Bacteria, microscopic single-celled organisms, exist in nearly every environment on Earth. Many bacterial species possess the ability to move independently, a characteristic known as motility. This movement is not random; it is a fundamental aspect of their survival, enabling them to seek nutrients and favorable conditions. Motility also allows bacteria to escape harmful substances or environments, important for their ecological success and interactions with other organisms.
Flagella: The Propellers
Many bacteria employ flagella, long, whip-like appendages, as their primary means of propulsion. A bacterial flagellum functions like a miniature propeller, pushing the cell through liquid environments. It consists of three parts: the filament, the hook, and the basal body. The filament, outside the cell, is a helical structure made of a protein called flagellin.
The hook connects the filament to the basal body, transmitting torque. The basal body, embedded within the cell membrane and wall, serves as the motor for the flagellum. This motor is powered by a flow of ions, typically protons, across the bacterial cell membrane, generating the proton motive force. This force rotates the basal body, which spins the hook and filament, propelling the bacterium forward.
Bacteria exhibit various flagellar arrangements, influencing their movement patterns:
Monotrichous bacteria have a single flagellum at one end.
Amphitrichous bacteria have a single flagellum at each end.
Lophotrichous bacteria have a tuft of flagella at one or both ends.
Peritrichous bacteria have multiple flagella distributed over their cell surface, allowing complex coordinated movement.
Pili: The Grappling Hooks
Pili are shorter and thinner hair-like appendages extending from the bacterial cell surface. While many pili function in attachment to surfaces or other cells, a specific type called Type IV pili also plays a role in bacterial movement. These pili are different from flagella in both structure and their mode of action.
Type IV pili mediate a jerky, stop-and-go movement called twitching motility. This involves the extension of a pilus from the bacterial cell, followed by its attachment to an external surface. Once attached, the pilus retracts, pulling the bacterium along the surface like a grappling hook. This movement is particularly effective on moist solid surfaces rather than in liquid environments.
Gliding: The Sliders
Some bacteria move smoothly over solid surfaces without the use of flagella or pili, a process called gliding motility. This locomotion is slower and more continuous than flagellar swimming or twitching. The exact mechanisms for gliding vary among species, and some aspects remain under research.
One proposed mechanism involves the secretion of a polysaccharide slime or adhesive molecules from the cell, allowing the bacterium to slide along a surface. Another mechanism, observed in some gliding bacteria like Myxococcus xanthus, involves the movement of proteins within the cell envelope. These internal protein complexes, potentially powered by the proton motive force, are thought to engage with the substrate and propel the cell forward. Examples include Myxococcus xanthus and various Flavobacterium species.
Axial Filaments: The Internal Motors
Spirochetes, spiral-shaped bacteria, utilize a distinct mechanism for motility involving internal structures called axial filaments. Unlike the external flagella of other bacteria, these filaments are located within the periplasmic space, which is the region between the inner and outer membranes of the cell. These filaments wrap around the cell body.
The rotation of these internal axial filaments causes the cell body to twist and flex. This corkscrew-like or undulatory motion allows spirochetes to move effectively through viscous environments, such as mucus or tissues, which might impede bacteria with external flagella. This internal motor provides an advantage for spirochetes in navigating complex biological environments.