Bacterial Chemotaxis

Abstract

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.

Keywords: flagella; flagellar motor; chemotaxis; motility; signal transduction; adaptation; bacterial mobility

Figure 1.

Flagella of E. coli observed in transmission electron microscope. Bar, 1 μm.

Figure 2.

E. coli or Salmonella flagellum. The actual diameters of the rod, L, P, M, S and C rings are ∼15, 33, 26, 29, 27 and 47 nm, respectively. CM, cytoplasmic membrane; OM, outer membrane; and PL, peptidoglycan layer.

Figure 3.

Schematic description of the stepwise assembly of E. coli and Salmonella flagella. Abbreviations: OM, outer membrane; PL, peptidoglycan layer; and CM, cytoplasmic membrane. (Modified with permission from Aizawa, .)

Figure 4.

Swimming behaviour of E. coli cells: (a) nonstimulated conditions and (b) stimulated conditions.

Figure 5.

Signal transduction in bacterial chemotaxis. The scheme is not drawn to scale. Black arrows stand for regulated interactions. CheA is a histidine kinase that phosphorylates CheB and CheY, CheB is a specific methylesterase that demethylates the chemotaxis receptors, CheR is a specific methyltransferase that methylates the receptors, CheW is a scaffolding protein that couples CheA to the receptors, CheY is the key response regulator in chemotaxis of E. coli, and CheZ is a phosphatase that enhances the spontaneous dephosphorylation of CheY. (Modified with permission from Bren and Eisenbach, .)

Figure 6.

Suggested role for the dual covalent modification of CheY. According to the model, the metabolic state of the cell determines the fraction of CheY molecules that are acetylated. Only nonacetylated CheY molecules can bind to CheA and be phosphorylated by it, and only they can bind to FliM with a resultant change in the direction of flagellar rotation. Thus, while acetylation, which is the slower process of these two covalent modifications, determines the fraction of CheY molecules that can participate in chemotactic signalling, phosphorylation, whose level is regulated by chemotactic and thermotactic stimuli, determines the extent of CheY binding to FliM. (Taken with permission from Liarzi et al., .)

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Further Reading

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Pantaloni D, Le Clainche C and Carlier M‐F (2001) Mechanism of actin‐based motility. Science 292: 1502–1506.

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Eisenbach, Michael(Dec 2011) Bacterial Chemotaxis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001251.pub3]