PhoU Protein Reveals How Klebsiella pneumoniae Builds Its Antibiotic-Tolerant Persister Cells

Klebsiella pneumoniae has long been one of the most formidable opponents in modern medicine, a bacterium that settles into hospitals and resists the drugs designed to erase it. Now a team of researchers in China has uncovered a previously underappreciated piece of the puzzle: a single protein from the phosphate transport system, called PhoU, helps determine how many antibiotic-tolerant persister cells this pathogen can produce. The finding, published in Molecular Genetics and Genomics, offers a fresh molecular explanation for why some infections seem to smolder through a full course of antibiotics and then flare back up once treatment stops.

Persister cells are not resistant bacteria in the classical genetic sense. Instead, they are metabolically quiescent variants within an otherwise genetically identical population that temporarily shut down the processes antibiotics usually attack. Because most antibiotics target actively growing cells—cell wall synthesis, DNA replication, protein production—dormant persisters simply wait out the assault and repopulate the infection afterward. The phenomenon was first described in the 1940s, yet the molecular switches that govern how many cells enter this state remain incompletely mapped, particularly in K. pneumoniae, a leading cause of hospital-acquired pneumonia, urinary tract infections, and bloodstream infections.

To find new regulators of persistence, the research team, led by Yanxin Sun and Yumei Zhang with collaborators across several institutions in Shandong and Zhejiang provinces, compared gene expression during the emergence and recovery of persister populations. One gene stood out: phoU, which encodes a protein belonging to the phosphate-specific transport system. PhoU had been implicated in persister formation in Escherichia coli before, but its role in K. pneumoniae—and the downstream pathway through which it might act—was unclear.

The researchers used CRISPR-Cas9 gene editing to delete phoU from K. pneumoniae strain ATCC 700603, creating a knockout strain they could compare directly with the wild-type parent. They also constructed a complemented strain in which phoU was restored, as well as overexpression strains carrying extra copies of phoU, metE, or phoA—two genes that would soon prove central to the story. This panel of strains allowed the team to separate the effects of losing the gene from the effects of the surrounding genome and to test whether specific downstream functions could rescue the knockout’s defects.

The first surprise was what did not change. The ΔphoU strain grew at essentially the same rate as the wild type and showed no meaningful shift in baseline antibiotic susceptibility. In other words, PhoU is not a general housekeeping factor and its absence does not make the bacterium more fragile in ordinary conditions. What changed was the knockout’s ability to form persisters. When cultures were exposed to levofloxacin, a fluoroquinolone that targets DNA gyrase, or tobramycin, an aminoglycoside that attacks the ribosome, the ΔphoU strain produced significantly fewer surviving persister cells than the wild-type or complemented strains. Two antibiotics with completely different cellular targets produced the same result, suggesting that PhoU influences a shared upstream state—dormancy itself—rather than a drug-specific defense.

To understand how a phosphate transport protein could control dormancy, the team turned to metabolomics and transcriptomics. Untargeted metabolomic profiling, later deposited in the EMBL-EBI MetaboLights database, and genome-wide expression analysis revealed that deleting phoU downregulated two genes in particular: metE, which encodes the cobalamin-independent methionine synthase responsible for producing the amino acid methionine, and phoA, which encodes alkaline phosphatase, a classic marker of the phosphate starvation response. Methionine biosynthesis has previously been tied to stress tolerance in other bacteria; impaired methionine synthesis can lead to homocysteine accumulation and heightened sensitivity to environmental stress. Alkaline phosphatase, meanwhile, connects PhoU to the Pho regulon, the phosphate-sensing network that is increasingly recognized as a regulator of bacterial virulence and survival physiology.

