Heteroresistance to colistin in oxacillinase-producing carbapenem-resistant Acinetobacter baumannii clinical isolates from Gorgan, Northern Iran.

Colistin resistance rates are rising globally among multidrug-resistant Gram-negative bacilli, including Acinetobacter baumannii (A. baumannii). A new type of resistance - heteroresistance - has also been reported to colistin in clinical A. baumannii isolates. This study investigated the presence of colistin heteroresistance in carbapenem-resistant A. baumannii clinical isolates. Different clinical specimens from hospitalised patients were investigated for A. baumannii. The MICs to imipenem, meropenem and colistin were determined by broth microdilution. PCR was performed to detect OXA-type carbapenemase genes (bla Overall, 71 A. baumannii isolates were recovered from clinical specimens. Of these, 27 (38.03%) and 44 (61.97%) isolates were carbapenem-susceptible and carbapenem-resistant, respectively. In addition, 67 (94.36%) isolates were susceptible to colistin, with MICs between 0.25-2 μg/mL. Among the 44 selected carbapenem-resistant colistin-susceptible isolates, the frequency of bla A. baumannii with a colistin heteroresistance phenotype was common. This could be of great concern since colistin is often used as a last-resort drug for treating A. baumannii infections, highlighting that care is necessary with colistin monotherapy. In addition, more effective strategies and surveillance are required to confine and prevent the inter-hospital and/or intra-hospital dissemination of A. baumannii between therapeutic centres.

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