Bacterial Motility and Chemotaxis

• Lecture covers bacterial motility and chemotaxis, describing the components of bacterial flagella and their function in flagellar swimming.
• Explains how flagellar rotation direction affects chemotaxis
• Highlights how MCPs control flagellar rotation via ligand binding and methylation, along with detailing swarming and twitching motility.

Bacterial Motility and Taxis

• Many bacteria are motile and use flagella or pili to move.
• Taxis involves directed movement toward or away from a stimulus.
• Positive taxis is movement toward a stimulus, like nutrients.
• Negative taxis is movement away from a stimulus, like toxic substances.
• Chemotaxis involves movement toward attractants or away from repellents.

Flagella Structure and Function

• Flagella are long, helical surface structures used for motility in liquid environments.
• The diameter is 20 nm and up to 20 μm long.
• Flagellar swimming comes from rapid flagella rotation, approximately 100-1000 times per second, enabling speeds of up to 100 μm/s.
• Atrichous lack flagella, monotrichous have a flagellum on one pole, lophotrichous have tufts of flagella, amphitrichous have a single flagellum on both poles, and peritrichous have flagella distributed over their surface.
• The basal body anchors the flagellum, containing the motor responsible for rotation.
• The filament is a long helical structure extending from the cell, and its rotation moves the cell.
• The hook acts as a flexible structure that transmits rotation from the basal body to the filament.
• Gram-negatives have L, P, and MS rings in the cell envelope, gram-positives do not.
• A C ring is in the cytoplasm.
• The basal body is involved in flagellum assembly by helping to build the hook and filament.
• The motor consists of a rotor (rod, MS ring), bushings/bearings (P, L rings), and stator (MotA, MotB).
• Rotation is usually powered by a proton motive force, via the MotA and MotB proton channel.
• A switch (C ring) determines the flagella rotation direction.
• Flagellum assembly occurs sequentially: basal body first, then hook, then filament.
• Subunits are exported through a hollow core and added at the distal end.
• The filament consists of thousands of flagellin proteins, assembled in a helical pattern, and incorporated under cap protein.

Flagella & Immune Response

• Flagella serve as good immune targets due to surface exposure.
• The innate immune system can recognize flagellin via Toll-like receptor 5 (TLR5).
• Flagellin binding to TLR5 induces NF-κB activation, leading to pro-inflammatory cytokine production.
• Adaptive immune system targets flagella, which are major antigenic structures, for example, E. coli O157:H7 with H7 being a flagellar antigen.
• There are thousands of protein subunits that increase the likelihood of antibody formation, ultimately leading to phagocytosis.
• Phase variation involves reversible changes in phenotype, specifically alternation between different flagellins.
• Salmonella enterica uses flagellins FljB and FliC.
• A reversible inversion of a promoter controls flagellin expression.

Chemotaxis & Flagellar Rotation

• A random walk happens when bacteria move without a chemical gradient.
• Bacteria swim forward (run), occasionally reorient (tumble), and then swim forward in a new random direction.
• Flagellum rotation direction determines if the bacterium runs or tumbles.
• Monotrichous bacteria: counterclockwise rotation results in a run, and clockwise in a tumble.
• Peritrichous bacteria: counterclockwise rotation results in a run, and clockwise in a tumble.
• Movement is a biased random walk within a chemical gradient.
• Bacteria move in a biased random walk.
• Runs are longer and tumbles are fewer if the direction is favorable.
• Tumbles are more frequent if the direction is unfavorable.

Methyl-Accepting Chemotaxis Proteins (MCPs)

• These are chemoreceptors in the cytoplasmic membrane that sense attractants and repellents to determine rotation direction.
• Flagella rotate counterclockwise by default, and this favors runs.
• MCPs can signal to flagella to change direction.
• The MCP ligand-binding domain forms a complex with a ligand.
• Ligand binding changes MCP signaling domain conformation.

Chemotaxis Signaling Pathway

• If no attractant is bound to MCP cells, tumbling/reorientation happens.
• CheW detects no attractant bound to MCP.
• Kinase CheA is auto-phosphorylated by CheW.
• Then, CheA-P phosphorylates CheY.
• CheY-P then binds to a flagellum switch, triggering CW rotation.
• CheY-P is dephosphorylated by CheZ over time.
• Runs are favored when attractant ligand is bound to MCP.
• CheW detects attractant.
• Auto-phosphorylation of CheA does not happen.
• Subsequently, CheY isn't phosphorylated and doesn't bind to a flagellum switch and CCW rotation is favored.

MCP Excitation & Adaptation

• Rotation direction is based on input from many MCPs.
• Ligand binding causes counterclockwise (CCW) bias by having more MCPs with bound ligands, resulting in less CheY-P formed.
• If ligand concentration increases: CCW bias occurs.
• If ligand concentration decreases: CW bias occurs.
• MCP methylation by CheR occurs a few seconds after the ligand binds.
• Methylation offsets the flagellar rotation impact of ligand binding, removing CCW bias.
• Temporal gradients explain how bacteria measure concentration gradients.
• Bacteria are considered to measure concentration using time, not space.
• Current concentration is seen in the number of MCPs with ligands. The methylation state of MCPs shows the past concentration.
• With the results given from a ~2 s methylation delay after the receptor binds, bacteria can avoid going from up gradients to down.
• CheB demethylates MCPs over time.
• As a result, if an MCP gets demethylated, what ligand it binds will cause CCW bias.
• Demethylation means bacteria will react to the signal.
• With rising attractant, you get CCW bias like normal.
• If demethylation did not happen, attractant binding would result in CCW bias.

Swarming Motility

• Flagella are utilized to move on solid media via coordinated movement of bacteria groups, referred to as rafts, across surfaces
• Requires peritrichous flagella and surfactants to reduce surface tension.
• Proteus mirabilis: cause of catheter-associated UTIs.
• They make swarmer cells on surfaces, produce many flagella (20-50 longer), form rafts, which can block urethral catheters.

Twitching Motility

• Type IV pili surface motility.
• Pilus extends, binds, and retracts, leading to jerky movement to colonize new environments, especially when pathogens colonize the host.
• Pilus extends via base subunits.
• Pilins after adhering, pilus retracts rapidly.
• Pilins are taken out from its base (~1500 per second).

Lecture on bacterial motility and chemotaxis, covering flagellar structure, function, and chemotaxis mechanisms. It explains how MCPs control flagellar rotation via ligand binding and methylation. Swarming and twitching motility are also detailed.