494. Fitness Cost of mcr-1-Mediated Colistin Resistance in Carbapenemase-Producing Klebsiella pneumoniae

BACKGROUND: The emergence of mobile colistin resistance gene mcr-1, a plasmid-borne polymyxin resistance mechanism, in carbapenem-resistant Klebsiella pneumoniae is an alarming concern. However, previous studies showed that the acquisition of mcr-1 was associated with a significant biological fitness cost in K. pneumoniae. We aimed to study the impact of mcr-1 on the biological fitness in clinical carbapenemase-producing K. pneumoniae strains. METHODS: Clinical carbapenemase-producing K. pneumoniae strains were collected consecutively at the Taipei Veterans General Hospital between November 2017 and December 2018. The strain positive for mcr-1 was subjected to whole-genome sequencing to delineate its genomic features. Escherichia coli J53 strain was used as the recipient strain in plasmid conjugation assay and the transconjugants were selected with sodium azide and colistin. Plasmid stability was tested by serial passaging in antibiotic-free LB broth for 28 days. The growth rate was compared between the parental mcr-1-bearing strain and the plasmid-cured strain. RESULTS: One ST11 strain isolated from a fatal case with bacteremia (KP2509) was found to harbor bla(KPC-2), bla(OXA-48), and mcr-1. This strain was resistant to colistin (MIC=8 mg/L) and imipenem (MIC≥16 mg/L). Whole-genome sequencing of KP2509 showed that mcr-1, bla(KPC-2) and bla(OXA-48) were located on an IncHI-FIB type plasmid of 319 Kb, an IncFII type plasmid of 96 Kb, and an IncL type plasmid of 64 Kb, respectively. Conjugation efficiency of mcr-1-bearing plasmid was 2.24 × 10(–4), and the colistin MIC of E. coli J53 transconjugant increased from 0.5 to 8 mg/L. The mcr-1-bearing plasmid in KP2509 showed high plasmid stability, and only ~1% were lost after 27-day passages. The resulting plasmid-cured strain (PC-KP2509) was susceptible to colistin (MIC=0.5 mg/L) and had a similar growth rate to that of parental mcr-1-bearing strain KP2509. CONCLUSION: We identified an ST11 K. pneumoniae strain carrying bla(KPC-2), bla(OXA-48), and mcr-1 genes causing a fatal bacteremia. The large mcr-1-bearing plasmid confers a moderate level of colistin resistance but without significant biological fitness cost in carbapenemase-producing K. pneumoniae, which could result in a serious threat clinically. DISCLOSURES: All authors: No reported disclosures.