Resistome and virulome accretion in an NDM-1-producing ST147 sublineage of Klebsiella pneumoniae associated with an outbreak in Tuscany, Italy: a genotypic and phenotypic characterisation

Background: Carbapenemase-producing Enterobacterales (CPE), particularly those producing metallo-β-lactamases, are among the most challenging antibiotic-resistant pathogens, causing outbreaks of difficult-to-treat nosocomial infections worldwide. Since November 2018, an outbreak of New Delhi metallo-β-lactamases-positive CPE (NDM-CPE) has emerged in Tuscany, Italy. In this study, we aimed to investigate the NDM-CPE associated with the outbreak and characterise the responsible Klebsiella pneumoniae clone. Methods: We used whole-genome sequencing and bioinformatic analysis to characterise NDM-CPE isolates that caused bloodstream infections in 53 patients at 11 hospitals in Tuscany and that were collected between Jan 1, 2018, and July 5, 2019 (ie, the early phase of the outbreak and preceding months). The CPE isolates characterised in this study were isolated and identified at the species level and as NDM producers by six diagnostic microbiology laboratories that serve the 11 hospitals. We used comparative genomic analysis, antimicrobial susceptibility testing, plasmid conjugal transfer assays, evaluation of virulence potential in the Galleria mellonella infection model, and serum bactericidal assays to further characterise the clone causing the outbreak. Findings: The outbreak was sustained by an ST147 K pneumoniae producing NDM-1, which had a complex resistome that mediated resistance to most antimicrobials (except cefiderocol, the aztreonam–avibactam combination, colistin, and fosfomycin). The clone belonged to a sublineage of probably recent evolution, occurred by the sequential acquisition of an integrative and conjugative element encoding the yersiniabactin siderophore, an FIB(pQil)-type multiresistance plasmid carrying blaNDM-1, and a transferable chimeric plasmid, derived from virulence elements of hypervirulent K pneumoniae, carrying several resistance and virulence determinants. Infection of G mellonella larvae revealed a variable virulence potential. The behaviour in serum bactericidal assays was different from typical hypervirulent K pneumoniae strains, with variable grades of serum resistance apparently associated with mutations in specific chromosomal loci (csrD, pal, and ramR). Interpretation: This description of a sublineage of ST147 K pneumoniae with a complex resistome and virulome that is capable of sustaining a large regional outbreak adds to existing research on the evolutionary trajectories within high-risk clones of K pneumoniae. Global surveillance programmes are warranted to track the dissemination of these lineages, and to prevent and control their spread. Funding: Italian Ministry of Health and Department of Experimental and Clinical Medicine, University of Florence.