The prevalence of IMPKp varies by regions and often induces outbreaks in clinical departments. A multicenter study showed that the MBLs were not equally common, with more than 60% of MBL-positive CRE isolates carrying the blaNDM, while only 9% of the isolates produced the IMP [3]. In China, the predominance of MBL among Enterobacteriaceae was NDMs, and the IMP has been sporadically detected [10]. We have reported the molecular characterization of some clinical IMPKp isolates [11]. In this study, a larger collection (n = 29) of IMPKp were collected and analyzed by more accurate methods such as whole-genome sequencing, and thus limited results in previous study are revised and supplemented. The high homology among some strains indicated the existence of small-scale outbreaks caused by various IMPKp clones (Fig. 1). Most IMPKp isolates exhibited low-level resistance or even susceptible to various carbapenems (Additional file 1: Table S1), indicating the low hydrolysis capacity of IMP, and may leading to a low epidemic level of IMPKp. In this study, IMP-4 (14/29, 48.28%) was the most popular enzyme type, followed by IMP-1 (10/29, 34.48%), and this was consistent with previous finding [12].

The ST type of IMPKp isolates in this study was relatively dispersed (Fig. 1). The K. pneumoniae CG258 are the most common carbapenem-resistant isolates reported worldwide, including the ST11 and the ST5422 in this study. The tonB allele of ST11 (tonB-4) is widely distributed in approximately 80 unrelated STs of CG258, but the tonB-79 allele of ST258 has only been observed among less than 10 STs (http://www.pasteur.fr/mlst). Some researchers considered that the tonB-79 was probably derived from tonB-4 by acquisition of site substitutions [13]. In this study, among the four novel STs (ST5422, ST5423, ST5426 and ST5427), only ST5422 [allelic profile (gapA, infB, mdh, pgi, phoE, rpoB, and tonB) was 3-3-1-1-1-1-9] may arise from ST11 (allelic profile: 3-3-1-1-1-1-4) by substituting tonB-4 with tonB-9. However, although it has high homology with ST11, completely different characteristics of IMP-coding genes suggested the independent evolutionary path of this novel identified clone (Fig. 1).

Consistent with previous study [14], we found that all known IMP-coding genes were located on IncN and IncHI5 plasmids in this study (Fig. 1). IncN and IncHI plasmids are both conjugative plasmids and have been reported to be part of a broad-host-range group [15]. Based on the nucleotide sequence homology over the backbones, the IncN group can be divided into three subgroups, including IncN1, IncN2 and IncN3 [16]. In this study, the predominant blaIMP-carrying plasmids belonged to N1 with a smaller size (Fig. 1). The blaIMP-1-carrying IncN3 plasmid was identified in UK [5], and this was the first report to identify this plasmid in China, showing the necessity of investigating prevalence and evolutionary history of IncN3 plasmids. The IncHI plasmids are important vectors in the dissemination of heavy metal resistance genes and antimicrobial resistance genes, and usually have a size larger than 200 kb [17]. In this study, all blaIMP-carrying IncHI5 plasmids carried conserved IncHI5 backbones, including repHI5B and a repFIB-like, parABC, and tra1, mediating replication, partition and conjugal transfer, respectively. Further studies are needed to continuously monitor the prevalence of blaIMP-carrying IncN and IncHI5 plasmids in different sources, especially among clinical settings [11, 14].

