Emerging infection and nosocomial transmission of carbapenem-resistant <i>Klebsiella pneumoniae</i> harboring <i>bla</i> _NDM-21 in a neonatal intensive care unit

Carbapenem-resistant Klebsiella pneumoniae (CRKP) represents a significant threat in neonatal intensive care units (NICU) because of its high antimicrobial resistance and association with increased mortality rates. In our study, we identified three blaNDM-21-harboring CRKP isolates from three critically ill pediatric inpatients; these isolates underwent comprehensive analysis. Our investigations included assessments of their clinical spatiotemporal distribution, antimicrobial susceptibilities, whole-genome sequencing (WGS), biological properties, and genomic and plasmid replicon typing. The three blaNDM-21-harboring CRKP strains, which belong to sequence type 35 (ST35), harbored the gene on IncX3 plasmids. Clinical spatiotemporal and WGS analyses indicated that the third case resulted from transmission from the second case within the NICU, whereas the first case remained epidemiologically unrelated. All three isolates displayed robust growth, formed strong biofilms, exhibited low mucoviscosity, possessed distinct capsule structures observable under an electron microscope, and demonstrated pathogenicity and virulence via in vitro experiments. Moreover, in mouse models, blaNDM-21-harboring CRKP showed significant multi-organ invasiveness and pathologic damage at an inoculation level of 106 CFU, leading to the death of all infected mice. This study provides evidence of the emergence of blaNDM-21-harboring CRKP with enhanced virulence-associated features in pediatric/NICU-associated infections, thereby highlighting the convergence of rare carbapenemase-mediated resistance, virulence potential, and a probable risk of nosocomial transmission in a vulnerable patient population.