ABSTRACT
Bacteria such as E. coli and R. sphaeroides respond to changes in extracellular attractant levels by changing the pattern of rotation of their flagella anchored across the cell membrane (Eisenbach et al., 2004). In the case of E. coli changes in attractant concentration are detected by membrane-spanning methyl-accepting chemotaxis proteins (MCPs) at the poles of the rod-like shaped bacteria. Through a number of intracellular phosphotransfer biochemical reactions, these changes are communicated to the FliM protein motors that switch the direction of flagellar rotation. In the case of E. coli, the default setting of clockwise rotations in the absence of an attractant gradient, interspersed with periodic switching to clockwise rotation, leads to a series of run and tumbles. During such movement the individual four to six flagella rotate together to form a bundle which leads to short bursts of directed motion, followed by tumbling as a result of one or more flagella switching. Tumbling re-orientates the bacterium on a different directional heading each time. Considered over the order of minutes this movement leads to three-dimensional random-walk-like behaviour. When a change in the external concentration level of an attractant is detected by the MCPs, the bacterial flagella rotate for longer in a counter-clockwise direction leading to extended periods of runs up the attractant gradient resulting in chemotaxis.