The team then asked whether PhoU’s effect on persisters ran through biofilms. Biofilms—structured bacterial communities wrapped in extracellular polymeric substances—are well known refuges for tolerant cells, and PhoU is a component of the phosphate system long linked to biofilm regulation. The ΔphoU strain indeed formed weaker biofilms and produced less extracellular polymeric substance than the wild type, the complemented strain, and the overexpression strains. But the researchers made a critical distinction here. When they restored metE or phoA expression in the knockout background, biofilm formation and persister levels both partially recovered, yet further analysis indicated that the reduction in persisters was primarily attributable to metabolic defects in the free-swimming, planktonic cells—specifically impaired methionine synthesis and a weakened stress response—rather than to the reduced biofilm mass itself. Biofilm deficiency, in their interpretation, is a parallel phenotype of phoU loss, not the cause of persister decline.

That distinction matters clinically. If persister formation depends on the metabolic state of individual planktonic cells rather than on the architecture of a biofilm, then anti-persister therapies need not penetrate or dismantle biofilms to be effective. Targeting PhoU or its downstream partners—MetE in methionine metabolism and PhoA in the phosphate stress response—could, in principle, strip K. pneumoniae populations of their dormancy capacity even outside the protective matrix of a biofilm. The authors frame PhoU and its transcriptional targets as potential therapeutic vulnerabilities: drug candidates that force persisters out of hiding, or adjuvants that make conventional antibiotics lethal to the full population rather than only its active fraction.

The study also fits into a broader reassessment of PhoU’s role in bacterial physiology. Once regarded largely as a passive scaffold in the phosphate transporter complex, PhoU has now been shown in several organisms to act as a regulatory node, influencing stress tolerance, antibiotic persistence, and even secondary metabolism. In K. pneumoniae, the new results position it as a transcriptional upregulator of metE and phoA, thereby supporting the stress-response capacity of planktonic cells and, ultimately, the pathogen’s ability to survive antibiotic challenge. Because PhoU is conserved across many bacterial species but absent from humans, it represents an attractive target for future drug development, although the authors note that restoring either downstream gene only partially rescues the persister defect—implying that PhoU’s influence extends beyond the two pathways characterized so far.

Limitations remain, as with any laboratory study. The work was performed in a single reference strain under defined culture conditions, and persister frequencies can vary enormously across clinical isolates and infection environments. The team also relied on a limited set of antibiotics, and it is not yet known whether PhoU governs tolerance to beta-lactams or other drug classes through the same pathway. Still, the convergence of genetic, metabolomic, and transcriptomic evidence gives the model considerable weight: by sustaining methionine synthesis and phosphate-linked stress responses, PhoU keeps a reservoir of K. pneumoniae cells poised to survive antibiotic treatment. As antibiotics continue to lose ground against multidrug-resistant Gram-negative pathogens, understanding the molecular choreography of persistence—and finding the proteins that choreograph it—may prove as important as discovering new drugs themselves. This study provides one more name for that choreography, and a promising one at that.

Subject of Research: The role of the phosphate transport protein PhoU in regulating antibiotic persister cell formation in Klebsiella pneumoniae.

Article Title: The phosphate specific transport system protein PhoU modulates persister cell formation of Klebsiella pneumoniae

Article References: Sun, Y., Xu, W., Chen, K., Hong, X., Sun, X., Ma, W., Wang, X., Cao, Q., Xue, Z., Zhou, B., Zhang, Y., Liu, Z., Cui, Z., Wang, D., Dong, Z., & Zhang, Y. (2026). The phosphate specific transport system protein PhoU modulates persister cell formation of Klebsiella pneumoniae. Molecular Genetics and Genomics, 301(1), Article 197. https://doi.org/10.1007/s00438-026-02509-w

Image Credits: AI Generated

DOI: 10.1007/s00438-026-02509-w

Keywords: Klebsiella pneumoniae, PhoU, persister cells, antibiotic tolerance, biofilm, metabolomics, CRISPR-Cas9, methionine synthesis, alkaline phosphatase, Pho regulon, antimicrobial resistance, Molecular Genetics and Genomics