Genomic characterization and stability of mcr-carrying plasmids in Klebsiella pneumoniae from Taiwanese hospitals: Identification of a distinct mcr-9-harboring plasmid in the high-risk ST11-KL47 clone

BACKGROUND: The spread of colistin-resistant Klebsiella pneumoniae carrying mobilized colistin resistance (mcr) genes via plasmids threatens last-line treatment options and complicates infection control in healthcare settings. Understanding the transferability and stability of these plasmids is crucial for assessing their potential for long-term persistence and hospital dissemination. This study characterizes the genetic contexts, transferability, and stability of mcr-carrying plasmids among colistin-resistant K. pneumoniae clinical isolates from tertiary hospitals in Taiwan, evaluating their implications for infection control and genomic surveillance. METHODS: A total of 105 colistin-resistant isolates collected from two hospitals were screened for mcr-1 to mcr-10. Positive isolates underwent whole-genome sequencing to define strain backgrounds and plasmid structures. Plasmid conjugative transfer and maintenance were evaluated using filter-mating assays and 14-day serial passages in the absence of antibiotic pressure. RESULTS: Five isolates harbored mcr genes: mcr-1.1 (n = 2), mcr-8.2 (n = 2), and mcr-9.1 (n = 1). Three mcr plasmids were readily transferable, whereas one mcr-1.1 and the mcr-9.1 plasmid were non-conjugative. Most mcr plasmids showed high stability (>70% retention after 14 days), including a distinct IncFIB/IncFII mcr-9 plasmid identified in the high-risk ST11-KL47 clone. In contrast, one mcr-8 plasmid (pKC457-3-mcr), accompanied by a ∼5 kb deletion, was highly unstable in Escherichia coli (<20% retention by Day 2, and ∼1% by Day 14). CONCLUSIONS: We report the first detection of mcr-9 in a clinical K. pneumoniae ST11-KL47 isolate in Taiwan and demonstrate that most mcr plasmids in this setting are highly stable even when transferability is limited. These findings underscore the risk of persistent reservoirs of colistin resistance in high-risk clones and highlight the need for ongoing genomic surveillance and targeted infection control measures in hospitals.