Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 18 2 2026 04 26 244 250 Murtaza Sadaqat Department of Cell and Molecular Biology, Faculty of Chemistry, University of Kashan, Kashan, Iran Elahe Mahmoodi Khaledi Department of Cell and Molecular Biology, Faculty of Chemistry, University of Kashan, Kashan, Iran Mehri Haeili Department of Animal Biology, Faculty of Natural Sciences, University of Tabriz, Tabriz, Iran 2025 09 29 2026 04 10 Background and Objectives: Colistin is considered as one of the last antibiotic choices for addressing infections resulting from multidrug-resistant Klebsiella pneumoniae. Nevertheless, the rising resistance to colistin is emerging as a growing threat to public health. The aim of the present study was to explore the molecular mechanisms underlying resistance to colistin in a clinical isolate of K. pneumoniae. Materials and Methods: Colistin resistance was confirmed through antimicrobial susceptibility testing, and the sequence type was identified using Multilocus sequence typing (MLST). The molecular mechanism of colistin resistance was investigated by sequence analysis of resistance-associated loci, including mgrB, pmrAB, phoPQ, crrAB, and PCR detection of plasmid-mediated mcr genes. Results: Among the studied 38 clinical K. pneumoniae isolates, one strain was colistin-resistant, which belonged to sequence type ST377. PCR results showed that the colistin resistance genes carried by plasmid (mcr-1 to mcr-4) were not present. While gene sequencing revealed wild-type pmrAB and phoPQ, the mgrB and crrA genes were found to be disrupted by insertion of IS elements in the promoter (position-45) and coding regions (position +365/+366), respectively. Moreover, a Q296L amino acid substitution was detected in CrrB. Conclusion: This study demonstrates that resistance to colistin in the K. pneumoniae ST377 isolate was mainly mediated by inactivation of MgrB and CrrA without the involvement of mcr plasmid genes or pmrAB or phoPQ genetic alterations. To our knowledge, no previous study has reported insertion sequence-mediated disruption of the crrA coding region in K. pneumoniae. The results highlighted the complexity of chromosomal resistance mechanisms and the importance of molecular surveillance in managing colistin-resistant infections. https://ijm.tums.ac.ir/index.php/ijm/article/view/5817 https://ijm.tums.ac.ir/index.php/ijm/article/download/5817/1882