Image created by Dr. Michael J. Miller
The capsule of Klebsiella pneumoniae is more than a protective coat. It is a major virulence factor, a useful epidemiological marker, and an important clue for selecting future vaccines, antimicrobial strategies, and phage therapies. However, capsule typing can be difficult to perform rapidly. Serological methods depend on costly antisera and can suffer from cross-reactivity, while PCR or sequencing methods cannot distinguish between live and dead bacteria.
A new study describes a modular reporter phage platform that turns capsule recognition into a measurable light signal. The researchers first engineered the K2-targeting bacteriophage RCIP0109 by inserting the NanoLuc luciferase reporter gene, creating ΦRCIP0109::nluc. When the reporter phage infects its matching K. pneumoniae hosts, it produces bioluminescence that can be measured in a standard microplate reader.
The key innovation is modularity. Because receptor-binding proteins (RBPs) help determine which capsule a Klebsiella phage recognizes, the team used RBP swapping to redirect the host range of the reporter phage chassis. This strategy generated reporter phages targeting K1, K47, K57, and K64 strains, adding to the K2-specific chassis and covering several clinically important capsular types associated with hypervirulence or carbapenem resistance.
“By swapping receptor-binding proteins, we can turn the natural specificity of phages into an adaptable diagnostic signal,” said Jie Feng, corresponding author of the study. “This work suggests a path toward rapid, scalable capsule typing that could be expanded as more phage-host recognition modules become available.”
The platform showed strong performance in multiple testing settings. For K2 detection, high bacterial concentrations were detected within 0.5 hours, and the limit of detection reached 10 CFU/mL after 2.5 hours. The RBP-swapped reporter phages selectively identified their corresponding capsular types with no detectable cross-reactivity. In synthetic urine, reporter phages detected K1, K2, K47, K57, and K64 K. pneumoniae strains at clinically relevant bacterial loads, with signals appearing within 0.5 hours at ≥105 CFU/mL while consistent luminescence kinetics were also observed at 103-104 CFU/mL. In polymicrobial urine-like samples containing other common uropathogens, the reporter phage panel still identified target strains within about 3 hours.
The researchers also demonstrated that directed evolution can tune reporter phage performance. An evolved K2 reporter phage produced 10- to 100-fold higher luminescence than its ancestral strain and shortened detection times at several bacterial concentrations. A separately evolved K57 reporter phage showed improved adsorption, higher titer, and a broader linear detection range, supporting the idea that phage evolution can be used to improve assay sensitivity.
Together, the findings present a scalable route for rapid K. pneumoniae capsular typing in near-clinical matrices. Because the assay can be read on standard microplate instruments and may also support visual inspection, it offers practical flexibility for future point-of-care development. Future integration with machine-learning prediction of phage-host interactions, AI-assisted protein design, and targeted mutagenesis could extend the approach to additional capsular types and other priority pathogens. This work was conducted by researchers at Shandong First Medical University and Shandong Academy of Medical Sciences; the Institute of Microbiology, Chinese Academy of Sciences; the University of Chinese Academy of Sciences; Yunnan University; and Peking University Third Hospital. Support was provided by the National Key Research and Development Program of China under Grant/Award 2024YFA0919400.
Reference
Yiyao Song, Shisong Jing, Yi Li, Yinglu Guo, Jiangqing Huang, Xianbiao Bi, Dawei Wei, Chao Wang, Gang Zhang, Jiajia Zheng, Zhongrui Ma, Jie Feng, A phage-based luminescent reporter platform for rapid typing of multiple capsular types of Klebsiella pneumoniae, hLife, 2026, ISSN 2949-9283, https://doi.org/10.1016/j.hlife.2026.06.001.
Abstract
Klebsiella pneumoniae is an important opportunistic pathogen, and its capsular polysaccharides are key virulence factors. Capsular typing is important for epidemiological surveillance, clinical diagnosis, and therapy development; however, methods for capsule-targeted detection remain limited. Here, we engineered a modular bioluminescent reporter phage platform leveraging bacteriophage receptor-binding proteins for specific K. pneumoniae capsular typing. Initially, the K2-specific bacteriophage ΦRCIP0109 was engineered with the nluc reporter gene to generate ΦRCIP0109::nluc for K2 detection. Using ΦRCIP0109::nluc as the chassis, we engineered reporter phages by swapping receptor-binding proteins (RBPs), thereby expanding the detection range to four additional clinically important K types (K64, K47, K1, and K57). Reporter phage specificity and sensitivity were evaluated against clinical isolates and in simulated polymicrobial environments. Directed evolution was employed to introduce RBP mutations for enhanced phage adsorption. These five reporter phages achieved 100% specificity against clinical isolates, detecting concentrations as low as 10 CFU/mL within 3.5–5.5 h and successfully differentiating host strains in polymicrobial synthetic urine. Directed evolution of an RBP yielded a 10- to 100-fold increase in luminescence compared to that of the wild-type phage. Together, these advances establish a scalable platform that can be expanded to additional capsular types and the detection of other pathogens, representing a platform with clear potential for integration into point-of-care diagnostics, guiding targeted antimicrobial therapy and precision phage therapy.