Chemotaxis and Flagella Rotation in Bacteria

Chemotaxis Driven by Flagella Rotation

• Flagella can move in two ways: clockwise (CW) or counterclockwise (CCW) rotation.
• The direction of flagella rotation is influenced by the concentration of a substance that the bacteria is attracted to.

Motility and Chemotaxis

• Normal movement of bacteria is composed of runs and tumbles.
• Chemotaxis is a response to different chemical gradients, where bacteria compare the chemical state of the environment to that sensed seconds before and move up or down the gradient accordingly.
• By altering the CW:CCW ratio, bacteria can move up or down the gradient.

Flagella Antigens and Phase Variation

• Salmonella spp. are typed by their O (LPS) and H (flagellar) antigens.
• There are over 2000 serotypes of Salmonella spp. due to different O and H antigens.
• Phase variation allows bacteria to differentially express protein antigens as either phase 1 or phase 2.

Flagellar Structure

Flagellar Basal Body

• The basal body is anchored in the cytoplasmic membrane and cell wall.
• The basal body consists of a small rod passing through a system of rings.
• Gram-negative bacteria have multiple rings, including an outer ring in the outer membrane/LPS, one in peptidoglycan, one in the cytoplasmic membrane, and one in the cytoplasm.
• Gram-positive bacteria have one fewer inner ring.

Flagellar Motor

• Two Mot (motor) proteins are anchored either side of the MS ring.
• The Mot proteins drive the flagellar motor, causing a torque that rotates the filament.
• The Fli proteins act as a motor switch.
• Energy is supplied by the passage of protons from outside the cell into the cytoplasm via the basal body mot complex.

Flagella Genetic Control

• Over 40 genes are necessary for motility in E. coli and S. typhimurium.
• The genes are grouped into fla, fli, and flg.
• These genes have several functions, including structural proteins, exporting of flagellar components, and biochemistry of synthesis.

Flagella Motility

• Flagella enables movement at a rate of up to 60 cell lengths per second.
• Movement is different in polar and peritrichous flagella.

Peritrichous

• Movement occurs in a straight line with counter-clockwise rotation.
• Tumbling occurs with clockwise smooth swimming.

Polar

• More rapid spinning occurs.

Flagella

• Flagella are long, thin appendages that originate from the cytoplasmic membrane and extend through the cell wall into the surrounding medium.
• They enable bacteria to be mobile.
• Flagella are very thin (20nm) and not visible by light microscope unless stained.

Flagellar Arrangement

• Monotrichous flagella have one flagellum (e.g., Vibrio cholerae).
• Lophotrichous flagella have multiple flagella on one end (e.g., Bartonella bacilliformis).
• Amphitrichous flagella have flagella at both ends (e.g., Spirllum serpens).
• Peritrichous flagella cover the surface of the bacteria (e.g., Escherichia coli).

Flagellar Structure

• Flagella have a fundamental helical structure with a constant distance between adjacent curves (wavelength).
• The wavelength is constant for a given organism.
• Flagella are composed of protein subunits of flagellin.