Declining old pole physiology gradually enhances gene expression asymmetry in bacteria

Abstract

AbstractGene expression is a heterogeneous process at the single-cell level. This heterogeneity is often coupled to individual growth rates, which are also highly stochastic, leading to the emergence of multiple physiological states within bacterial populations. Although recent advances have shown that cellular aging acts as a deterministic driver of growth heterogeneity, the relationship between aging and the maintenance of gene expression heterogeneity is often overlooked. Here, we show that the maturation and subsequent decrease in physiological activity at the old cell poles enhances cell-to-cell phenotypic heterogeneity inEscherichia coli. We use single-cell microscopy and microfluidics to quantify the expression of RpoS, a transcription factor that inhibits growth while activating stress responses, throughout the aging process. By tracking mother and daughter cells over generations, we show that the maternal old poles progressively decline in their contribution towards the cellular physiology, creating a source of intracellular variance that leads to phenotypic asymmetry among mother and daughter cells. Thus, we demonstrate that gene expression, similarly to growth, is a function of the age of cell poles inherited by each cell. Our results show that the mother-daughter asymmetry is built into the declining physiology of the mother cell across generations, illustrating the deterministic nature of aging in bacterial systems. These findings provide further evidence for cellular aging as a mechanism to enhance phenotypic heterogeneity in bacterial populations, with possible consequences for stress response and survival.SignificanceGrowth and gene expression are highly stochastic aspects of bacterial physiology. This heterogeneity can be advantageous for stress survival, as it creates distinct physiological states within a population. Nonetheless, the variance in growth can be partly explained by cellular aging processes, which lead to a progressive physiological decline in maternal cell lineages. Here, we show that aging also contributes to gene expression heterogeneity. As the mother cell ages, the contribution of its old poles to the production of RpoS (a growth-modulating transcription factor) declines, creating an intracellular gradient in gene expression. This gradient persists across generations, creating a physiological asymmetry between mother and daughter cells. Therefore, our findings show that the mother-daughter asymmetry that produces aging and rejuvenation patterns in a bacterial population is built into the intracellular asymmetry of mother cells. These findings offer insights on the maintenance of phenotypic heterogeneity in bacterial populations, which has implications for stress survival.