Aging a little: The optimality of limited senescence in Escherichia coli

TL;DR

This paper models the asymmetric division in E. coli to explain limited senescence, showing that two similar growth states can be evolutionarily optimal due to fitness benefits of heterogeneity.

Contribution

It introduces a mathematical model based on a generalized Fibonacci recurrence to explain the stability of two distinct growth states in bacteria.

Findings

Limited senescence can be evolutionarily advantageous.

Two stable growth states differ by only a few percent.

Asymmetry in cell division can be beneficial for population fitness.

Abstract

Recent studies have shown that even in the absence of extrinsic stress, the morphologically symmetrically dividing model bacteria Escherichia coli do not generate offspring with equal reproductive fitness. Instead, daughter cells exhibit asymmetric division times that converge to two distinct growth states. This represents a limited senescence / rejuvenation process derived from asymmetric division that is stable for hundreds of generations. It remains unclear why the bacteria do not continue the senescence beyond this asymptote. Although there are inherent fitness benefits for heterogeneity in population growth rates, the two growth equilibria are surprisingly similar, differing by a few percent. In this work we derive an explicit model for the growth of a bacterial population with two growth equilibria, based on a generalized Fibonacci recurrence, in order to quantify the fitness benefit of a limited senescence process and examine costs associated with asymmetry that could generate the observed behavior. We find that with simple saturating effects of asymmetric partitioning of subcellular components, two distinct but similar growth states may be optimal while providing evolutionarily significant fitness advantages.