Genes involved in colistin resistance of gram-negative isolates in the northwest of Iran

Background: Colistin is used as a last line antibiotic for treating infections caused by multidrug-resistant gramnegative bacteria. The emergence of colistin-resistant isolates is the main concern of public health in recent years. The aim of this study was to determine the responsible mechanisms for colistin resistance in gram-negative isolates in Tabriz in the northwest of Iran. Methods: Nine hundred clinical Enterobacteriaceae isolates were collected from hospitals in Tabriz in the northwest of Iran. Antibiotic susceptibility of the isolates was determined by disk diffusion and agar dilution methods. The colistin-resistant isolates were investigated for the presence of mcr-1, mcr-2 and crrB genes. Results: Quantitative real-time PCR was performed for the determining expression level of phoP and phoQ genes. Of the 900 clinical isolates, 3.33% were identified as a colistin resistance and 26.6% of them were highly resistant. All of the isolates were negative for mcr-1 and mcr-2 genes. None of the K. pneumoniae had crrB gene. The results of qRT-PCR showed up-regulated of phoP had no significant difference with colistin susceptible isolate. Conclusion: Up-regulation of phoQ gene was the unique responsible mechanism for resistance to colistin in gramnegative isolates in the northwest of Iran. These results indicate despite the distribution of mcr genes worldwide, they are not present in clinical isolates in this region.