Not all bacteria possess flagella, the whip-like appendages primarily associated with independent movement. While many bacterial species use flagella to navigate, a significant number either lack them entirely or employ different mechanisms for movement. The presence or absence of flagella, and the specific type of motility, allows bacteria to adapt and thrive in diverse ecological niches.
What Are Bacterial Flagella?
Bacterial flagella are thin, hair-like appendages extending from the cell surface, acting as propellers for movement. Each flagellum has three main parts: a long, helical filament, a hook, and a basal body. The filament, made of flagellin protein, is the whip-like part.
The hook connects the filament to the basal body, which is embedded within the bacterial cell wall and membrane. This basal body functions as a molecular motor, powering the filament’s rotation like a boat propeller to generate thrust. Energy for this rotation often comes from a flow of protons across the cell membrane, allowing flagellated bacteria to swim through liquid environments.
Why Some Bacteria Have Flagella
Flagella offer bacteria significant advantages for survival and adaptation. Motility allows bacteria to actively seek favorable conditions, like nutrient-rich areas (chemotaxis), or move away from harmful substances. This directed movement is crucial for bacteria such as E. coli and Vibrio cholerae.
The number and arrangement of flagella vary among species, reflecting environmental adaptations. Some bacteria, like Vibrio cholerae, have a single flagellum at one pole (monotrichous). Others may have a tuft at one or both ends (lophotrichous or amphitrichous), or flagella distributed over their surface (peritrichous). These diverse arrangements aid in colonization and competition within their habitats.
How Some Bacteria Move Without Flagella
Many bacteria move or persist without flagella, using diverse alternative strategies. Some, like Myxococcus xanthus, exhibit gliding motility, moving smoothly across solid surfaces. This is often powered by slime secretion or protein movement within the cell wall.
Myxococcus xanthus uses two distinct gliding systems: adventurous (A-motility) for individual cells and social (S-motility) for coordinated group movement, often involving type IV pili.
Another method is twitching motility, seen in bacteria like Pseudomonas aeruginosa. This relies on the extension and retraction of hair-like type IV pili. The pili extend, attach to a surface, then retract, pulling the bacterium along in a jerky, crawling motion. This mechanism is important for surface colonization and biofilm formation. Many bacteria are non-motile, relying on external forces like water currents or growth to spread.