  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.
