Calculating instantaneous growth rate, µ

One widely used method to calculate growth rate is to consider single-cell growth an exponential process^12,48 and solve for instantaneous growth rate, µ. This definition of growth rate is used throughout this study. From the length and width measured during particle tracking, the instantaneous volume of each individual cell was approximated as a cylinder with hemispherical caps¹⁹. The approximated volumes were then used to compute instantaneous single-cell growth rates in terms of volume doublings per hour. Using V(t + ∆t) = V(t) × 2^µ∆t, we calculated µ between each pair of time steps, with ∆t = 117 s (imaging frame rate). Specifically, we took the natural logarithm of each volume trajectory and calculated the slope between each point. Dividing the slope by the natural log of 2 changes the base of the exponential from e to 2. Thus, µ represents the exponential rate at which volume doubles.