Cell Locomotion in Prokaryotes

Introduction to Motility

Motility is a fundamental property that allows prokaryotic cells to move within their environment, enabling them to access nutrients, escape harmful conditions, and colonize new niches. The two major types of prokaryotic cell movement are swimming and gliding. Directed movement in response to specific stimuli is termed taxis (plural: taxes).

• Swimming Motility: Movement through liquid environments, typically powered by flagella.

• Gliding Motility: Movement across solid surfaces, independent of flagella.

• Taxis: Directed movement toward or away from a stimulus (e.g., chemotaxis, phototaxis).

Swimming Motility and Flagella

Flagella Structure and Function

Flagella are long, thin appendages (15–20 nm wide) that act as rotary motors, propelling bacterial cells through liquid environments. In Bacteria, these structures are called flagella, while in Archaea, the analogous structure is termed archaella.

• Flagella: Enable rapid movement; can rotate up to 1000 revolutions per second, achieving speeds up to 60 cell lengths/second.

• Archaella: Similar function in Archaea, but structurally distinct and powered by ATP hydrolysis rather than proton motive force.

Flagellar Arrangements

Bacteria exhibit different flagellar arrangements, which influence their swimming behavior:

• Polar: Flagella attached at one or both ends of the cell.

• Lophotrichous: Tuft of flagella at one end.

• Amphitrichous: Tufts of flagella at both poles.

• Peritrichous: Flagella distributed around the cell surface.

Flagellar Structure and Activity

Flagella are helical filaments with a constant wavelength, characteristic for each species. The main protein component is flagellin, which determines the shape and function of the flagellum.

• Flagellin: Highly conserved protein; its amino acid sequence affects flagellar properties.

• Flagellar Filament: Composed of many flagellin subunits; grows from the tip.

• Basal Body: Anchors the flagellum in the cell envelope and contains the motor apparatus.

Flagellar Motor Structure

The flagellar motor is a complex structure that spans the cell envelope, especially in Gram-negative bacteria:

• L ring: Anchored in the outer membrane.

• P ring: Anchored in the peptidoglycan layer.

• MS ring: Located in the cytoplasmic membrane.

• C ring: Located in the cytoplasm.

• Gram-positive bacteria: Only the inner rings (MS and C) are present.

Flagellar Motor Function

The flagellar motor consists of two main components:

• Rotor: Central rod and rings (L, P, MS, C) forming the basal body.

• Stator: Mot proteins that generate torque via proton flow.

Protons are translocated through the Mot complex, creating electrostatic forces that rotate the rotor. The speed of rotation is determined by the rate of proton flow.

Flagellar Synthesis

Flagellar assembly is a highly ordered process:

1. MS ring is synthesized and inserted into the cytoplasmic membrane.

2. Other anchoring proteins and the hook are assembled.

3. Flagellin subunits are transported through a central channel and added to the tip of the growing filament, assisted by a cap protein.

Swimming Motility in Archaea

Archaella are thinner than bacterial flagella (10–13 nm) and composed of several different proteins. Their rotation is powered by ATP hydrolysis, not proton motive force, indicating independent evolutionary origins.

• Archaella: Capable of both clockwise and counterclockwise rotation.

• Energy Source: ATP hydrolysis (in contrast to bacterial flagella, which use proton motive force).

Surface Motility: Gliding and Twitching

Gliding Motility

Gliding motility allows cells, typically filamentous or rod-shaped, to move smoothly across solid surfaces. This movement is slower than swimming and often involves secretion of extracellular polysaccharides.

• Adhesion Complexes: Specialized proteins in the membranes facilitate movement.

• Energy Source: Proton motive force drives movement of gliding-specific proteins.

• Examples: Myxobacteria, filamentous cyanobacteria (Oscillatoria).

Twitching Motility

Twitching motility involves the extension and retraction of type IV pili, dragging the cell along the surface. This mechanism is powered by ATP hydrolysis and is observed in many bacteria and some archaea.

• Type IV Pili: Repeated extension and retraction enables movement.

• Example: Pseudomonas species.

Mechanisms of Gliding Motility

Three Main Mechanisms

1. Polysaccharide Secretion: Cells secrete slime, which propels them forward.

2. Adhesion Complexes: Surface proteins interact with the substrate to generate movement.

3. Motor Proteins: Movement of proteins in the cytoplasmic membrane, powered by proton motive force, is transmitted to the outer membrane.

Taxes: Directed Movement in Response to Stimuli

Types of Taxes

Prokaryotes exhibit several types of taxis, allowing them to respond to environmental gradients:

• Chemotaxis: Movement in response to chemical gradients.

• Phototaxis: Movement in response to light.

• Aerotaxis: Movement in response to oxygen concentration.

• Osmotaxis: Movement in response to osmotic strength.

• Hydrotaxis: Movement in response to water availability.

• Scotophobotaxis: Movement away from darkness.

Chemotaxis in Peritrichously Flagellated Bacteria

Escherichia coli is a model organism for studying chemotaxis. In the absence of a chemical gradient, cells exhibit 'run and tumble' behavior:

• Run: Smooth forward movement; flagella rotate counterclockwise.

• Tumble: Random reorientation; flagella rotate clockwise and the bundle disperses.

In the presence of an attractant, runs become longer and tumbles less frequent, resulting in movement up the concentration gradient.

Chemical Gradient Sensing

• Bacteria sample their environment periodically and compare current chemical concentrations to those sensed previously (temporal sensing).

• Chemoreceptors in the membrane detect attractants or repellents.

• Signal transduction cascades alter flagellar rotation direction.

Chemotaxis in Polarly Flagellated Bacteria

Polarly flagellated bacteria may reverse flagellar rotation to change direction, avoiding tumbles. Some species, like Rhodobacter, reorient via Brownian motion when the flagellum stops.

Measuring Chemotaxis in the Laboratory

• Capillary Assay: A capillary tube containing attractant or repellent is placed in a medium with motile bacteria. Bacteria move toward attractants and away from repellents.

• Microscopy: Video tracking of cell movement allows quantification of chemotactic behavior.

Phototaxis and Scotophobotaxis

• Phototaxis: Movement toward light, enabling phototrophic organisms to optimize photosynthesis.

• Scotophobotaxis: Movement away from darkness; cells reverse direction upon entering dark regions.

Aerotaxis, Osmotaxis, and Hydrotaxis

• Aerotaxis: Movement toward or away from oxygen.

• Osmotaxis: Movement in response to osmotic strength; important for maintaining cellular water balance.

• Hydrotaxis: Movement toward water; allows cyanobacteria to seek hydration in dry environments.

Table: Types of Prokaryotic Motility and Their Mechanisms

Table: Types of Taxes in Prokaryotes

Key Equations

• Flagellar Rotation Speed:

• Chemotactic Response: (Cells respond to temporal changes in attractant concentration)

Summary

Prokaryotic motility is essential for survival and adaptation. Swimming and gliding motility are powered by distinct structures and energy sources, while various forms of taxis enable cells to navigate complex environments. Understanding these mechanisms provides insight into microbial ecology, pathogenesis, and biotechnological applications.