Study setting and study populations
A total of 24 microbiological laboratories from major acute-care hospitals in the Aichi prefecture participated in the study on a voluntary basis. The hospitals that participated in the study were certified as meeting the national infection control standards and they are currently participating in the prefectural infection control network (Prefectural Infection Control Kasan-1 Network Inter–Conference). This study comprises of 24 hospitals, 3 with more than 800, 6 municipal hospitals, 3 national hospitals and 12 private hospitals.
Bacterial isolates
The present study has a timeframe between January and March of the years ranging 2015 to 2019, in which the laboratories involved collected all non-duplicate Escherichia coli, Klebsiella spp. (K. pneumoniae, K. oxytoca, K. aerogenes) and E. cloacae complex isolates which met the susceptibility criteria attached below (Table 1). Even if the isolates were from the same species, multiple ones from the same patient were considered for this analysis under the condition of being identified more than one year apart. All isolates were sent to our laboratory for their due analysis. The participating laboratories provided the following data: specimen source, bacterial identification, antimicrobial susceptibility testing results, number of isolates of each Enterobacteriales species during each study period / individual timeframe.
Bacterial identification and antimicrobial susceptibility profile
The identification of the species of all isolates enrolled in this study was re-confirmed by matrix-assisted laser desorption ionization–time of flight (MALDI–TOF) by using the Vitek MS system (bioMérieux) as previously described [7]. If the data collected from this re-identification is not consistent with the original one, the result of re-identification is selected and adapted into the corresponding susceptibility criteria for the isolate. If the isolate did not meet the criteria, it was excluded from the study. Minimum inhibitory concentrations (MICs) of cefotaxime (CTX), ceftazidime (CAZ), cefmetazole (CMZ), flomoxef (FMOX), imipenem (IPM), meropenem (MEPM), cefepime (CFPM), cefozopran (CZOP), or cefpirome (CPR) were determined according to the version of the Clinical Laboratory Standards Institute (CLSI) M100 document applied by each laboratory.
Detection of carbapenemase
All collected strains were screened for carbapenemase production by the modified carbapenem inactivation method (mCIM) according to the CLSI description [8]. According to the CLSI guidelines, the results were classified as negative for a zone diameter ≥19 mm and as positive for a zone diameter of 6–15 mm, and the result was regarded as intermediate if it detected the presence of pinpoint colonies within a 16–18 mm zone.
PCR and DNA sequence analysis of bla genes
The DNAs of all isolates were obtained by using a a Cica GeneusR DNA extraction reagent (Kanto Chemical Holdings, Tokyo, Japan) and they were used as PCR templates. The carbapenemase genes (blaKPC, blaGES, blaIMP, blaVIM, blaNDM, blaOXA-48, blaOXA-23, blaOXA-24, and blaOXA-58group) were screened in all mCIM positive and intermediate strains and carbapenem-resistant strains regardless of the results of mCIM. Common ESBL genes (blaCTX-M-1, blaCTX-M-2, blaCTX-M-9, blaCTX-M-8/25, blaTEM, and blaSHV) and AmpC genes (blaMOX, blaDHA, blaCIT) were screened in all clinical strains using previously described primers [9–11]. The positive PCR products were screened by electrophoresis on 2.0% agarose gel, and the subsequent analysis (of PCR products for carbapenemase genes) was sent to Eurofins Genomics Inc. (Ohta-ku, Tokyo, Japan). Nucleotide sequences were compared and analyzed using BLAST (http://www.ncbi.nlm.nih.gov/BLAST).
Multi-locus Sequence typing
The multi-locus sequence typing (MLST) analysis was performed with all carbapenemase-producing K. pneumoniae and E. cloacae complex isolates and the DNA was isolated with Cica GeneusÒ DNA Extraction Reagent (Kanto Chemical, Tokyo, Japan). Seven housekeeping genes were amplified using primer sets according to the method previously reported [12, 13]. The DNA sequencing was performed at a commercial laboratory (FASMAC, Kanagawa, Japan) and the consensus regarding the sequence type (ST) was determined with Enterobacter cloacae locus/sequence definitions database (https://pubmlst.org/bigsdb?db=pubmlst_ecloacae_seqdef), and Klebsiella pneumoniae database (https://bigsdb.pasteur.fr/klebsiella/klebsiella.html).
Whole-genome sequencing
In this study 29 representative isolates from carbapenemase-producing E. cloacae complex and K. pneumonia were subject to a whole-genome sequencing analysis on a MiniSeq system (Illumina, San Diego, CA, USA) and on a MinION nanopore sequencer (Oxford Nanopore Technologies, Oxford, UK) by using the SQK-RBK004 kit and R9.4 flowcells in order to obtain complete sequences of plasmids carrying the blaIMP-1 gene. De novo sequence assembly was performed by using Unicycler or Miniasm and error correction was provided by Illumina reads with Unicycler or CLC Genomics Workbench v9.5.3 (QIAGEN, Hilden, Germany). Coding sequence (CDS) was annotated and registered with the PATRIC server (https://www.patricbrc.org) and, specifically, the linear comparison of blaIMP-1-carrying plasmid sequences was performed by using BLAST and visualized with Easyfig (http://mjsull.github.io/Easyfig/). Finally, we have also indicated the blaIMP-1 gene and other antimicrobial resistance genes, type IV secretion system-associated genes for conjugation were detected by using the T346Hunter server, whereas mobile gene elements were detected through CDS annotations.