Fluoroquinolones, which include widely used drugs such as ciprofloxacin and levofloxacin, kill bacteria by trapping DNA gyrase and topoisomerase IV on the chromosome, converting these essential enzymes into agents of DNA destruction. The resulting double-strand breaks and associated lesions are lethal to most cells. But in stationary-phase cultures, where nutrients are scarce and most bacteria have stopped dividing, persisters survive fluoroquinolone treatment at high frequency. Previous work from the same research group demonstrated something surprising: transient growth inhibition after the antibiotic was removed significantly increased the recovery of persisters from stationary-phase Escherichia coli populations, but only if two specific genes were intact. One was recA, the central mediator of homologous recombination, and the other was uvrD, a DNA helicase that functions in nucleotide excision repair. That finding suggested that persister recovery depends on coordinated action between two repair systems that biologists usually study in isolation.

To dissect this relationship, the team turned to epistasis analysis, a classical genetic approach in which researchers construct double mutants and compare their behavior to that of the corresponding single mutants. If two genes operate in the same pathway, deleting both typically produces no worse a defect than deleting either one alone. If they operate in parallel, compensatory, or synergistic pathways, the double mutant reveals a different phenotype. By deleting DNA repair enzymes known to physically or functionally interact with RecA or UvrD, and then measuring how the resulting mutant strains recovered from fluoroquinolone treatment, the researchers could chart the boundaries of the persister recovery network with unprecedented resolution.

The results confirmed that the network extends well beyond the two original genes. Within nucleotide excision repair, the excision nuclease components uvrA and uvrB, along with mfd, a transcription-repair coupling factor that directs repair machinery to RNA polymerase-stalled sites, were all found to epistatically interact with recA during persister recovery. On the homologous recombination side, recB and recC, which encode the subunits of the RecBCD nuclease-helicase complex that processes DNA ends for recombination, emerged as additional epistatic partners of uvrD. These findings establish that both pathways genuinely contribute to the recovery phenomenon rather than serving as incidental bystanders, and they suggest that fluoroquinolone-induced damage in persisters generates substrates that require both excision of bulky lesions and recombinational repair of strand breaks.

Yet the data also revealed that the network is more nuanced than a simple story of one pathway compensating for the loss of the other. Loss of recA had far-reaching epistatic consequences that were not mirrored by the loss of other homologous recombination genes, indicating that RecA performs functions in persister recovery that its pathway partners do not fully duplicate. Similarly, loss of uvrD had a greater impact on the network than the loss of any other nucleotide excision repair gene tested. This asymmetry is significant because it implies that RecA and UvrD are not interchangeable cogs in a repair pipeline but rather hub proteins whose specific biochemical activities are indispensable for the recovery program. The double-mutant combinations across the two pathways produced patterns that could not be explained by straightforward redundancy, hinting at regulatory or structural roles that go beyond canonical repair chemistry.

To test whether the canonical enzymatic functions of these two hub proteins were actually required, the researchers deployed separation-of-function mutants. RecA(N304D) is a variant that retains some activities of the protein but is specifically defective in its recombination function, while UvrD(R284A) carries a mutation that abolishes its ATP-dependent helicase activity. If persister recovery depended merely on the physical presence of these proteins, for example as DNA-binding scaffolds, these mutants should have supported recovery. They did not. The recombination-proficient form of RecA and the helicase-active form of UvrD were both strictly required for their participation in fluoroquinolone persister recovery. This result anchors the phenomenon in biochemistry: the actual motor and strand-exchange activities of these enzymes, not just their occupancy of the DNA, drive the repair that allows persisters to resume growth after antibiotic treatment ends.

The findings carry weight for a long-standing debate in the microbiology of antibiotic tolerance. For years, persisters were assumed to survive because their sluggish metabolism rendered them invisible to antibiotics. Fluoroquinolones complicate that picture, because these drugs can form lesions even in metabolically quiescent cells, and the new work reinforces the view that survival after fluoroquinolone exposure is an active, repair-dependent process. In other words, persisters are not merely hiding from the drug; they are fighting back with the same DNA repair arsenal that dividing cells use, albeit under conditions of transient growth arrest that appear to give the repair machinery time to work. The post-treatment growth inhibition that boosts recovery in the laboratory may mimic conditions persisters encounter in infected tissues, where nutrient limitation and immune pressures can stall bacterial growth after antibiotic levels decline.

There are also therapeutic implications worth considering. If RecA and UvrD sit at the center of the persister recovery network, they become attractive targets for adjunctive drugs designed to sensitize persisters. Inhibitors that compromise RecA-mediated recombination or UvrD helicase activity could, in principle, be paired with fluoroquinolones to prevent the repair that underpins survival, thereby reducing the reservoir of tolerant cells that can seed recurrent infections. The epistasis map constructed in this study provides a rational starting point for such efforts, because it identifies which combinations of repair functions must be blocked to collapse the network. The asymmetry in the data suggests that targeting the hubs, RecA and UvrD, might be more effective than targeting peripheral components such as UvrA, UvrB, Mfd, RecB, or RecC.

