Hypervirulent K. pneumoniae, first recognized in Taiwan and now reported worldwide, is distinguished by its capacity for metastatic spread and its ability to cause serious infections in otherwise healthy individuals. The pathotype is conventionally associated with two functional groups of virulence factors. The first comprises regulators of the mucoid phenotype, most notably the plasmid-encoded rmpA and rmpA2 genes, which sit on a large virulence plasmid of roughly 200 kilobases known as pLVPK and orchestrate the overproduction of extracellular capsular polysaccharide. The second group consists of siderophores, iron-scavenging molecules including aerobactin, salmochelin, enterobactin, and yersiniabactin, which allow the bacteria to strip iron from host tissues. Aerobactin in particular has been shown to increase strain virulence by up to 100-fold in mouse lethality models, and its biosynthesis gene iucA is frequently used as a molecular signature of hypervirulence.

Complicating the diagnostic picture, the hypermucoviscous phenotype long used as a shorthand for hypervirulence has proven unreliable. Not all hypermucoviscous strains cause metastatic infection, not all hypervirulent strains display the stringy mucoid appearance, and some classical isolates exhibit the phenotype as well. Against this backdrop, the Egyptian research team adopted a combinatory genotypic definition: an isolate was classified as hypervirulent only if it carried at least one mucoid-regulator gene, either rmpA or rmpA2, together with at least one siderophore gene, either iucA or iroN. This dual requirement is grounded in established genomic frameworks showing that the coexistence of capsular regulation and enhanced iron acquisition predicts the hypervirulent pathotype with accuracy approaching 94 percent.

The molecular screening yielded striking results. Among the 82 confirmed isolates, obtained from sputum, blood, pus, urine, ear discharge, and vaginal swabs, the rmpA gene was present in 51.2 percent of strains and rmpA2 in 35.4 percent, with at least one of the two genes detected in 57.3 percent of the collection. The iucA aerobactin gene appeared in 37.8 percent of isolates, while iroN, associated with salmochelin biosynthesis, was comparatively rare at 7.3 percent. Critically, of the 36 isolates carrying at least one siderophore gene, 34 also carried at least one mucoid regulator, meaning that 41.5 percent of the clinical isolates met the genotypic definition of hypervirulence while the remaining 58.5 percent were classified as classical strains. The distribution of isolates across specimen types varied significantly, with sputum, urine, and pus yielding the largest numbers.

Beyond the core classification markers, the researchers probed additional virulence determinants to identify which, if any, correlated with the hypervirulent genotype. The wzi gene, which encodes an outer membrane protein involved in anchoring capsular polysaccharides and is conserved across all 77 known K. pneumoniae capsular serotypes, was detected in 86.6 percent of isolates overall, including 97 percent of hypervirulent and 79.2 percent of classical strains. Statistical analysis using Fisher’s exact test revealed a significant association between wzi presence and the hypervirulent genotype, though the authors caution that the gene’s high prevalence in both groups limits its discriminatory power as a standalone marker.

The most compelling individual correlate emerged from the peg-344 gene, a putative metabolite transporter repeatedly implicated in hypervirulence. Peg-344 was found in 26.8 percent of all isolates but in 58.8 percent of genotypically hypervirulent strains versus only 4.2 percent of classical ones, producing an extremely significant statistical association. However, the team’s diagnostic performance analysis revealed an important nuance: while peg-344 positivity carried high specificity of 95.83 percent and a positive predictive value of 90.91 percent, its sensitivity was only moderate at 58.82 percent. Fourteen of the 34 hypervirulent isolates tested negative for the gene, demonstrating that peg-344 absence does not exclude the hypervirulent pathotype and that the marker should be interpreted as one component of a combinatory molecular panel rather than a sole screening tool.

