Bacterial Chemotaxis

Introduction

During the 17th century, the advent of the light microscope allowed scientists to observe “tiny particles” that were proposed to be living due to their seemingly purposeful motion.[3] It was only in the 19th century when directed bacterial movements were first characterised by Wilhelm Pfeffer.[3] Pfeffer’s work described the ability of bacteria to navigate through complex environments, in response to changes in temperature (thermotaxis), osmolarity (osmotaxis), light (phototaxis) and chemical substrates (chemotaxis).[3]

Bacterial chemotaxis is a regulated response that involves the processing of chemical substrates as input signals, into physical movements that result in bacterial motility.[2] This response allows for bacteria to selectively move along chemical gradients[4] (Fig 1), directing them towards substances that are favourable to their survival, and away from noxious substances.[5] Therefore, chemotaxis confers an important survival advantage to bacteria, particularly in their natural, non-mixed environment in which chemical gradients exist.[4,5]

Fig 1: The movement of a bacterial cell in chemotaxis-stimulated conditions[1]

Motions of chemotaxis

Motile bacteria are endowed with flagella, which are long, helical projections that are anchored to the cell surface5 (Fig 2). At the base of the flagella, there is a rotary motor that is powered by the electrochemical energy that is generated by a transmembrane ion flux.3,6 This motor device induces reversible rotation of the flagella, which serves as the impetus for bacterial cell propulsion.3 In the well studied E. coli, there is an array of flagella at one pole of the cell that function collectively to induce cell motility.6 During the course of its movement, two types of motions are exhibited: running and tumbling. Running is associated with the flagella rotating in a counter-clockwise direction, forming a bundle that works to propel the cell forward in a directed manner. Alternatively, tumbling is the result of the flagella rotating in a clockwise direction, disrupting the flagella bundle and causing the bacterial cell to fall in solution (Fig 3). This allows for the cell to reorientate itself and for the direction of its runs to be changed, if deemed necessary. The frequency of each of these two motions varies, depending on the environmental signals that are transduced to the flagella motors. Chemotactic bacteria are able to make spatial as well as temporal comparisons of the concentration of the substance that they encounter, allowing them to regulate their motion in response to an increasing or decreasing concentration gradient.5 In the event that the bacterial cell is moving towards an attractant or away from a repellent, the movement is direct and is characterised by longer runs and fewer tumbles. However, in a uniform environment, the two motions alternate in such a way that the cell moves in a random walk.3

Fig 2: (A) The polar flagella of E. coli viewed by transmission electron microscopy (B) A schematic of the organisation of the componenets associated with the flagella in E. coli 1

Molecular mechanism of chemotaxis

Adler et al3 were the first to describe the intracellular signal transduction network involved in chemotaxis that is responsible for the relay of information from the environment, through the cell and to the flagella motor proteins. This led on to the elucidation of the existence and the function of a group of proteins that are responsible for this intracellular signalling known as the Che proteins (Fig 4).

Fig 4: The components involved in the signalling pathway responsible for chemotaxis in E. coli 2

This signalling cascade begins at the cell surface, where there is an interaction between a ligand and a bacterial chemoreceptor. E. coli has five types of transmembrane chemoreceptors that exist as large polar clusters.2,3 These receptor proteins have highly variable periplasmic domains, allowing E. coli cells to respond to over 30 different compounds even with a minimal number of receptor types.3,5 The intracellular domain, on the other hand, is highly conserved and provides the scaffolding for the intracellular membrane associated Che protein complex. CheW directly binding to these conserved regions of the chemoreceptor to serve as an adapter between the chemoreceptor and the CheA protein, to form the intracellular Che protein complex.3

CheA is a large and complex histidine kinase that is able to phosphorylate its own histidine residues, using ATP as its substrate. This autophosphorylation of CheA results in the phosphorylation of CheY, which is a monomeric protein which is usually bound to CheA. Phospho-CheY (p-CheY) has a decreased affinity for CheA, resulting in its dissociation from the membrane associated Che protein complex. In this unbound state, p-CheY rapidly diffuses to the flagella switch proteins (FliM) and functions as an allosteric regulator to induce clockwise rotation of the flagella. The overall affect is the tumbling of the bacterial cell.2,3 This process is the native state of the system and occurs without chemoreceptor stimulation. However, when the bacterial cell encounters an attractant or a repellent that binds to a specific chemoreceptor, a ligand-induced conformational change occurs in the chemoreceptor. These changes may include piston-like movements and rotation of the receptor protein, which serves as the signal to the intracellular Che protein complex.3 This inhibits the kinase activity of the CheA protein, which will abrogate the production of p-CheY. Flagella motor proteins, in the absence of p-CheY regulation, produce counter clockwise flagella movements which result in directed bacterial swimming.3

Regulation of Chemotaxis

The complexity of the signal transduction network that is responsible for chemotaxis lies within the mechanisms of its regulation. Factors that regulate the intracellular p-CheY concentration dictate the motion of the flagella and ultimately, the movement of the bacterial cell.3 p-CheY concentrations are regulated by transmembrane signals that modulate the kinase activity of CheA. However, there are other Che proteins that are peripheral to the main signalling cascade that can also regulate the p-CheY concentration.

CheR and CheB are methyl transferases and demethylases, respectively.3 They are responsible for the methylation state of the cytoplasmic domain of the chemoreceptors. Methylation of the chemoreceptor induces clockwise rotation of the flagella, whereas demethylation causes counter-clockwise rotation.1 This is thought to occur via conformational changes of the chemoreceptor as a result of methylation or demethylation.1 The state of methylation of the receptor is important as it modifies the sensitivity of the chemoreceptor, to prevent sensory saturation as the bacteria travel towards higher or lower concentrations.7 Moreover, it provides a mechanism of short term memory, allowing for temporal comparisons of the stimulant concentration.3

Another protein that can regulate bacterial chemotaxis is CheZ. This is a phosphatase that dephosphorylates p-CheY, freeing it from the flagella FliM protein and restoring the counter clockwise rotation of the flagella to elicit running.2 Therefore, CheZ influences the intracellular concentration of p-CheY directly, and is capable of modulating the motion that the bacterial chemotaxis.

References

1. Eisenbach M. Bacterial Chemotaxis. In: Enyclopedia of Life Sciences; 2001.

2. Topp S, Gallivan JP. Guiding bacteria with small molecules and RNA. J Am Chem Soc 2007;129:6807-11.

3. Baker MD, Wolanin PM, Stock JB. Signal transduction in bacterial chemotaxis. Bioessays 2006;28:9-22.

4. Vladimirov N, Lebiedz D, Sourjik V. Predicted auxiliary navigation mechanism of peritrichously flagellated chemotactic bacteria. PLoS Comput Biol 2010;6:e1000717.

5. Adler J. Chemotaxis in Bacteria. Annual Review of Biochemistry 1975;44:341-56.

6. Berg HC. The rotary motor of bacterial flagella. Annual Review of Biochemistry 2003;72:19-54.

7. Sourjik V. Receptor clustering and signal processing in E. coli chemotaxis. Trends Microbiol 2004;12:569-76.