Coexistence of mcr-9 and bla NDM-1 in a multidrug-resistant Enterobacter hormaechei strain recovered from a bloodstream infection in China Sir, Enterobacter hormaechei is an important global pathogen contributing to increased morbidity and mortality in hospitalised patients, particularly when infected with carbapenem-resistant isolates owing to limited alternative therapeutic options [1] . Since the carbapenemase New Delhi metallo-β-lactamase 1 (NDM-1) was first reported in Klebsiella

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... in 2009, NDMproducing Enterobacterales have been rapidly spreading not only in clinical settings but also in food animals globally [2] . The emergence and wide spread of the plasmid-borne mcr genes conferring resistance to colistin also represent a serious threat to public health. In May 2019, a novel mcr homologue (mcr-9) was identified in a multidrug-resistant (MDR) Salmonella enterica serotype Typhimurium isolate recovered from a human patient in the USA. Here we report the genomic characteristics of a MDR E. hormaechei co-carrying mcr-9 and bla NDM-1 genes recovered from a bloodstream infection in China. We retrospectively screened by PCR amplification for the presence of acquired carbapenemase genes (bla KPC , bla NDM , bla IMP , bla VIM and bla OXA-48 ) and mcr genes (mcr-1 to mcr-9) in all Enterobacterales recovered from a tertiary hospital in Hangzhou, China. Only one isolate, designated E. hormaechei 3804, tested positive for both bla NDM-1 and mcr-9 genes. This isolate was cultured from a blood sample of a male patient with a hospitalisation record of fever and bloodstream infection. The patient was treated empirically with an injection of ceftizoxime and was discharged alive from hospital prior to the availability of blood culture results. The bacterial species was identified by matrixassisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF/MS) using a MALDI Biotyper 1 (Bruker Daltonics, Billerica, MA, USA) and 16S rRNA gene sequencing. Enterobacter hormaechei 3804 was subjected to antimicrobial susceptibility testing by the microdilution broth method for the following antimicrobial agents: amikacin; ampicillin/sulbactam; aztreonam; cefazolin; cefepime; cefotetan; ceftazidime; ceftriaxone; ciprofloxacin; colistin; gentamicin; imipenem; levofloxacin; piperacillin/tazobactam; tetracycline; and tigecycline. Minimum inhibitory concentrations (MICs) were interpreted according to Clinical and Laboratory Standards Institute (CLSI) 2020 standards, and the breakpoints of colistin and tigecycline for Enterobacterales were interpreted according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) criteria (v.9.0). Escherichia coli ATCC 25922 was used as the quality control strain.