Caveats remain, as they always do in laboratory genetics. The study was conducted in Escherichia coli grown to stationary phase in culture, and persister physiology in vivo, within the context of an actual infection, may involve additional stress responses, host factors, and heterogeneous metabolic states that alter the repair network’s composition. Moreover, epistasis analysis reveals functional relationships but does not by itself identify the molecular substrates being repaired or the sequence in which the pathways act. Future work combining the genetic framework established here with direct measurements of DNA damage, repair kinetics, and single-cell recovery dynamics will be needed to convert the network map into a mechanistic model.

Even so, the study represents a substantial advance in understanding how bacteria survive one of medicine’s most important antibiotic classes. By systematically expanding the persister recovery network and demonstrating that the core enzymatic activities of RecA and UvrD are non-negotiable requirements, Soto-Echevarria, Lemma, and Brynildsen have transformed a curious observation about post-treatment growth arrest into a genetically grounded framework. The work deepens appreciation of the interplay between homologous recombination and nucleotide excision repair in non-dividing cells, and it sharpenens the rationale for attacking persister survival at its enzymatic roots. As antibiotic resistance continues to erode treatment options worldwide, strategies that target tolerance rather than resistance are gaining urgency, and this study offers a detailed molecular blueprint for where such strategies might strike.

Subject of Research: Epistatic interactions between nucleotide excision repair and homologous recombination in fluoroquinolone persister recovery in Escherichia coli

Article Title: Genetic analysis of epistasis between nucleotide excision repair and homologous recombination in the recovery of persisters after fluoroquinolone treatment

Article References: Soto-Echevarria, N., Lemma, A. S., & Brynildsen, M. P. (2026). Genetic analysis of epistasis between nucleotide excision repair and homologous recombination in the recovery of persisters after fluoroquinolone treatment. PLOS Genetics, 22(9), e1012305. https://doi.org/10.1371/journal.pgen.1012305

Image Credits: AI Generated

DOI: 10.1371/journal.pgen.1012305

Keywords: antibiotic persisters, fluoroquinolones, DNA repair, RecA, UvrD, homologous recombination, nucleotide excision repair, epistasis, Escherichia coli, antibiotic tolerance, stationary phase, PLOS Genetics

The team combined a microfluidic device known as the dual input mother machine with time-lapse fluorescence microscopy and automated image analysis to follow thousands of individual cells through an abrupt switch from glucose-rich growth medium to a carbon-free version of the same medium. Each strain carried a transcriptional fluorescent reporter for one of 22 different promoters, spanning native genes and synthetic constitutive promoters chosen to cover a wide range of expression levels and regulatory behaviors. This multiplexed setup allowed the researchers to quantify, for every single cell, its size, growth rate and the rate at which it produced a fluorescent protein from each promoter, corrected for the confounding effects of photobleaching, fluorescent protein maturation and, crucially, protein degradation.

The first finding is a matter of minutes. Upon removal of the carbon source, every cell stopped growing essentially immediately, within the three-minute resolution of the microscopy. Contrary to any picture of the stationary phase as a mixture of growing and dying cells, the population entered growth arrest as one. Most remarkably, after growth had already ceased, roughly 38 percent of cells went on to divide during the following ten hours, undergoing what biologists call reductive division. These divisions shrank the average cell size of the population and occurred preferentially in longer cells, likely those that had already progressed furthest in their cell cycle, perhaps having started or completed DNA replication. Even after 60 hours without food, more than 90 percent of cells were still capable of regrowing when nutrients returned.

With growth arrested, attention turned to gene expression. Here the diversity across promoters was dramatic. Ribosomal protein promoters such as rplN, which drive the cell’s protein synthesis machinery during exponential growth, were shut down almost completely within an hour of the switch. Other promoters, like hupA, showed a sharp initial drop followed by a slower, sustained trickle of production. A third class, exemplified by hslV and the RpoS-regulated promoter bolA, did something counterintuitive: production briefly increased, sometimes above its level during growth, before decaying exponentially over the following hours. By late starvation, the relative production rates across the 22 promoters spanned more than two orders of magnitude, from about 20 percent down to 0.1 percent of their growth-phase values, underscoring how profoundly the regulatory network is rewired when nutrients vanish.

