Mutation in mgrB is the major colistin resistance mechanism in Klebsiella pneumoniae clinical isolates in Tehran, Iran

BACKGROUND AND OBJECTIVESColistin is considered as one of a last resort antimicrobial agent against multidrug-resistant Gram-negative bacteria including Escherichia coli and Klebsiella pneumoniae. However, the recent emergence of colistin resistance (ColR) worldwide that severely restricts therapeutic options is a serious threat to global public health. In this study we have investigated the molecular determinants in ColR K. pneumoniae isolates collected from clinical specimens.MATERIALS AND METHODSA total of ۹۸ E. coli and ۱۹۵ K. pneumoniae clinical isolates were collected from two hospitals from August ۲۰۱۸ to December ۲۰۱۹ in Tehran, Iran. Colistin susceptibility and minimum inhibitory concentrations (MIC) were determined according to the Clinical and Laboratory Standards Institute by disk diffusion method, and microdilution method, respectively. For isolates with colistin MIC ≥۴ mg mL-۱ , PCR was performed for the detection of mcr-۱ to mcr-۴ genes. Moreover, nucleotide sequences of mgrB, phoP, phoQ, pmrA, and pmrB genes were determined by sequencing. Finally, the transcriptional level of pmrK and pmrC genes was evaluated by quantitative reverse transcription PCR (RT-qPCR).RESULTS AND DISCUSSIONNone of the E. coli isolates were resistant to colistin while ۲۱ out ۱۹۵ K. pneumoniae isolates were identified as resistant, ۱۹ of which carried mutation in the mgrB gene. Three different mutations were observed in the pmrB gene in ۳ K. pneumoniae isolates. None of the ColR isolates showed alternations in pmrA, phoP, and phoQ genes. Furthermore, none of the plasmid-encoding genes were detected. Transcriptional level of the pmrK gene increased in all ColR isolates meanwhile, pmrC overexpression was detected in ۱۶ out ۲۱ (۷۶.۱۹%) isolates. Eventually, all ColR isolates were susceptible to tigecycline.CONCLUSIONOur results demonstrated that the alternation of mgrB gene is the main mechanism related to colistin resistance among ColR K. pneumoniae isolates in this study.