Emergence and regional spread of extended-spectrum β-lactamase-producing <i>Klebsiella pneumoniae</i> ST307 at a Japanese tertiary-care hospital

The global emergence of extended-spectrum β-lactamase-producing <i>Klebsiella pneumoniae</i> (ESBL-Kp), especially antimicrobial-resistant high-risk clones, is a public health concern. However, data on their molecular epidemiology in Japan remain limited. In this study, we conducted molecular analysis of 119 ESBL-Kp clinical isolates collected between January 2020 and April 2023 at a Japanese tertiary-care hospital. We identified 42 sequence types (STs), of which the most common was ST307 (<i>n</i> = 60, 50.4%), an antimicrobial-resistant high-risk clone. All ST307 isolates carried CTX-M-15, and 53 (88%) co-harbored OXA-1. Of the 60 ST307 isolates, 54 (90%) transferred IncFIIK plasmids carrying CTX-M-15 to <i>Escherichia coli</i> J53 recipients in conjugation experiments. ST307 isolates showed higher resistance rates than non-ST307 isolates, particularly to levofloxacin (100% vs 19%). Whole-genome sequencing of nine ST307 isolates revealed that <i>bla</i>_CTX-M-15, <i>bla</i>_OXA-1, and other antimicrobial-resistance genes were located on the same plasmid. All nine isolates had the GyrA S83I and ParC S80I substitutions. ST307 was found in both inpatients and outpatients, and core-SNP analysis revealed a narrow distance between inpatient and outpatient isolates, suggesting regional spread.IMPORTANCESequence type (ST) 307 is an emerging antimicrobial-resistant high-risk clone of <i>Klebsiella pneumoniae</i> with global clinical relevance. This study is the first to document large-scale dissemination of ST307 in Japan, revealing its predominance among extended-spectrum β-lactamase-producing <i>K. pneumoniae</i> at a tertiary-care hospital. Detection in both inpatient and outpatient settings suggests community circulation. ST307 harbors multiple resistance genes on plasmids capable of spreading antimicrobial resistance between bacteria, with potential for interspecies transmission and limited treatment options. Comparative analysis of plasmid structures revealed high similarity to those reported globally, indicating possible international transmission routes. The identification of a conserved plasmid structure associated with ST307 may provide a useful genetic marker for tracking its spread. Continued molecular surveillance of ST307 and its plasmids is essential to understand the global epidemiology of its evolution and distribution and to inform infection control strategies.