A physicist looks at bacterial chemotaxis

What is distinctive about bacterial chemotaxis, as compared to, for example, taste in the elephant, is the time over which decisions must be made. The lower limit is set by diffusion of chemicals to and from the cell surface, which demands long times for statistically significant counts. The upper limit is set by diffusion of the cell itself, which demands short times for well-defined swimming paths. For an organism the size of E. coli, temporal comparisons of the concentrations of chemicals in the environment must be made within a few seconds. Although such short time spans might be difficult for the biochemist, they are not so difficult for E. coli, because diffusion can carry a small molecule across the cell in about 1 msec. E. coli has the opposite problem: How does it integrate inputs from many receptors over periods 1000 times as long? The mechanisms for this signal processing are beginning to be understood. We know how most chemical attractants are identified, how temporal comparisons might be made, and how the behavioral output is effected. We know less about how sensory information crosses the cytoplasmic membrane, how the reactions that link the receptors to the flagella generate such high gain, and what actually controls the direction of flagellar rotation. One thing is quite clear: E. coli demands our admiration and respect.