The clinical problem the study addresses is formidable. K. pneumoniae is wrapped in a thick capsule composed of long-chain polysaccharides that shields the cell from immune recognition and from many antibiotics. The chemical composition of this capsule defines the K type, and more than 70 distinct K types have been described. Hypervirulent and multidrug-resistant lineages of the bacterium have spread globally, and the enzymes that degrade these capsules, known as depolymerases, have emerged as a promising class of anti-infective agents precisely because they are highly specific: a given depolymerase typically attacks only one capsular serotype, sparing the rest of the bacterial community and, importantly, sparing the patient’s own cells and microbiota.

The phage at the center of the study was isolated against the K57-type K. pneumoniae strain 21AA3004 and proved strictly lytic, meaning it kills its host rather than integrating quietly into the bacterial genome. Transmission electron microscopy revealed its particle morphology, and whole-genome sequencing performed on the DNBSEQ-T7 platform showed a double-stranded DNA genome of 40,795 base pairs. Based on this sequence, the authors classified vB_KpnP_JYSS5 as a novel phage within the order Autographivirales, a group of T7-like viruses well known for their compact genomes and efficient lytic cycles. Genomic analysis also flagged open reading frame 11, ORF11, as the likely depolymerase gene, a prediction the team went on to test experimentally.

Growth parameters matter enormously for any phage being considered as a therapeutic, and vB_KpnP_JYSS5 performed well on these standard benchmarks. Using the double-layer agar plate method, the researchers determined that the optimal multiplicity of infection, the ratio of phage particles to bacterial cells, was 0.001, meaning that a single phage could efficiently clear a large bacterial population. One-step growth experiments showed an average burst size of approximately 114 plaque-forming units per infected cell, a measure of how many progeny virions each infected bacterium releases. The phage also displayed reassuring robustness under environmental stress: it retained infectivity across temperatures from 4 degrees Celsius up to 60 degrees Celsius and remained active across a pH range from 5 to 11, conditions that comfortably encompass the physiological environments where treatment would need to work.

Biofilms represent one of the hardest challenges in modern infectious disease medicine. Within these self-produced matrices of polysaccharide, protein, and extracellular DNA, bacteria become dramatically more resistant to antibiotics and to immune attack, and catheters, ventilators, and implants provide ideal surfaces for their formation. The team evaluated the anti-biofilm activity of both the intact phage and the recombinant Dep11 enzyme using three complementary readouts: plate counting to quantify viable bacteria, scanning electron microscopy to visualize structural damage, and confocal laser scanning microscopy to map live and dead cells within the biofilm architecture. Both agents effectively inhibited the formation of new biofilms by the host strain and, critically, could also degrade biofilms that had already established themselves, breaking open a structure that conventional antibiotics often penetrate poorly.

Dep11, the protein product of ORF11, emerged as the study’s most versatile candidate. When expressed recombinantly and applied to K57-type bacteria, the enzyme visibly degraded the capsular polysaccharide, confirming its predicted function. Loss of the capsule has consequences that ripple through every layer of host defense. The researchers showed that Dep11 treatment suppressed biofilm formation and eliminated mature biofilms, but the enzyme’s benefits extended further into synergistic antibacterial therapy. By removing the capsule, Dep11 made the bacteria more vulnerable to two clinically important antibiotics, polymyxin B and gentamicin, potentiating their activity against the target strain. This kind of enzyme-antibiotic synergy is particularly attractive in an era when dose escalation of toxic drugs such as polymyxins carries significant risks for patients.

The immunological experiments added another dimension to the therapeutic case. Macrophages, the tissue-resident scavenger cells of the innate immune system, struggle to engulf encapsulated bacteria because the slippery polysaccharide coat prevents firm attachment and internalization. When the team treated K. pneumoniae with Dep11 and then exposed the bacteria to macrophages, phagocytic uptake increased measurably, as assessed by plate counting, enzyme-linked immunosorbent assay, and flow cytometry. In other words, the enzyme does not kill bacteria directly in the way an antibiotic does; instead, it disarms them, converting an invisible, ungraspable pathogen into an accessible target for the host’s own immune machinery. This mechanism of action also suggests a lower selective pressure for the emergence of widespread resistance compared with conventional bactericidal drugs, although capsular variation among circulating strains remains a constraint that any depolymerase-based therapy must confront.

The most striking results came from the animal model. The researchers established systemic infection by injecting K. pneumoniae intraperitoneally into mice and then treating the animals with Dep11. A 50-microgram dose of the enzyme raised the survival rate of infected mice to 70 percent, a substantial improvement over untreated controls. Beyond survival, the treatment dampened the inflammatory storm that accompanies severe Klebsiella sepsis: concentrations of four pro-inflammatory cytokines, interleukin-1 beta, interleukin-6, tumor necrosis factor alpha, and interferon gamma, were all significantly lowered in the lung tissue of treated animals. This dual effect, improved survival together with reduced inflammatory signaling, indicates that capsule removal not only facilitates bacterial clearance but may also blunt the tissue damage driven by an overexuberant immune response.

The study was approved by the Ethics Committee of the First Affiliated Hospital of Hebei North University and by the institution’s Laboratory Animal Ethics Committee, and the authors report no competing interests. The work was funded by the Zhangjiakou Basic Research and Talent Cultivation Program and by cultivation research projects at Hebei North University. As with any preclinical phage or enzyme study, the path from mouse model to clinical use involves hurdles that the current experiments do not address, including dosing in humans, pharmacokinetics, immunogenicity of a repeatedly administered bacterial enzyme, and the practical need to identify the capsular type of a patient’s infecting strain before a serotype-specific agent like Dep11 can be deployed.

Nevertheless, the findings sharpen a strategy that is gaining momentum in the field of anti-infective research. Rather than relying on whole phage particles alone, which face regulatory complexity and the possibility of bacterial resistance through receptor modification, many groups are now isolating the depolymerase enzymes that phages use to pierce the capsule at the start of infection. These enzymes act as precision tools: they are chemically defined, can be produced recombinantly at scale, do not replicate, and can be combined with antibiotics or with the immune system. The characterization of vB_KpnP_JYSS5 and Dep11 adds a validated reagent for the K57 serotype to this growing toolbox and provides a template for how phage genomics, protein biochemistry, microscopy, and animal modeling can be woven together to move a candidate enzyme from sequence to therapeutic proof of concept.

Subject of Research: Characterization of the lytic phage vB_KpnP_JYSS5 and its depolymerase Dep11 targeting the K57 capsular type of Klebsiella pneumoniae for anti-biofilm and anti-infection therapy

Article Title: The virulent phage vB_KpnP_JYSS5 and its depolymerase Dep11 targeting K57 Klebsiella pneumoniae: characterization and therapeutic potential against biofilm and systemic infection

Article References: The virulent phage vB_KpnP_JYSS5 and its depolymerase Dep11 targeting K57 Klebsiella pneumoniae: characterization and therapeutic potential against biofilm and systemic infection. (n.d.). https://doi.org/10.1186/s12985-026-03288-x

Image Credits: AI Generated

DOI: 10.1186/s12985-026-03288-x

Keywords: Klebsiella pneumoniae, bacteriophage, depolymerase, Dep11, K57 capsular type, biofilm, capsular polysaccharide, phage therapy, macrophage phagocytosis, antibiotic synergy, systemic infection, Autographivirales