Identification and characterisation of colistin-resistant Acinetobacter colistiniresistens co-producing IMP-1 and OXA-58 carbapenemases

Background: Carbapenem-resistant Acinetobacter is of increasing global concern because infections are challenging to treat with standard antibiotics. Here, we identified a previously uncharacterised Acinetobacter sp. clinical isolate as Acinetobacter colistiniresistens co-producing IMP-1 and OXA-58. We also examined expression of genes related to antibiotic susceptibility and drug resistance, including bla _IMP.

Methods: The isolate was deposited at the National Institute of Technology and Evaluation (NITE) as Acinetobacter sp. NBRC 110496. Susceptibility was defined according to the Clinical and Laboratory Standards Institute (CLSI) breakpoints. Genomic and clonal analyses were performed to identify species and resistance genes.

Results: The isolate was resistant to β-lactams, including broad-spectrum cephalosporins and carbapenems, polymyxins, and trimethoprim/sulfamethoxazole. Genomic analysis identified the isolate as A. colistiniresistens harbouring bla _IMP-1, bla _OXA-58, bla _OXA-670, aac(6')-Ib, aac(6')-Ij, ant(3")-I I, aph(3')-VI, msrE, mphE, and sul1. Colistin resistance was associated with the eptA-like gene, which encodes a lipid A-modifying enzyme. SNP-based phylogenetic analysis revealed that the strain clustered with other strains isolated in Japan. The IMP-1/OXA-58-producing strain described in this study has a novel integron structure surrounding bla _IMP-1, aacA and sul1.

Conclusions: Colistin-resistant IMP-1/OXA-58-co-producing A. colistiniresistens was identified in a patient. This isolate could serve as a reservoir for carbapenemase-producing organisms. This study suggests that screening for colistin-resistant isolates is crucial to preserve colistin as a therapeutic agent for multidrug-resistant bacteria. Identification of this MDR isolate in Asia, and the danger of it spreading worldwide, should raise serious concerns.