Cell size control in bacteria is modulated through extrinsic noise, single-cell- and population-growth

Abstract

Living cells maintain size homeostasis by actively compensating for size fluctuations. Here, we present two stochastic maps that unify phenomenological models by integrating fluctuating single-cell growth rates and size-dependent noise mechanisms with cell size control. One map is applicable to mother machine lineages and the other to lineage trees of exponentially-growing cell populations, which reveals that population dynamics alter size control measured in mother machine experiments. For example, an adder can become more sizer-like or more timer-like at the population level depending on the noise statistics. Our analysis of bacterial data identifies extrinsic noise as the dominant mechanism of size variability, characterized by a quadratic conditional variance-mean relationship for division size across growth conditions. This finding contradicts the reported independence of added size relative to birth size but is consistent with the adder property in terms of the independence of the mean added size. Finally, we derive a trade-off between population-growth-rate gain and division-size noise. Correlations between size control quantifiers and single-cell growth rates inferred from data indicate that bacteria prioritize a narrow division-size distribution over growth rate maximisation.