Salmonella enterica serovar Typhimurium is a common foodborne pathogen responsible for significant gastrointestinal illness in humans. Its ability to move is fundamental to its life cycle, survival, and capacity to initiate a robust infection. Motility allows S. Typhimurium to rapidly seek out nutrients and evade local threats. This mechanical capability is directly linked to the pathogen’s overall virulence, enabling it to successfully navigate and breach the host’s protective barriers.
The Flagellar Motor: Structure and Assembly
The movement of S. Typhimurium is powered by the flagellum, a complex rotary device that acts as both a propeller and an engine. This apparatus is a supramolecular assembly built from approximately 30 different protein types, with over 40 genes governing its construction. The flagellum is divided into three main functional parts: the filament, the hook, and the basal body.
The filament is the long, helical structure extending outside the cell, functioning as the propeller that generates thrust. It is composed of thousands of repeating subunits of the protein flagellin.
Connecting the filament to the motor within the cell envelope is the hook, a short, flexible segment. The hook acts as a universal joint to efficiently transmit the rotational force (torque).
The basal body is the motor itself, embedded within the bacterial cell wall and membranes. It consists of a central rod surrounded by a series of rings: the L-ring and P-ring act as bushings in the outer membrane and peptidoglycan layer. Deeper inside, the MS-ring and the C-ring are situated in the inner membrane and cytoplasm, forming the core components of the rotary engine.
Powering the Movement: The Mechanics of Rotation
The Salmonella flagellar motor converts electrochemical energy into mechanical rotation. The energy source is the proton motive force (PMF), which is the difference in proton (H+) concentration and electrical charge across the inner cell membrane. Protons flow through specialized channels in the motor, similar to water turning a turbine.
The motor’s mechanical components are the stator and the rotor. The rotor is formed by the inner rings of the basal body, including the MS-ring and the C-ring, which contains the protein FliG. The stator units surround the rotor and are the force-generating components, each made of five MotA subunits and two MotB subunits.
The stator unit is anchored by the MotB protein, which binds to the rigid peptidoglycan layer. The MotA subunits act as the proton channel and force generator, interacting directly with the FliG protein on the rotor.
The influx of protons through the MotA/MotB channel drives conformational changes in MotA. This rapid cycle of interaction and release between the stator (MotA) and the rotor (FliG) creates the torque that spins the basal body and attached filament, allowing the flagellum to rotate at speeds up to several hundred revolutions per second.
Navigating the Environment: Chemotaxis and Directional Control
Movement in S. Typhimurium is a directed navigation process called chemotaxis. This system allows the bacterium to sense and move toward favorable chemical attractants, such as nutrients, and away from harmful repellents, like toxins. The bacteria utilize two primary motor actions for navigation: the “run” and the “tumble.”
During a “run,” the flagella rotate counter-clockwise (CCW). This rotation allows the numerous flagella to coalesce into a tight, helical bundle that pushes the bacterium smoothly in a straight line.
When the flagella switch to clockwise (CW) rotation, the bundle is forced apart. This causes the cell to momentarily stop and randomly reorient in a “tumble,” after which it begins a new run in a different direction.
The frequency of switching between these two modes is controlled by an internal signaling cascade. The sensory apparatus consists of Methyl-Accepting Chemotaxis Proteins (MCPs) embedded in the membrane, which bind to external chemical signals. When an MCP senses an increasing concentration of an attractant, it signals a decrease in the phosphorylation of the response regulator protein, CheY.
Low levels of phosphorylated CheY (\(CheY \sim P\)) maintain the motor in the CCW (run) state. Conversely, sensing a repellent increases the concentration of \(CheY \sim P\). This phosphorylated protein binds to the FliM component on the motor’s C-ring, forcing the motor to switch to CW rotation and initiating a tumble. By biasing the duration of runs and the frequency of tumbles, the bacterium effectively directs its movement toward the most favorable conditions.
Motility’s Critical Role in Pathogenesis
Motility is a prerequisite for S. Typhimurium to establish a successful infection in the host gut. The intestinal tract is lined with a thick layer of mucus, which serves as a protective physical barrier. Motility is necessary for the bacterium to penetrate this layer and reach the underlying epithelial cells.
Non-motile strains of S. Typhimurium are excluded by the dense inner mucus layer and are less virulent than motile counterparts. Motile bacteria use near-surface swimming to find permissive sites within the mucus layer for passage. Once through the mucus, flagellar movement allows the bacteria to rapidly search for and adhere to host epithelial cells, often targeting specialized M cells in the Peyer’s patches.
This swift, directed movement enables the pathogen to overcome initial physical defenses and reach the invasion site quickly. The presence of the flagella and motility can also trigger the expression of other virulence factors necessary for the next stage of infection.