The bacterium Escherichia coli (E. coli) is a common inhabitant of the digestive tract, yet certain strains can cause significant disease. Like many single-celled organisms, E. coli possesses the ability to move independently through liquid environments, a property known as motility. This movement is a highly coordinated mechanism that allows the bacterium to actively seek out nutrients and favorable environments. This locomotive capability relies on a complex mechanical structure, a unique energy source, and a sophisticated navigation system. Understanding this molecular machinery offers insight into the bacterium’s survival strategies and its capacity to cause infection.
The Flagellum: Anatomy of the Bacterial Motor
The structure responsible for E. coli motility is the flagellum, a remarkable biological machine that functions as a rotary engine. The flagellum is composed of three main parts that work together to generate thrust. The filament is a long, helical structure extending out from the cell body that acts as the propeller, pushing the bacterium through the medium.
The filament is connected to the hook, which functions as a flexible universal joint, transmitting the torque from the motor to the propeller. Anchored within the cell envelope is the basal body, a complex arrangement of protein rings that constitutes the motor itself. This motor is divided into two functional parts: the stator and the rotor.
The rotor is the spinning component, made up of several protein rings, including the FliF, FliG, FliM, and FliN proteins, which span the inner cell membrane and the cytoplasm. Surrounding the rotor are the stator units, complexes made of MotA and MotB proteins, which are fixed in the cell membrane. The stator units generate the necessary force to turn the rotor, allowing the entire flagellum to rotate at high speeds.
Powering Movement: The Proton Motive Force
The energy that drives the flagellar motor is not supplied by adenosine triphosphate (ATP), the typical energy currency of the cell, but by the Proton Motive Force (PMF). The PMF is an electrochemical gradient created by a higher concentration of protons (hydrogen ions, H+) in the periplasmic space outside the inner cell membrane compared to the cytoplasm inside. This difference in concentration and electrical charge creates a potential energy source across the membrane.
The stator units (MotA/MotB) act as channels that allow these accumulated protons to flow back into the cell, down their concentration and electrical gradient. The movement of protons through these channels generates a mechanical force. This flow of ions is directly coupled to the rotation of the FliG ring, a key part of the rotor.
In the flagellum, the proton flow generates torque, causing the rotor to spin and the flagellar filament to rotate. The speed of the flagellar rotation is directly proportional to the strength of the PMF, allowing the bacterium to adjust its speed according to its energy state.
Navigating the Environment: Chemotaxis
The mechanical rotation of the flagella is controlled by a sensory system that enables the bacterium to navigate its environment effectively, a process known as chemotaxis. E. coli achieves directional movement through a pattern of alternating motions called “runs” and “tumbles.” When the flagella rotate counter-clockwise (CCW), they coalesce into a single, rotating bundle that propels the cell forward in a smooth, straight “run.”
If the flagellar motor switches its rotation to clockwise (CW), the flagellar bundle flies apart, causing the bacterium to briefly stop and reorient randomly in a rapid “tumble.” This run-and-tumble cycle constitutes a random walk in a uniform environment, but becomes biased when the cell detects a chemical gradient.
The sensory input is managed by chemoreceptors, which are transmembrane proteins that monitor the concentration of attractants (like sugars) or repellents (like toxins). If the bacterium is swimming toward an attractant and the concentration is increasing, the chemoreceptors signal the motor to prolong the CCW rotation, extending the “run.” Conversely, if the cell is moving in the wrong direction, the tumble is triggered sooner, allowing for a new, more favorable direction to be chosen.
Motility’s Critical Role in Infection Progression
The ability of E. coli to swim and navigate via chemotaxis is a factor in the progression of many infections. Motility allows the pathogen to overcome physical barriers within the host. For instance, in Uropathogenic E. coli (UPEC), which causes most uncomplicated urinary tract infections, movement is essential for navigating the urinary tract.
Motility allows the bacteria to ascend from the bladder to the kidneys, a process called ascending dissemination. It also helps the bacteria penetrate the host’s thick mucus layers, which serve as a primary physical defense mechanism. Furthermore, the ability to rapidly move enables the bacteria to evade localized immune responses, such as phagocytic cells.
Studies have shown that non-motile UPEC strains are outcompeted by their motile counterparts during co-infection, demonstrating the fitness advantage provided by the flagellum. The coordinated movement system allows E. coli to locate specific colonization sites within the host environment, enhancing the bacterium’s capacity to establish and maintain an infection.