Bacillus subtilis, a common bacterium found in soil and the gut, is a workhorse of scientific research. This non-pathogenic microbe is studied for its remarkable ability to move. Powering this movement is the flagellum, a whip-like appendage that functions as a biological motor. The flagellum is a complex molecular machine composed of dozens of distinct proteins. This article explores how this motor is constructed and how it propels the bacterium.
The Architecture of the Flagellar Motor
The flagellum of B. subtilis is a nanomachine with a structure separated into three main parts: the basal body, the hook, and the filament. The entire apparatus generates and transmits torque for propulsion. Because B. subtilis has multiple flagella distributed around its cell surface, a trait known as peritrichous flagellation, it can coordinate their motion for directed movement.
The engine of the flagellum is the basal body, a structure embedded within the bacterial cell envelope that acts as the anchor and rotational motor. This motor consists of a stationary component, the stator, and a rotating component, the rotor. The stator is composed of protein complexes, primarily MotA and MotB, fixed in the cell membrane. The rotor includes the C-ring, a cytoplasmic structure connected to a central rod that passes through the cell wall.
Connecting the basal body to the long external filament is the hook, which functions as a flexible universal joint. This short, curved tube is made of a single type of protein called FlgE. Its flexibility allows the torque generated by the basal body to be transmitted to the filament, which extends away from the cell at an angle.
The most visible part of the flagellum is the filament, a long, helical structure that acts like a propeller. It is a polymer constructed from thousands of copies of a protein known as flagellin. The filament grows from its tip, as new flagellin subunits travel through a narrow internal channel to be added at the far end. The helical shape of this filament allows its rotation to displace fluid and push the bacterium forward.
Mechanism of Flagellar Rotation and Propulsion
The rotation of the flagellar motor is powered not by ATP, but by the proton motive force (PMF). The PMF is an electrochemical gradient generated by the cell’s metabolic activities, which pump protons across the cell membrane. This creates a higher concentration outside than inside. This stored energy is harnessed by the flagellum, operating much like a hydroelectric dam.
This flow of protons is channeled through the stator proteins, the MotA/MotB complexes that form pores in the membrane. As protons pass through these channels, they induce conformational changes in the stator proteins. These changes exert sequential electrostatic forces on the rotor’s proteins. This interaction generates a continuous torque that drives the counter-clockwise rotation of the rotor and the attached filament.
This rotation directly translates into bacterial movement. When the multiple flagella of B. subtilis rotate counter-clockwise, they coalesce into a single, coordinated bundle behind the cell. This synchronized bundle propels the bacterium forward in a relatively straight line, a behavior known as a “run.”
To change direction, the cell reverses the rotation of its flagella to a clockwise direction. This switch causes the flagellar bundle to fly apart, and the uncoordinated filaments push the cell in various directions. This results in a random tumbling motion that reorients the bacterium. Once the motors switch back to counter-clockwise rotation, the cell begins a new run in this new direction.
The Role of Flagella in Bacterial Behavior
The “run and tumble” system is the basis for a behavior called chemotaxis. This process allows B. subtilis to navigate its environment by sensing chemical gradients. The bacterium can move toward beneficial substances, such as sugars and amino acids (attractants), and away from harmful ones, like toxins (repellents).
Chemotaxis works by modulating the frequency of tumbling. When the bacterium is moving up a gradient of an attractant, its internal signaling system suppresses the switch to clockwise rotation. This results in longer runs in the favorable direction. If the cell is moving toward a repellent, the signaling pathway promotes more frequent tumbles, allowing the bacterium to reorient and find a better path.
Beyond individual navigation, flagella enable collective behaviors like swarming motility. This is a rapid and coordinated movement of a large population of bacteria across a solid surface. This requires the cells to produce a greater number of flagella, a state known as hyper-flagellation. This allows the bacterial community to expand outward as a cohesive group.
Flagellar motility is also involved in the initial stages of biofilm formation. Biofilms are structured communities of bacteria encased in a self-produced matrix, adhering to a surface. The ability to move towards and explore a surface is a prerequisite for attachment. Flagella allow the bacteria to make the initial contact needed to begin secreting the biofilm matrix.
Assembly and Regulation of Flagella
The construction of a flagellum is an organized process that occurs in a hierarchical sequence, building the structure from the inside out. The process begins with the basal body components embedded in the cell membrane. First, the MS-ring is assembled, followed by the cytoplasmic C-ring and the export apparatus. Subsequently, the rod proteins are assembled, forming the axle that spans the cell envelope.
Once the basal body and rod are complete, the hook protein subunits are assembled to form the universal joint. The filament is the final major component to be built. This step-by-step assembly ensures that each part is in place before the next is added, preventing faulty structures.
This assembly line is controlled at the genetic level by a regulatory cascade. The expression of the nearly 50 genes required to build a flagellum is initiated by a master regulator protein. In B. subtilis, this role is filled by a protein called SwrA, which activates the transcription of the flagellar genes. This ensures that all necessary components are synthesized in a coordinated fashion.
The cell decides whether to build these structures based on environmental signals. Factors such as nutrient availability, cell density, and surface contact can influence the activity of the master regulator. This regulatory network allows B. subtilis to adapt its lifestyle, building flagella to explore new environments or forgoing their production to conserve energy when conditions are stable.