Bacteria are single-celled organisms, many of which can move independently. This motility is crucial for their survival, allowing them to seek nutrients and favorable conditions, such as optimal oxygen levels or temperatures. It also enables them to escape harmful substances, like toxins or antibiotics, and colonize new surfaces or hosts.
The Bacterial Flagellum
The bacterial flagellum is a complex, whip-like appendage that functions as a primary means of propulsion for many bacteria. This structure is composed of three main parts: the filament, the hook, and the basal body. The filament is the longest and most visible part, extending outward from the cell surface as a rigid, helical propeller.
Connecting the filament to the cell surface is the hook, a short, curved segment that acts as a universal joint. Embedded within the cell membrane and cell wall is the basal body, which serves as the rotary motor for the flagellum. This motor is powered by a flow of protons across the cell membrane, generating what is known as the proton motive force.
The basal body’s rotation causes the filament to spin, much like a boat propeller, generating thrust that pushes the bacterium through its liquid environment. Flagella arrangement varies among species. Some bacteria have a single flagellum at one end, others a tuft at one end, or a single flagellum at each end. Many, like Escherichia coli, have flagella distributed all over their cell surface.
Other Motility Mechanisms
Beyond flagella, bacteria employ several other distinct mechanisms for movement. One such method is twitching motility, which is commonly observed in bacteria like Pseudomonas aeruginosa. This movement is mediated by structures called type IV pili, which are long, thin protein appendages. These pili extend from the bacterial cell, attach to a surface, and then retract, pulling the cell forward in a jerky, “twitching” motion.
Gliding motility involves bacteria sliding smoothly over solid surfaces without flagella or pili. Some achieve this by secreting a slime layer for propulsion, while others use specific adhesion proteins that interact with the substrate to facilitate movement.
Spirochetes, a unique group of spiral-shaped bacteria, exhibit a distinctive corkscrew-like movement. Their motility is achieved through internal flagella, often referred to as axial filaments, which are located within the periplasmic space between the inner and outer cell membranes. The rotation of these internal flagella causes the entire cell body to twist and flex, enabling the spirochete to bore through viscous environments.
Directing Bacterial Movement
Bacteria do not simply move aimlessly; they actively direct their movement in response to environmental signals. This controlled movement is broadly termed taxis, which allows bacteria to navigate towards favorable conditions or away from harmful ones. Chemotaxis is a common form of taxis, where bacteria sense and respond to chemical gradients in their surroundings.
When a bacterium detects an increasing concentration of an attractant, such as a nutrient, it will adjust its movement to move closer to the source. Conversely, if it senses a repellent, like a toxic substance, it will reorient itself to move away. For flagellated bacteria, this often involves a “run-and-tumble” behavior. A “run” occurs when flagella rotate in a coordinated manner, propelling the bacterium in a relatively straight line.
A “tumble” occurs when the flagella momentarily reverse their rotation, causing the bacterium to randomly reorient itself. By modulating the frequency and duration of runs and tumbles, bacteria can effectively bias their movement towards attractants and away from repellents. Another form of directed movement is phototaxis, where bacteria respond to light stimuli, moving towards optimal light conditions or away from damaging light intensities.