Bacteria, which are single-celled prokaryotic organisms, do not have legs, arms, or any appendage resembling those found in animals. While they lack complex, multi-cellular structures, they exhibit remarkable mobility through specialized, microscopic mechanisms. This ability to move allows them to navigate their environment, which is essential for their survival and proliferation.
The Direct Answer: No Legs, Just Microscopic Life
Bacteria are defined by their simple, single-celled structure, which lacks a nucleus. These organisms exist on the scale of a few microns, making a complex structure like a leg anatomically impossible and energetically inefficient. Legs are complex systems requiring coordinated muscle groups and skeletal support that a single cell cannot manage. Instead of pushing off a surface, bacterial movement relies on generating thrust or tension at the cellular level to propel them through liquid or across surfaces.
Propeller-Like Structures: Flagella and Axial Filaments
Swimming is powered by a rotating, whip-like structure known as the flagellum. This appendage is a rigid, helical filament composed of the protein flagellin, which acts as a miniature propeller extending from the cell surface. The flagellum is driven by a molecular motor embedded in the bacterial cell wall and membrane. This motor is powered by the flow of hydrogen ions, or protons, across the cell membrane, causing it to spin continuously, sometimes reaching speeds of up to 100,000 revolutions per minute.
In liquid environments, bacteria with flagella exhibit a pattern of movement known as “run and tumble.” When the flagella rotate counterclockwise, they form a unified bundle that propels the bacterium forward in a straight-line “run.” A change in the motor’s direction to clockwise rotation causes the flagellar bundle to fly apart, resulting in a random, erratic “tumble” that reorients the cell.
Axial Filaments
A variation of the propeller system is found in spiral-shaped bacteria called spirochetes, which use internal structures called axial filaments. These filaments are essentially flagella located within the periplasmic space between the inner and outer bacterial membranes. The rotation of these internal filaments generates a twisting motion of the entire cell body, enabling the bacterium to bore through viscous materials in a corkscrew-like fashion. This movement is crucial for the survival of pathogenic spirochetes, such as the bacteria that cause Lyme disease and syphilis, allowing them to penetrate host tissues.
Other Ways Bacteria Get Around
Bacteria employ distinct mechanisms to move when navigating solid or semi-solid surfaces. One is twitching motility, a type of crawling movement mediated by the extension and retraction of hair-like appendages called Type IV pili. These pili extend from the cell, attach to the surrounding substrate, and then rapidly retract, pulling the cell body forward in a jerky, “grappling hook” action. This mechanism is important for surface colonization and the formation of biofilms.
Other bacteria utilize gliding motility, which allows them to slide smoothly across a surface without the use of flagella or pili. In some cases, this movement is achieved by the coordinated secretion of a slime-like substance, which propels the cell forward, similar to a jet of fluid. In other gliding bacteria, movement is driven by adhesion complexes that move along the cell membrane, creating traction against the surface. Bacteria can also exhibit passive movement, where they are simply carried along by environmental forces such as water currents or air flow.
The Purpose of Bacterial Motility
The ability to move is a directed behavior, termed “taxis,” which is movement in response to an environmental stimulus. One of the most studied forms is chemotaxis, involving sensing and responding to chemical gradients. Chemotaxis allows bacteria to move toward beneficial chemicals, such as nutrients, or away from harmful substances like toxins. For photosynthetic bacteria, phototaxis directs them toward optimal light intensity. This navigation is important in the context of disease, as motile pathogens use these mechanisms to locate host tissues for colonization.