Escherichia coli (E. coli) is a common and well-studied bacterium. It is known for its ability to move, which is significant for its survival and adaptability in diverse environments. This directed movement allows E. coli to navigate its surroundings, seeking favorable conditions and avoiding harmful ones. This mobility is fundamental to its biology, enabling it to thrive in various ecological niches.
The Flagellar Structure
E. coli’s movement relies on its flagella, which are long, whip-like appendages extending from the bacterial surface. Each flagellum is composed of a filament made from many copies of a protein called flagellin, arranged helically to form a hollow tube. This filament connects to a curved hook, which acts as a universal joint. The hook links the filament to the basal body, a complex structure embedded within the bacterial cell wall and membrane.
The basal body consists of several protein rings that securely anchor the flagellum to the cell envelope, allowing for its rotation. In Gram-negative bacteria like E. coli, these rings include the MS ring in the cytoplasmic membrane, the C ring extending into the cytoplasm, and the P and L rings which anchor the flagellum to the peptidoglycan layer and outer membrane. E. coli typically possesses multiple flagella, often arranged around its entire cell surface.
The Rotating Motor
E. coli’s movement is powered by a bacterial flagellar motor, a complex protein assembly embedded within the cell’s inner membrane and cell wall. This motor functions like a propeller, converting energy into rotational force to spin the flagellum. The energy for this rotation comes from a proton gradient, known as the proton motive force, which exists across the bacterial cell membrane. Protons flow through channels within the stator components of the motor, generating torque that drives the rotor.
The motor rotates in two directions: counter-clockwise (CCW) and clockwise (CW). When the flagella rotate counter-clockwise, they form a coordinated bundle behind the cell, propelling the bacterium forward in a smooth, straight path known as a “run.” A switch to clockwise rotation disrupts this bundle, causing the cell to reorient randomly in a “tumble.” This alternating pattern of runs and tumbles allows the bacterium to change direction and explore its environment.
Sensing and Steering: Chemotaxis
E. coli actively directs its movement through chemotaxis. Chemotaxis is the ability of bacteria to sense chemical gradients in their surroundings and respond by moving towards beneficial substances, known as attractants, or away from harmful ones, called repellents. Attractants often include nutrients like amino acids and sugars, while repellents can be toxic compounds.
The bacterium employs specialized receptors on its surface, called methyl-accepting chemotaxis proteins (MCPs), to detect these chemical signals. When an attractant binds to these receptors, it typically leads to a decrease in the activity of an associated kinase protein, CheA. This, in turn, reduces the phosphorylation of another protein, CheY, which then influences the flagellar motor. Lower levels of phosphorylated CheY promote counter-clockwise flagellar rotation, extending the duration of “runs” and allowing the bacterium to move efficiently towards the attractant.
Conversely, encountering a repellent or a decrease in attractant concentration increases CheA activity, leading to higher levels of phosphorylated CheY. This promotes clockwise flagellar rotation, increasing “tumbling” frequency and allowing the bacterium to reorient and potentially move away from the unfavorable condition. This sophisticated signaling pathway enables E. coli to navigate its environment with remarkable precision.
Purpose of Movement
Motility allows the bacterium to actively explore its surroundings, increasing its chances of locating nutrient-rich areas. By directing its movement towards attractants, E. coli can efficiently find and exploit food sources necessary for its growth and reproduction.
Movement also provides a mechanism for E. coli to escape from harmful substances or environments that could impede its survival. This includes moving away from toxins or areas with unfavorable pH levels. Furthermore, motility enables E. coli to colonize new areas, spreading to different niches where resources might be more abundant or competition less intense.