Acinetobacter calcoaceticus from a fatal case of pneumonia harboring bla_NDM-1 on a widely distributed plasmid

We have recovered one bla_NDM-1-harboring bacterial strain, designated as XM1570, from a sputum sample obtained from a fatal case of pneumonia in China.

Biochemical profiling, 16S rRNA sequencing and antimicrobial susceptibility testing were performed. Conjugation experiments were conducted to determine transmissibility of resistance. Pulsed-field gel electrophoresis and whole genome sequencing were performed to identify strain-specific features.

The isolate XM1570 was identified as Acinetobacter calcoaceticus. Whole genome sequencing identified two plasmids, pXM1 and pXM2. Comparative analysis showed >99% similarity between XM1570 and A. calcoaceticus PHEA-2. Plasmid pXM1 carried the carbapenemase gene bla_NDM-1 and displayed high homology with previously described plasmids isolated from different Acinetobacter spp., which were collected from human or livestock distributed in China and worldwide. The bla_NDM-1 gene was located on this conjugative plasmid in a transposon-like region flanked by two copies of the insertion sequence ISAba125; and resistance to all tested β-lactams was observed. Transferability of resistance from pXM1 to the transconjugants was identified. Plasmid pXM2 had an insertion sequence ISAba125 and a −35 region of the bla_NDM-1 gene promoter but the bla_NDM-1 gene was not present. A chromosomally located carbapenemase-encoding gene bla_OXA-75 was detected; however, this gene was interrupted by an insertion sequence ISAba22 belonging to IS3 family.

Location of bla_NDM-1 on different self-transmissible plasmids could facilitate geographically broad dissemination and host range expansion of the bla_NDM-1 gene via horizontal gene transfer. Our findings of this normally environmental species A. calcoaceticus XM1570 further underline the significant clinical challenge and the essential need for surveillance including molecular methods and plasmid analyses.