# Characterization of multidrug-resistant ST978-KL103 Klebsiella quasipneumoniae subsp. quasipneumoniae clinical strains co-harboring tmexCD2-toprJ2 and blaIMP-4

## TL;DR

This study identifies a dangerous new strain of Klebsiella quasipneumoniae resistant to most antibiotics and capable of spreading resistance genes.

## Contribution

The discovery of ST978-KL103 Klebsiella quasipneumoniae co-harboring multiple resistance genes on transferable plasmids.

## Key findings

- ST978-KL103 Klebsiella quasipneumoniae isolates are extensively drug-resistant, with polymyxin B as the only effective agent.
- The isolates carry tmexCD2-toprJ2 and blaIMP-4 on IncU-type plasmids, and some also carry blaNDM-1 on IncC-type plasmids.
- These isolates show strong biofilm formation and serum resistance but low acute virulence in infection models.

## Abstract

Klebsiella quasipneumoniae is an emerging opportunistic pathogen increasingly associated with multidrug-resistant infections. In this study, we characterized four clinical isolates of K. quasipneumoniae subsp. quasipneumoniae assigned to the ST978-KL103 clonotype, all linked to poor clinical outcomes. Antimicrobial susceptibility testing revealed an extensively drug-resistant phenotype, including resistance to β-lactams, carbapenems, aminoglycosides, fluoroquinolones, tigecycline, and trimethoprim/sulfamethoxazole, with polymyxin B as the only effective agent. Whole-genome sequencing and comparative analysis showed that all ST978-KL103 isolates harbored multiple plasmids, including a conserved IncU-type plasmid carrying blaIMP-4 and the tmexCD2-toprJ2 tigecycline resistance cluster. Two isolates also carried IncC-type plasmids encoding blaNDM-1. Conjugation assays confirmed efficient horizontal transfer of both plasmid types to E. coli recipients. Although classical virulence loci such as yersiniabactin, aerobactin, and rmpA/rmpA2 were absent, the isolates exhibited strong biofilm formation and serum resistance, indicating potential for persistence and immune evasion. Cytotoxicity assays and Galleria mellonella infection models demonstrated low acute virulence compared to the hypervirulent K. pneumoniae strain NUTH-K2044. Phylogenetic analysis placed the ST978 isolates within a distinct and globally distributed clonal lineage. The close genetic relatedness among isolates, particularly between KP57 and KP58, suggests recent nosocomial transmission. Resistome profiling of nine global ST978 genomes revealed over 40 resistance genes, with our isolates uniquely harboring the tmexCD2-toprJ2 cluster. These findings highlight ST978 K. quasipneumoniae as a globally disseminated and increasingly drug-resistant lineage with the potential to emerge as a high-risk clone in clinical settings.

The emergence of extensively drug-resistant Klebsiella quasipneumoniae strains poses a significant threat to global health yet remains underrecognized due to diagnostic limitations. Our study identifies ST978-KL103 as a cryptic high-risk lineage co-harboring blaIMP-4, blaNDM-1, and tmexCD2-toprJ2 on transferable plasmids. These findings highlight the species’ underappreciated role as a reservoir and disseminator of last-line resistance. The strong biofilm formation and serum resistance further suggest persistence potential in clinical settings. Early recognition and surveillance of such emerging clones are critical to prevent silent dissemination and therapeutic failure in healthcare environments.