Investigation of Colistin Resistance and Heteroresistance in Acinetobacter spp. Isolates from Various Clinical Specimens

Introduction: Current studies draw attention to increased rates of resistance and heteroresistance of Acinetobacter spp. to colistin. This study aimed to investigate the colistin susceptibility of Acinetobacter spp. and detect heteroresistance in colistin-susceptible isolates. Materials and Methods: Acinetobacter spp. isolated from clinical samples submitted to the medical microbiology laboratory were analyzed. Colistin susceptibility of the isolates was investigated by the broth microdilution technique according to European Committee on Antimicrobial Susceptibility Testing standards. In isolates within the susceptibility limits, heteroresistance was screened using Mueller-Hinton agar (MHA) plates containing 4 mg/L colistin. The mcr-1 gene was amplified using polymerase chain reaction. Results: Of the 110 isolates of Acinetobacter spp., 17 (15.5%) were resistant to colistin. After 24 h of incubation in the MHA medium containing 4 mg/L of colistin, growth was observed in 18 isolates, whereas growth was observed in eight isolates after 48 h. The minimum inhibitory concentration values of those isolates was ≥4 mg/L. Thus, of the 93 colistin-susceptible Acinetobacter spp. isolates, 26 (27.9%) were detected as heteroresistant. We did not observe mcr-1 carriage in Acinetobacter baumannii (A. baumannii) isolates. Conclusion: The high rates of colistin heteroresistance in A. baumannii isolates may result in the failure of treatment with colistin because of the selection of colistin-resistant subpopulations. The prediction of potential unresponsiveness before starting colistin therapy appears necessary, and the use of this method, which is more convenient in practice and can test several isolates, may contribute to clinical practice by allowing the clinician to consider various drug options and combinations for treatment.