Antimicrobial resistance profiles and genetic elements involved in carbapenem resistance in Acinetobacter baumannii isolates from a referral hospital in Southern Iran

• ISAba elements were associated with resistance to carbapenem in A. baumannii.
• We report the emergence of decreasing susceptibility to colistin among A. baumannii.
• Carbapenem resistant XDR strains were showed the greatest sensitivity to colistin.
• Ninety-seven percent of carbapenem resistant isolates were carbapenemases producers.

In this study, antimicrobial resistance patterns, carbapenemase production, carbapenem-hydrolysing class D OXA-type β-lactamase (CHDL)-encoding genes and ISAba insertion elements were characterised in Acinetobacter baumannii isolates from Southern Iran. A total of 85 A. baumannii isolates from patients in a tertiary care hospital were isolated. Antibiotic susceptibility profiles to 14 antimicrobials were determined by disc diffusion assay. Minimum inhibitory concentrations (MICs) to colistin and tigecycline were determined by the broth microdilution method. Isolates were analysed for OXA-type carbapenemase-encoding genes, ISAba elements upstream of blaOXA genes, and carbapenemase production by the CarbAcineto NP test. Of the 85 A. baumannii isolates, 73 (86%) were non-susceptible to imipenem. The MIC50 and MIC90 values for imipenem were 16 mg/L and 64 mg/L, respectively. Fourteen isolates (16%) were resistant to colistin. Among the 79 carbapenem-non-susceptible A. baumannii isolates, 77 (97%) were carbapenemase-producers as determined by the CarbAcineto NP test. The most frequently identified OXA-type carbapenemase gene was blaOXA-23-like, which was identified in 44 isolates (52%), followed by blaOXA-24-like with 13 isolates (15%). Statistical analysis showed that ISAba elements upstream of the blaOXA gene were significantly related to imipenem-non-susceptible isolates. The presence of blaOXA genes and ISAba1 and ISAba4 elements can explain the resistance of the isolates to carbapenems. The concerns raised following the emergence of colistin-resistant A. baumannii isolates highlight the need for a comprehensive national programme to collect and examine A. baumannii isolates from different parts of Iran.