Carbapenem-resistant Acinetobacter baumannii isolates carrying blaOXA genes with upstream ISAba1: First report of a novel OXA subclass from Iran

Abstract

Objectives: Carbapenem-resistant Acinetobacter baumannii (CRAB) have emerged as a serious threat to public-health worldwide. This study aimed to determine the antimicrobial susceptibility of A. baumannii isolates in Iran and to investigate oxacillinase-encoding determinants and their association with insertion sequence ISAba1 in CRAB isolates.

Methods: This study was performed on A. baumannii isolates recovered from patients with burn wound infections during 2013. All isolates were evaluated for antimicrobial susceptibility by the disk diffusion method. Minimum inhibitory concentrations (MICs) of five antibiotics (imipenem, meropenem, polymyxin B, colistin and tigecycline) were determined for all CRAB isolates. PCR was performed to determine the distribution of bla_OXA determinants and ISAba1 insertion upstream of each corresponding gene in the CRAB isolates.

Results: A total of 65 A. baumannii isolates were recovered during the 1-year period, with CRAB accounting for 63 (96.9%) of isolates. Polymyxin B, colistin and tigecycline were the most effective agents against CRAB isolates, with susceptibility rates of 100%, 87.3% and 65.1%, respectively. The proportion of CRAB isolates carrying oxacillinase determinants was as follow: bla_OXA-51-like, 100%; bla_OXA-23-like, 74.6%; bla_{OXA-24/40-like}, 47.6%; and bla_OXA-235-like, 12.7%. ISAba1, ISAba1-bla_OXA-23-like and ISAba1-bla_OXA-51-like were detected in 100%, 41.3% and 1.6% of CRAB isolates, respectively. Co-occurrence of bla_OXA determinants or inserted ISAba1 upstream of the corresponding genes was associated with increased carbapenem MICs (≥128μg/mL).

Conclusion: The emergence of high-level CRAB with bla_OXA and ISAba1-bla_OXA family in burn patients is a matter of increasing clinical concern, emphasising the need for infection control efforts to limit such problematic bacteria.

Keywords: Acinetobacter baumannii; Antimicrobial resistance; Insertion sequence; OXA carbapenemase; Oxacillinase.