begin a new run in a different direction. The bacterium biases its motion by continually measuring the chemoattractant concentration, and extending the time of runs for which the change in concentration is positive [@celani2010bacterial; @tu2013quantitative; @berg1977physics; @dahlquist1976studies]. The change in concentration $\Delta \bar{c} = Tv\bar{g}$ over a run time $T$ depends on the concentration gradient $\bar{g} = \partial \bar{c} / \partial x$ and the bacterium’s velocity $v$. Berg and Purcell argued that for a change in concentration to be detectable, it must be larger than the measurement uncertainty, $\Delta \bar{c} > \sigma_c $. Together with Eq. \[eq:singleConc\], this places a lower limit on the run time, $T > [ \bar{c}/(aDv^2\bar{g}^2)]^{1/3}$. Using typical values [@berg1977physics] for the sensory threshold of *E. coli* of $\bar{c} = 1$ mM, $\partial \bar{c} / \partial x = 1$ mM/cm, $a=1$ $\mu$m, $v=15$ $\mu$m/s, and $D=10^{-5}$ cm$^2$/s, we find $T > 0.1$ s. Actual run times are on the order of $1$ s. Thus we see that *E. coli* chemotaxis is consistent with this physical bound. Although the end goal of concentration sensing in *E. coli* is chemotaxis by temporally sampling changes in the chemical concentration, we would like to focus the reader’s attention on the 