What makes these dynamics especially noteworthy is their consistency across cells. The researchers had anticipated that individual bacteria might respond to starvation in idiosyncratic ways, as has been observed for other stresses such as the SOS response to DNA damage, where the population-level induction conceals almost no coordination between cells. Instead, during early starvation, single cells followed nearly identical trajectories. For promoters showing a burst of expression, more than half of all the variance observed across cells and time was captured simply by the population average, a degree of coordination rarely seen in bacterial gene regulation. This homogeneity suggests that entry into starvation triggers a tightly controlled program rather than a stochastic drift into dormancy.

Production rates, however, are only half the story; protein concentrations are what determine physiology. Because growth arrest halts the dilution of existing proteins while degradation continues, and because production does not stop instantly, many proteins accumulated substantially even as their synthesis slowed. The researchers measured the degradation rate of fluorescent proteins directly and found that it too decays exponentially during starvation, dropping from initial values around 0.02 to 0.03 per hour. This slowdown makes mechanistic sense: most protein degradation in E. coli is carried out by ATP-dependent proteases, and ATP levels plummet when carbon is gone. Integrating production, dilution and degradation in a simple mathematical model, the team showed that promoter concentrations either fell by as much as half or rose by up to twelvefold, with nearly all of these changes occurring within the first five to ten hours. After that, protein levels were essentially frozen, locking in a phenotype that persisted for days.

The model also revealed an elegant set of principles governing the final proteome. The concentration fold-change for each gene depends on just three effective ratios: the total degradation capacity, the balance between the fold-change in production and the fold-change in degradation relative to growth rate, and the relative time scales over which production and degradation decay. Even without a burst of expression, a promoter whose production decays gradually over about five hours can achieve a fourfold increase in protein concentration, simply because dilution has stopped while synthesis continues. For RpoS-regulated promoters such as bolA, an initial burst pushes the increase further, to around sevenfold. Experiments with an rpoS deletion strain confirmed the central role of this alternative sigma factor, although not all starvation-induced promoters depended on it, pointing to additional regulators still to be identified.

Importantly, the response did not require cells to anticipate starvation in advance. In a separate set of experiments, the researchers let a batch culture exhaust its glucose gradually while flowing the same medium through the microfluidic chip, so that cells inside the chip experienced a slower, roughly 45-minute transition into growth arrest instead of an abrupt five-minute switch. The gene expression dynamics and the resulting protein concentration changes were remarkably similar between the gradual and abrupt transitions, suggesting that the program is triggered by the cessation of growth itself rather than by sensing dwindling nutrients. Even when protein synthesis was temporarily blocked with chloramphenicol during the first five hours of starvation, the expression program simply resumed once the drug was removed, implying that the eventual decline in production reflects exhaustion of internal resources rather than a specific transcriptional shutoff.

The most consequential result concerns what this early remodeling is for. When the researchers subjected starving cells to a strong oxidative stress with hydrogen peroxide at the 20-hour mark, nearly 98 percent of normally starved cells survived. But when translation was inhibited from the very start of starvation, survival fell to 19 percent, whereas inhibiting protein production only after the first five hours was far less damaging, allowing 65 percent of cells to survive. The protective early expression tracked the accumulation of RpoS-regulated reporters, and a strain lacking rpoS altogether fared even worse than translation-inhibited wild-type cells, with survival dropping almost to zero. Together, these experiments demonstrate that the protein production of the first five hours is what arms the cells against stresses arriving much later, when their capacity to respond has essentially vanished.

The findings also suggest how starving cells manage to keep producing proteins at all. With ribosomal gene expression shut off, the reduced translation capacity is redirected away from growth machinery toward protective proteins, effectively raising the relative output of many genes even as total synthesis falls. The energy likely comes from endogenous metabolism, the controlled self-digestion of abundant cellular constituents: the team’s degradation measurements imply that roughly 40 percent of the proteome may be broken down within the first day of starvation, releasing metabolites and fueling ATP production. In this light, the first hours of starvation emerge as a resource-limited but precisely coordinated investment phase, in which a bacterium cannibalizes part of itself to build the molecular toolkit it will need to endure an uncertain and potentially hostile future. For a organism long caricatured as a simple growth machine, E. coli turns out to be a far more deliberate survivor than its reputation suggests.

Subject of Research: Proteome remodeling and gene expression dynamics in single E. coli cells during carbon starvation

Article Title: E. coli prepares for starvation by dramatically remodeling its proteome in the first hours after loss of nutrients

Article References: E. coli prepares for starvation by dramatically remodeling its proteome in the first hours after loss of nutrients. (n.d.). https://doi.org/10.1038/s44320-026-00226-5

Image Credits: AI Generated

DOI: 10.1038/s44320-026-00226-5

Keywords: E. coli, carbon starvation, proteome remodeling, gene expression, microfluidics, single-cell microscopy, RpoS, stationary phase, protein degradation, stress tolerance, bacterial survival, Molecular Systems Biology