Bacterial Chemotaxis

Bacteria can move by a variety of means, the most common one by rotating their flagella. This movement is often directed towards favourable chemicals (chemoattractants) or away from unfavourable chemicals (chemorepellents), a process termed chemotaxis. This modulation of swimming direction is the outcome of controlled changes in the direction of flagellar rotation. Therefore, mechanistically, the essence of bacterial chemotaxis is to control the direction of flagellar rotation. This control is done by a sophisticated signal transduction system, involving a small protein, CheY, which shuttles back and forth between the receptor complexes clustered at the pole of the cell and the flagellar motor complexes around the cell. These interactions are modulated by phosphorylation and acetylation. The excitatory signalling process involves amplification. The adaptation signalling involves methylation of the receptors. Even though bacterial chemotaxis is considered the best understood signalling system at the molecular level, many major questions are still waiting to be resolved. Key Concepts: In bacteria, chemotaxis is the motile response to stimuli and it serves as a means of cell–environment and cell-to-cell communication. Bacteria sense stimulant gradients (chemoattractants and chemorepellents) temporally rather than spatially. The chemotaxis-specific receptors are clustered at the bacterial poles. This clustering is essential for signalling and amplification. Bacteria swim by rotating their flagella. The direction of rotation determines the swimming mode. Therefore, the essence of bacterial chemotaxis is to control the direction of flagellar rotation. The direction of flagellar rotation is modulated by the signalling molecule, CheY, according to changes in receptor occupancy. CheY function is modulated by phosphorylation at both the excitatory and adaptation signalling phases. The latter also involves receptor carboxy methylation. Phosphorylated CheY binds to the switch at the base of the flagellar motor and, thereby, changes the direction of rotation.