A link between aging and persistence

Abstract

ABSRACTDespite the various strategies that microorganisms have evolved to resist antibiotic treatments, most chronic infections are caused by subpopulations of susceptible bacteria in a transient state of dormancy. This phenotype, known as bacterial persistence, arises due to a natural and ubiquitous heterogeneity of growth states in bacterial populations. Nonetheless, the unifying mechanism of persistence remains unknown, with several pathways being able to trigger the phenotype. Here, we show that asymmetric damage partitioning, a form of cellular aging, produces the underlying phenotypic heterogeneity upon which persistence is triggered. Using single-cell microscopy and microfluidic devices, we demonstrate that deterministic asymmetry in exponential phase populations leads to a state of growth stability, which prevents the spontaneous formation of persisters. However, as populations approach stationary phase, aging bacteria -- those inheriting more damage upon division -- exhibit a sharper growth rate decline, increased probability of growth arrest, and higher persistence rates. These results indicate that persistence triggers are biased by bacterial asymmetry, thus acting upon the deterministic heterogeneity produced by cellular aging. This work suggests unifying mechanisms for persistence and offers new perspectives on the treatment of recalcitrant infections. IMPORTANCEWhenever bacterial cultures are treated with antibiotics, a fraction of the population survives despite exhibiting no active resistance mechanisms. These "persisters" are cells in a state of slow growth or dormancy, already present in the population prior to antibiotic exposure. Although various stressors or mutations increase persistence rates, a unifying persistence mechanism has not been established. Here, we show that cellular aging can represent such a mechanism. Bacteria age through the inheritance of intracellular damage, which occurs even in unstressed populations. As populations approach stationary phase, aging Escherichia coli have a steeper decline in elongation rates and earlier division arrest compared to younger cells. Upon antibiotic treatment, aging bacteria have higher persistence rates. These results show that stationary phase, a well-established persistence trigger, operates on the phenotypic heterogeneity produced by cellular aging. Because aging is a deterministic and ubiquitous process, it could represent a fundamental mechanism for the formation of persisters.