Bacterial chemotaxis

Chemotaxis is the directed motion of an organism toward environmental conditions it deems attractive and/or away from surroundings it finds repellent. Movement of flagellated bacteria such as Escherichia coli can be characterized as a sequence of smooth-swimming runs punctuated by intermittent tumbles. Tumbles last only a fraction of a second, which is sufficient to effectively randomize the direction of the next run. Runs tend to be variable in length extending from a fraction of a second to several minutes. As E. coli cells are only a few microns long, they behave essentially as point sensors, unable to measure gradients by comparing head-to-tail concentration differences. Instead, they possess a kind of memory that allows them to compare current and past chemical environments. The probability that a smooth swimming E. coli cell will stop its run and tumble is dictated by the chemistry of its immediate surroundings compared to the chemistry it encountered a few seconds previously. The tendency to tumble is enhanced when the bacterium perceives conditions to be worsening — when attractant concentrations decrease or repellent concentrations increase. Conversely, tumbling is suppressed and cells keep running when they detect that conditions are improving. Thus, when a bacterium runs up a gradient of attractants or down a gradient of repellents it tends to continue on course (Figure 1).