Insertion element mediated mgrB disruption and presence of ISKpn28 in colistin-resistant Klebsiella pneumoniae isolates from Saudi Arabia Taher Uz Zaman,1 Maha Albladi,1 Mohammed Ismail Siddique,2 Sameera M Aljohani,3,4 Hanan H Balkhy1,5 1Infectious Diseases Research Department, King Abdullah International Medical Research Center, King Saud bin Abdulaziz University for Health Sciences, Riyadh, Saudi Arabia; 2Deccan College of Medical Sciences, Hyderabad, India; 3Microbiology Section, King Abdulaziz Medical City, Riyadh, Saudi Arabia; 4Department of Microbiology, King Saud bin Abdulaziz University for Health Sciences, Riyadh, Saudi Arabia; 5Department of Infection Prevention and Control, King Abdulaziz Medical City, National Guards Health Affairs, Riyadh, Saudi Arabia Background: In Klebsiella pneumoniae, mgrB and components of pmrHFIJKLM operon play a major role in colistin resistance. Methods: We analyzed 23 nonduplicating colistin-resistant K. pneumoniae isolates, collected during the years 2011–2015, for the possible mechanism underlying their nonsusceptibility to colistin. Isolates were tested for their minimum inhibitory concentrations and antibiotic resistance determinants and genotyped by multilocus sequence typing (MLST). The MLST genes, antibiotic-resistant genes, and the genes of two component system (TCS), including mgrB, PhoQ/PhoP, pmrA/B, and CrrAB, were investigated by PCR amplification and Sanger sequencing. Results: All isolates were distributed in eight sequence types (STs) and showed mutations either in mgrB or PhoP genes. ISKpn14 was found in 10, ISKpn28 in four, and IS903 in three isolates. One isolate showed deletion of a single nucleotide in mgrB open reading frame causing premature stop codon. L26Q substitution in PhoP was found in five isolates. Conclusion: The mutations in mgrB were mostly mediated by insertion elements (IS). ISKpn14 is the major IS while ISKpn28 is reported for the first time in mediating mgrB disruption. IS903, an IS5 family member, involved in mgrB disruption in three ST-152 NDM-1-positive isolates, was previously responsible for omp-36 disruption in our carbapenem-resistant K. pneumoniae and appears to contribute to transform the isolates into a pan-drug ones. Also, the abundance of insertion sites in mgrB indicates the plasticity of this gene. In our isolates, IS-mediated colistin resistance appears to be a later phenomenon than mutation in PhoP gene. Keywords: colistin resistance, mgrB gene mutations, insertion elements, Klebsiella pneumoniae