Diversity of amino acid substitutions in PmrCAB associated with colistin resistance in clinical Acinetobacter baumannii isolates

Abstract This study aimed to investigate the mechanisms of colistin resistance in 64 Acinetobacter baumannii isolates obtained from patients with ventilator-associated pneumonia hospitalized in Greece, Italy and Spain. In total, 31 A. baumannii isolates were colistin-resistant. Several novel amino acid substitutions in PmrCAB were found in 27 colistin-resistant A. baumannii isolates. Most substitutions were detected in PmrB, indicating the importance of the histidine kinase for colistin resistance. In two colistin-resistant isolates, 93 amino acid changes were observed in PmrCAB compared to A. baumannii ACICU and homologous recombination across different clonal lineages was suggested. Analysis of gene expression revealed increased pmrC expression in isolates harbouring pmrCAB mutations. The complementation of ATCC 19606 and ATCC 17978 with a pmrAB variant revealed increased pmrC expression but unchanged colistin MICs, indicating the need for additional unknown factors. Moreover, a combination of PmrB and PmrC changes was associated with very high colistin MICs, suggesting the accumulation of mutations as the mechanism for high-level colistin resistance. The pmrC-homologue eptA was detected in 29 colistin-susceptible and 26 colistin–resistant isolates. In 8 colistin-susceptible and one colistin-resistant isolate, ISAba1 was found upstream of eptA, and in two colistin-resistant isolates eptA was disrupted by ISAba125. While in most isolates an associated of eptA with colistin resistance was excluded, in one isolate an amino acid substitution R127L in EptA combined with a point mutation in ISAba1 upstream of eptA contributed to elevated colistin MICs. Overall, this study helps to gain insight into the diversity and complexity of colistin resistance in A. baumannii.