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Team:WITS-CSIR SA/Project/Motility
From 2011.igem.org
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
