Microbial Motility

Cell Locomotion

Microorganisms exhibit various forms of motility, allowing them to move toward favorable environments and away from harmful conditions. The most studied mechanism is flagellar motility, but other forms exist as well.

• Flagella-Enabled Motility: Flagella are long, whip-like appendages that protrude from the cell body. They rotate to propel the cell through liquid environments. The flagellum consists of three main parts: the filament, the hook, and the basal body. The basal body anchors the flagellum to the cell membrane and acts as a rotary motor powered by the proton motive force.

• Flagellar Structure:

  • Filament: Composed of the protein flagellin, forms the visible part of the flagellum.

  • Hook: Connects the filament to the basal body, acts as a flexible coupling.

  • Basal Body: Embedded in the cell envelope, contains rings and the motor apparatus.

• Flagellar Synthesis: Flagella are assembled from the inside out, beginning with the basal body, followed by the hook, and finally the filament. Flagellin monomers are transported through the hollow core and added to the tip.

• Other Forms of Motility:

  • Gliding Motility: Movement across solid surfaces without flagella, often seen in cyanobacteria and myxobacteria.

  • Twitching Motility: Uses type IV pili to pull the cell forward in short, jerky movements.

  • Swarming Motility: Coordinated movement of groups of cells across surfaces, often involving flagella.

• Chemotaxis and Other Forms of Taxis: Chemotaxis is movement in response to chemical gradients. Other taxis include phototaxis (light), aerotaxis (oxygen), and magnetotaxis (magnetic fields).

Example: Escherichia coli uses peritrichous flagella to swim toward nutrients via chemotaxis.

Regulation of Chemotaxis

Flagellar Rotation and Chemotaxis: Runs and Tumbles

Bacterial chemotaxis is regulated by alternating between two types of movement: runs and tumbles. This allows bacteria to navigate chemical gradients efficiently.

• Runs: Straight-line movement when flagella rotate counterclockwise, forming a bundle.

• Tumbles: Random reorientation when flagella rotate clockwise, causing the bundle to fall apart.

• Biased Random Walk: Bacteria modulate the frequency of runs and tumbles based on the concentration of attractants or repellents. When moving toward an attractant, runs are lengthened; when moving away, tumbles increase.

Example: In a gradient of glucose, E. coli increases run duration when moving toward higher glucose concentrations.

Measuring Chemotaxis

Chemotaxis can be measured using capillary assays, agar plate assays, or microfluidic devices. These methods quantify the movement of bacteria toward or away from chemical sources.

• Capillary Assay: A capillary tube containing attractant is placed in a bacterial suspension; bacteria accumulate in the tube if attracted.

• Agar Plate Assay: Bacteria are placed on agar with a chemical gradient; migration patterns are observed.

• Microfluidic Devices: Allow precise control of gradients and real-time observation of bacterial movement.

Regulation of Chemotaxis: Sensing and Adaptation

Sensing Signal Concentration and Altering Motility

Bacteria sense changes in chemical concentration using membrane-bound chemoreceptors. These receptors transmit signals to the flagellar motor, altering motility patterns.

• Chemoreceptors: Proteins that detect specific chemicals and initiate signal transduction cascades.

• Signal Transduction: Involves phosphorylation of proteins such as CheA and CheY, which regulate flagellar rotation.

Adaptation and Its Role in Chemotaxis

Adaptation allows bacteria to reset their sensitivity to persistent stimuli, enabling them to respond to new changes in concentration. This is achieved by reversible methylation of chemoreceptors.

• Adaptation Mechanism: Methylation and demethylation of chemoreceptors adjust their responsiveness.

• Role in Chemotaxis: Prevents saturation of the response, allowing bacteria to detect temporal changes in gradients.

Example: E. coli adapts to constant attractant levels by methylating its chemoreceptors, maintaining sensitivity to new gradients.

Key Equations

• Proton Motive Force (PMF): Drives flagellar rotation.

• Biased Random Walk Probability: Probability of run or tumble depends on attractant concentration. Additional info: K is a constant representing receptor sensitivity.

Comparison Table: Types of Bacterial Motility

Additional info: Adaptation mechanisms and signal transduction pathways are covered in more detail in advanced molecular microbiology chapters.