In contrast, two capsular serotype markers showed no significant relationship with hypervirulence in this collection. The magA gene, associated with the K1 serotype, appeared in only 6.1 percent of isolates, distributed between 11.8 percent of hypervirulent and 2.1 percent of classical strains, while wzyK2, encoding the K2 serotype, was entirely absent. The wabG gene, involved in capsular lipopolysaccharide production, was highly prevalent at 90.2 percent but showed no statistically significant correlation with the hypervirulent genotype, echoing earlier findings that its presence does not track with hypervirulence. These results underscore the variable detection rates of capsular serotype genes among hypervirulent strains worldwide and the need to explore correlations across a broader range of the 77 recognized K antigens.

The study also delivered a rigorous head-to-head evaluation of the two phenotypic assays most commonly used to detect the hypermucoviscous phenotype. In the string test, a bacteriological loop is drawn through a colony to assess whether a viscous string of at least 5 millimeters forms; only 13 isolates, or 15.8 percent, tested positive, including 11 hypervirulent and 2 classical strains. The test achieved high specificity of 95.83 percent but dismal sensitivity of 32.35 percent, with Cohen’s kappa of 0.31 indicating only fair agreement with molecular classification. The low-speed sedimentation assay, which measures how much of a standardized bacterial suspension remains in the supernatant after brief centrifugation, performed considerably better: 21 isolates tested positive, including 61.8 percent of the hypervirulent strains and no classical strains, yielding perfect specificity, sensitivity of 61.76 percent, accuracy of 84.15 percent, and substantial kappa agreement of 0.65.

Neither phenotypic method proved adequate as a standalone diagnostic. The string test’s very low sensitivity produces frequent false negatives, a problem compounded by reports of string test-negative hypervirulent strains, including carbapenem-resistant isolates from liver abscess cases in multiple countries, and by the observation that hypermucoviscosity can arise in classical strains through mutations in wzi independent of conventional hypervirulence genes. The sedimentation assay, while more reliable, still missed more than a third of true hypervirulent isolates. The authors conclude that molecular detection of virulence markers, particularly the combination of rmpA or rmpA2 with iucA or iroN, remains essential for definitive identification, and they call for standardized genotypic criteria to enable consistent detection and tracking of phenotypically unrecognizable hypervirulent strains globally.

The findings carry practical weight for infection control in resource-limited settings, where accessible PCR-based panels could replace unreliable phenotypic screening. The authors acknowledge limitations, including the single-hospital sampling frame, the moderate sample size of 82 isolates, the absence of functional virulence validation such as serum resistance or infection models, and the lack of whole-genome sequencing or multilocus sequence typing to resolve clonal lineages and plasmid repertoires. Larger multicenter cohorts with functional assays and genomic profiling will be needed to validate and refine the combinatory biomarker framework. In the meantime, the study reinforces a message increasingly echoed across the field: as hypervirulent K. pneumoniae continues its global spread, laboratories that rely on the string test alone risk overlooking some of the most dangerous strains circulating in their patient populations.

Subject of Research: Molecular identification of hypervirulent Klebsiella pneumoniae virulence determinants and evaluation of phenotypic diagnostic methods

Article Title: Molecular Insights Into Hypervirulent Klebsiella pneumoniae: Identification and Correlation of Virulence Determinant

Article References: Elkady, F. M., Basher, N. S., Abdulrahman, M. S., Hashem, A. H., Metwally, R. A., Tawab, A. A., Saied, E., Ibrahim, N. A., & Aufy, M. (2026). Molecular Insights Into Hypervirulent Klebsiella pneumoniae : Identification and Correlation of Virulence Determinant. MicrobiologyOpen, 15(5), Article e70433. https://doi.org/10.1002/mbo3.70433

Image Credits: AI Generated

DOI: 10.1002/mbo3.70433

Keywords: hypervirulent Klebsiella pneumoniae, rmpA, iucA, peg-344, hypermucoviscosity, string test, low-speed sedimentation assay, siderophores, virulence genes, PCR diagnostics, capsular serotypes, infection control