To date, various genetic context of blaIMP-4 has been identified, e.g. blaIMP-4-qacG-aacA4-aphA15, blaIMP-4-Kl.pn.I3-mobC and blaIMP-4-Kl.pn.I3-qacEΔ-sul1. Of note, the structure of blaIMP-4-Kl.pn.I3 seems unique in isolates from China revealed by blasting in GenBank, thus it could be used as an epidemiological marker for blaIMP-4 detection in China. In823 harboring blaIMP-4 has been identified in IncHI5 plasmid [18]. In this study, all In823 harboring the blaIMP-4 was presented in the IncN1 plasmid, and the blaIMP-4-attCDΔ::Kl.pn.I3 was the most common cassette. Meanwhile, the integrons in IncHI5 were In809 and the novel integron In2146 (Figs. 1 and 2). In809 with four cassettes (blaIMP-4-qacG2-aacA4-catB3) is widely disseminated in Enterobacteriaceae and Acinetobacter spp. in Asia–pacific region [19]. In2146 may be derived from In809, and is considered as an In809-like integron (blaIMP-4-Kl.pn.I3-qacG2-aacA4-catB3Δ), of which a group II intron Kl.pn.I3 was inserted into the attC site of the blaIMP-4 cassette (Fig. 2). The blaIMP-1-carrying In994 and In1223 has been identified in previous studies [16], and it has also been found in this study (Fig. 1). In addition, the integron In837 (intI1-blaIMP-26-attCDΔ::Kl.pn.I3-3'CS, Fig. 2) showed low homology with other blaIMP-26-carrying integrons, and had different genetic structure such as intI1-blaIMP-26-ltrA-qacEΔ1-sul1, intI1-blaIMP-26-qacG-aacA4-aac(6ʹ)-orf-catB3, and intI1-blaIMP-26-qacG-aac(6ʹ)-Ib-aac(6ʹ)-orf3-orf4-catB3-dfrA1-tnpA-istB-orf5, respectively [20]. In general, country-specific blaIMP subtypes corresponded to the specific integron types previously characterized in that country, i.e., blaIMP-4-carrying In809 in Australia [21]; blaIMP-6-carrying In722 in Japan [22]; blaIMP-8-carrying In73 in Philippines [23]; and blaIMP-14-carrying In687 in Thailand [24]. In this study, we identified 7 different blaIMP-carrying integron types, including 2 novel cassette combinations (In2146 and In2147; Fig. 2). All the four class I integrons have a complete set of IRi/IRt, intI1, and attI1. All integrons carried the strong promoters PcS (In2147 and In809) or the PcWTGN-10 (In837, In994, In823, In1223 and In2146) (Additional file 1: Table S1), and these promoters can drive the high-level expression of cassette-borne genes [25, 26].

IMP-90 is a novel variant with an amino acid substitution at Ser262Gly compared with IMP-8. The IMP-90-producing isolates showed a sensitive feature to imipenem (MIC = 1 mg/L), while the blaIMP-90-carrying E. coli DH-5α (competent cell) showed a reduced resistance to imipenem, compared with the blaIMP-8 (data not published). Previous studies show that isolates producing the IMP-type metallo-beta-lactamases with Ser262Gly substitution all exhibited susceptibility to imipenem, including the IMP-6 and IMP-68, which were the Ser262Gly substitution variants of IMP-1 and IMP-11, respectively [9, 27]. These results indicate that the Ser262Gly substitution may affect the hydrolysis ability of IMP variants to imipenem. The blaIMP-90 presented in the novel integron In2147 located on a IncM1 plasmid (Figs. 2 and 3). Plasmids belonging to the IncL/M are the important mobile genetic platforms for dissemination of clinically important resistance genes, e.g. blaCTX-M and blaOXA-48 [28]. Although L and M plasmids showed high level of DNA homology (approximately 94% overall nucleotide identity), the ExcA, TraY, and TraX proteins exhibited evident division (35%, 59%, and 75% amino acid identity, respectively). IncM plasmids have a wide host range, while IncM1 and IncM2 showed the 99% amino acid identity of the entry exclusion ExcA and TraY proteins [29]. This was the first report about the blaIMP-carrying IncM1 plasmid. Conjugation between isolates carrying various IncM plasmids is compromised by inhibitory interactions of the exclusion system, which is highly conserved across all IncM plasmids (Fig. 3) [29]. Therefore, more attention should be paid to the transmission of drug resistance mediated by the IncM-type plasmids, and continuous monitoring shall also be carried out.