Do E. coli care about single molecules?

Berg and Purcell derived a fundamental limit on how accurately concentration can be sensed from the stochastic arrival of ligand molecules to a cell’s surface by diffusion. However, it has remained unclear to what extent molecule counting noise is a meaningful limitation on sensing accuracy and downstream functions in cells. Answering this question has been challenging, even for E. coli chemotaxis, one of the paper’s original motivations. This is in part because E. coli chemotaxis depends on information the cell “gathers” about the fluctuating, instantaneous time derivative of concentration, not the absolute concentration, as we recently demonstrated. Here, we introduce an information rate that quantifies how much behaviorally-relevant information (in bits) is encoded per unit time by the receptor associated kinases. We show that the stochastic arrival of single particles limits this rate, which in turn limits E. coli’s ability to climb shallow gradients.