Prevalence of Mobile Colistin Resistance (<i>mcr</i>) Genes in <i>Acinetobacter baumannii</i>: Isolates of Clinical Infection from an Iranian University Hospital

Abstract

Background: The emergence of colistin resistance is a global issue that threatens the effectiveness of colistin treatment against multidrug-resistant Gram-negative bacteria. Objectives: The present study reports on the prevalence of colistin resistance and mcr-encoding genes in Acinetobacter baumannii strains isolated from patients referred to Shahid Rahimi Hospital, Khorramabad, Iran, from January to August 2019. Methods: A total of 47 non-duplicate A. baumannii isolates were obtained from clinical specimens of patients. Colistin susceptibility of the isolates was assessed using the broth microdilution method, and the presence of mcr-encoding genes was determined through the use of PCR. Results: Out of the total 47 isolates belonging to A. baumannii, 14 isolates (29.8%) were resistant to colistin. The minimum inhibitory concentration (MIC) of colistin ranged from 2 to 256 μg/mL, and the MIC50 and MIC90 values for colistin were 0.25 μg/mL and 16 μg/mL, respectively. The presence of mcr-1 and mcr-2 was detected in 6 (12.7%) and 2 (4%) isolates, respectively. However, no isolates harbored the mcr-3 gene. Except for one isolate, none of the mcr-positive isolates exhibited phenotypic resistance to colistin. Conclusions: The higher phenotypic prevalence of colistin resistance compared to genotypic characterization, along with the discrepancy with the presence of mcr genes, suggests the need to monitor and consider all molecular mechanisms of colistin resistance as a last-resort treatment option.