Identification of a novel circular DNA carrying blaNDM-1 gene in an extensive drug resistant Providencia huaxiensis strain.

Providencia, as an opportunistic pathogen, can cause serious infections and increasingly poses a life-threatening risk to public health. Carbapenems were initially considered effective agents conquering such infectious; however, the emergence of carbapenem-resistant Providencia strains has been attributed to the rapid spread of carbapenemase genes. In this study, we first reported a uniquely blaNDM-1 genetic structure that can evolve 'circular DNA'. Among 901 carbapenem-resistant Enterobacteriaceae strains examined, only one P. huaxiensis strain was identified using VITEK MS, 16S rRNA sequencing, average nucleotide identity (ANI) and the DNA-DNA hybridization (DDH). Susceptibility testing using the broth microdilution method revealed that the P. huaxiensis strain was highly resistant to carbapenems, cephalosporin, quinolone and aminoglycosides. By whole-genome sequencing (WGS) and large fragment PCRs, we characterized more than 7 resistance genes including blaNDM-1, blaOXA-10, qnrB, aadB, sul1, tetA and aadA1 in the P. huaxiensis strain PH1503, in which 2 copies of each the blaNDM-1 and blaOXA-10 genes were identified to be integrated into 3 plasmids. The blaNDM-1 gene was integrated into a novel structure of ISCR1-trpF-bleMBL-blaNDM-1-CatB3-Arr-3-DfrA27-AadA16-qacEdelta1-Sul1, which was present in both pPH1503-1-NDM and pPH1503-5-NDM. Moreover, the DNA sequences of the novel structure and pPH1503-5-NDM was 100% identical to each other, indicating that the novel structure may cyclize into a 'circular DNA'. To the best of our knowledge, this is the first description of a circular DNA carrying the blaNDM-1 gene, which was identified in clinically isolated P. huaxiensis strains and appears to be stably maintained.