Bacillus subtilis is a rod-shaped, Gram-positive bacterium found in soil and used in probiotic supplements and fermentation processes. Addressing the core question, B. subtilis is definitively motile, possessing a sophisticated system that enables it to navigate its environment effectively. Its capacity for active movement is a regulated lifestyle choice, allowing it to seek out resources before shifting its strategy for survival.
Motility Status and the Flagellar Mechanism
The ability of Bacillus subtilis to move through liquid media, a process called swimming, relies on specialized appendages known as flagella. These structures are distributed across the entire cell surface in a peritrichous pattern. A typical motile cell assembles approximately 26 flagellar basal bodies, which anchor the complex motor to the cell membrane.
The flagellar motor is a rotary engine driven by an electrochemical gradient, utilizing both proton (H\(^+\)) and sodium (Na\(^+\)) ions for power. Rotation of these numerous flagella allows the cell to propel itself. The movement pattern is characterized by alternating phases of “runs” and “tumbles,” which together create a biased random walk.
During a “run,” the flagella rotate counter-clockwise, forming a unified bundle that pushes the cell in a straight line. To change direction, the flagellar motor switches to a clockwise rotation, causing the bundle to fly apart, resulting in a chaotic “tumble.” The cell then reorients itself randomly before the flagella re-bundle for the next straight “run.”
Environmental Significance of Active Movement
Active movement is a fundamental survival strategy for B. subtilis, enabling it to sense and respond to chemical cues in its ecosystem. This directed movement toward or away from specific chemicals is known as chemotaxis. The bacterium possesses sensory systems that detect gradients of attractants, such as sugars and amino acids, and repellents, like certain toxins.
By sensing a favorable gradient, the bacterium suppresses the frequency of its tumbles, extending its straight “runs” in the desired direction. Conversely, sensing a repellent increases the frequency of tumbles, allowing the cell to quickly reorient and move away from the unfavorable condition.
Motility is linked to the initial stages of colonization and the formation of biofilms. Before settling down to form a robust biofilm on a surface, motile cells use their flagella to swim toward and explore it. This initial movement is a prerequisite for a lifestyle switch, where the cells stop moving and begin to excrete a protective, extracellular matrix.
The Non-Motile State of Spore Formation
In contrast to its motile, vegetative state, B. subtilis possesses an alternative survival strategy: the formation of an endospore. This process, called sporulation, is initiated when the bacterium encounters severe environmental stress, such as prolonged starvation or extreme temperatures.
Upon committing to sporulation, the cell undergoes an asymmetric division, sequestering one copy of its chromosome into a small compartment that will develop into the spore. The cell effectively sheds its flagella and ceases all active movement. The resulting endospore is a highly resistant, metabolically dormant structure that is completely non-motile.
The spore remains viable for decades, tolerating conditions that would instantly kill the vegetative cell, including desiccation and harsh chemicals. This non-motile, dormant state represents the ultimate survival mechanism when active strategies, like motility and chemotaxis, have failed to locate adequate resources.