. As a bacterium swims, receptors on its surface constantly sample the concentration of chemicals in the environment. If conditions are improving (more nutrients, fewer harmful compounds), the cell keeps running. If conditions worsen, the cell tumbles sooner, picking a new random direction.
The molecular switch controlling this behavior is a signaling protein called CheY. When CheY is activated by a chemical signal from the receptors, it binds to the flagellar motor and triggers clockwise rotation, causing a tumble. When attractive chemicals are increasing in concentration, the signal to CheY is suppressed, runs last longer, and the bacterium drifts up the gradient. A separate system of enzymes constantly resets the receptors, giving the cell a fresh baseline to compare against. This adaptation step is what creates the “memory,” allowing the bacterium to detect changes over time rather than just absolute concentrations.
Twitching: The Grappling Hook
On solid surfaces, many bacteria switch to a completely different strategy. Species like Pseudomonas aeruginosa extend long, thin protein fibers called type IV pili from their surface. These work like grappling hooks: the pilus shoots out, sticks to a surface, then retracts, pulling the cell forward. The retraction is powered by molecular motors that burn ATP, and it generates remarkable force for something so small.
This “twitching” motility is jerky and slower than swimming, but it lets bacteria crawl across tissues, medical devices, and other surfaces where swimming isn’t possible. It also plays a role in infection, helping bacteria press through host tissues to reach deeper sites.
Gliding Without Appendages
Some bacteria glide smoothly across surfaces without any visible propulsion structure. How they do this has puzzled scientists for decades, and the answer turns out to vary by species. In Myxococcus xanthus, a soil bacterium, gliding relies on more than 37 genes and appears to involve multiple small motor elements arrayed along the entire length of the cell body. These motors sit between the inner and outer membranes, anchored to the rigid cell wall, and they push adhesion points along the outer membrane like treads on a tank.
Other gliding bacteria use different strategies entirely. Some filamentous cyanobacteria secrete slime through specialized pore structures at their cell junctions, and the force of that secretion pushes them forward. Still others appear to propagate waves along their cell surface. The common thread is smooth, steady movement across a solid surface without flagella or pili.
Corkscrew Motion Through Thick Fluids
Spirochetes, the group of bacteria that includes the species causing Lyme disease and syphilis, have a unique solution for moving through viscous environments like mucus, connective tissue, and bodily fluids. Their flagella are internal, running between the inner and outer membranes of the cell. When these internal flagella rotate, they cause the entire cell body to undulate or roll in a corkscrew motion.
This design gives spirochetes an unusual advantage. Most bacteria slow down as their surroundings get thicker and stickier, but spirochetes actually speed up in gel-like, polymer-rich fluids. The corkscrew shape bores through viscous material the way a drill bit moves through wood. This is one reason spirochetes are so effective at penetrating tissues during infection. The speed boost doesn’t work in all thick fluids, only in those with a gel-like, heterogeneous structure similar to the mucus and connective tissue found in the body.
Swarming as a Group
Under the right conditions, some bacteria stop swimming as individuals and begin moving collectively across surfaces in coordinated swarms. This transition requires several triggers. The surface must be moist but not wet enough to swim through. Cell density needs to be high. And many species produce surfactants, soap-like molecules that reduce surface tension and create a thin film of fluid the swarm can glide across. In Bacillus subtilis, for example, cells must reach a critical population density before they begin producing enough surfactant to enable group movement.
There’s a lag period when bacteria first contact a surface during which they’re non-motile, apparently sensing and preparing for the transition. Some species detect surface contact through their flagella: when the motor’s rotation is physically impeded by a surface, changes in ion flow through the motor trigger a genetic program that activates swarming. Swarming bacteria often become elongated, grow extra flagella, and display significantly greater resistance to antibiotics compared to their free-swimming counterparts.
Hijacking Host Cells From the Inside
A few intracellular pathogens have evolved a way to move that doesn’t involve any bacterial motor at all. Listeria monocytogenes, which causes the foodborne illness listeriosis, hijacks the structural scaffolding inside human cells. Once inside a host cell, Listeria produces a single surface protein called ActA that activates the host cell’s own machinery for building actin filaments, the protein fibers cells use to maintain their shape and move.
The host’s actin-building machinery assembles a dense, branching network of filaments on one side of the bacterium, forming a structure called a comet tail. As new filaments grow, they don’t push the bacterium directly. Instead, they accumulate around the bacterial surface, building up mechanical stress that eventually releases as a burst of forward force, like compressing a spring. This propels the bacterium through the host cell’s interior and can even push it into neighboring cells, allowing the infection to spread without the bacteria ever being exposed to the immune system in the space between cells.
Why Movement Matters for Infection
Bacterial motility isn’t just a curiosity of microbiology. It’s a key factor in how infections establish, spread, and resist treatment. Swimming allows bacteria to disperse from biofilm colonies and colonize new tissues. Twitching motility helps bacteria penetrate deeper into wounds and damaged skin. Swarming populations show heightened antibiotic resistance, making surface infections harder to clear. Even bacteria traditionally considered non-motile, like round-shaped cocci, can slide across surfaces using passive mechanisms that aid in wound colonization.
Biofilm formation, one of the biggest challenges in treating chronic infections and contaminated medical devices, depends heavily on motility in its early stages. Bacteria must first reach a surface, attach, and then use surface-based movement to organize into the structured communities that become biofilms. Disrupting any of these motility systems is an active area of interest for developing new ways to prevent and treat bacterial infections.