Insertion element mediated disruption and presence of ISKpn28 in colistin-resistant isolates from Saudi Arabia.

Background: In and components of pmrHFIJKLM operon play a major role in colistin resistance.

Methods: We analyzed 23 nonduplicating colistin-resistant 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 , and , were investigated by PCR amplification and Sanger sequencing.

Results: All isolates were distributed in eight sequence types (STs) and showed mutations either in B 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 B open reading frame causing premature stop codon. L26Q substitution in PhoP was found in five isolates.

Conclusion: The mutations in B were mostly mediated by insertion elements (IS). ISKpn14 is the major IS while ISKpn28 is reported for the first time in mediating B disruption. IS903, an IS5 family member, involved in B disruption in three ST-152 NDM-1-positive isolates, was previously responsible for -36 disruption in our carbapenem-resistant and appears to contribute to transform the isolates into a pan-drug ones. Also, the abundance of insertion sites in B indicates the plasticity of this gene. In our isolates, IS-mediated colistin resistance appears to be a later phenomenon than mutation in gene.