Declining old pole physiology gradually enhances gene expression asymmetry in bacteria

Gene 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 asymmetry, the relationship between aging and gene expression heterogeneity remains elusive. Here we show that old poles undergo a progressive decline in gene expression as mother cells age, contributing to enhance phenotypic heterogeneity in bacterial populations. We quantified the activity of promoters with distinct activity profiles: a constitutive promoter, whose expression positively correlates with growth, and the promoter of RpoS, the general stress response sigma factor, for which growth and expression are mutually inhibitory. We demonstrate that mother cells have lower gene expression for both promoters. This asymmetry could not be explained by metabolic rate differences, but rather by the increasing intracellular heterogeneity of mother cells. As a mother ages, the declining activity of its old pole produces intracellular gradients in gene expression. This intracellular asymmetry manifests in the next generation as mother-daughter asymmetry, thus representing a source of phenotypic heterogeneity for the population. Our results show that bacterial asymmetry is built into the declining physiology of mother cells across generations, illustrating the deterministic nature of aging in bacterial systems. These findings provide further evidence for cellular aging as a mechanism to enhance the variance of metabolic states found in bacterial populations, with possible consequences for stress response and survival.