Escherichia coli swimming is robust against variations in flagellar number.

Bacterial chemotaxis is a paradigm for how environmental signals modulate cellular behavior. Although the network underlying this process has been studied extensively, we do not yet have an end-to-end understanding of chemotaxis. Specifically, how the rotational states of a cell's flagella cooperatively determine whether the cell 'runs' or 'tumbles' remains poorly characterized. Here, we measure the swimming behavior of individual E. coli cells while simultaneously detecting the rotational states of each flagellum. We find that a simple mathematical expression relates the cell's run/tumble bias to the number and average rotational state of its flagella. However, due to inter-flagellar correlations, an 'effective number' of flagella-smaller than the actual number-enters into this relation. Data from a chemotaxis mutant and stochastic modeling suggest that fluctuations of the regulator CheY-P are the source of flagellar correlations. A consequence of inter-flagellar correlations is that run/tumble behavior is only weakly dependent on number of flagella. DOI: http://dx.doi.org/10.7554/eLife.01916.001.