Colistin Heteroresistance and Involvement of the PmrAB Regulatory System in Acinetobacter baumannii

Abstract

Multidrug-resistant infection has recently emerged as a worldwide clinical problem, and colistin is increasingly being used as a last-resort therapy. Despite its favorable bacterial killing, resistance and heteroresistance (HR) to colistin have been described. The purpose of the present study was to investigate the role of the PmrAB regulatory pathway in laboratory-selected mutants representative of global epidemic strains. From three unrelated clinical strains (sequence types 2, 3, and 20), eight colistin-resistant mutants were selected. Half of the mutants showed HR to colistin according to the reference method (population analysis profiling), whereas the other half exhibited stable resistance. M12I mutation within and M308R, S144KLAGS, and P170L mutations for were associated with HR to colistin, while T235I, A226T, and P233S mutations within were associated with stable resistance. The transcript levels of the operon were upregulated in all the mutants. Compensatory mutations were explored for some mutants. A single mutant (T235I mutant) displayed a compensatory mutation through IS mobilization within the gene that was associated with the loss of colistin resistance. The mutant resistance phenotype associated with T235I was partially restored in a-complementation assay turning to HR. The level of colistin resistance was correlated with the level of expression of in the-complemented strains. This report shows the role of different mutations in the PmrAB regulatory pathway and warns of the development of colistin HR that could be present but not easily detected through routine testing.

Resistance and Heteroresistance to Colistin in Pseudomonas aeruginosa Isolates from Wenzhou, China

“…its favorable bacterial killing, resistance and heteroresistance to colistin have been described (3,5). Heteroresistance is a phenomenon in which subpopulations of seemingly isogenic bacteria exhibit variable susceptibilities to a particular antibiotic (9). It has drawn extensive attention clinically, because the resistant proportion of bacterial isolates may survive and become predominant during therapy, leading to treatment failure and even lethal infections (25).…”

“…Comparative results did not show discrepancies for both colistin resistance and heteroresistance in P. aeruginosa being mainly caused by alterations in the PmrAB regulatory system, resulting in upregulation of the LPS modification system. The mechanisms involved in colistin heteroresistance are diverse and complicated, as has been described for several bacteria (9,(18)(19)(20)(21)(22) and shown for P. aeruginosa. The heteroresistance of bacteria is considered an indication of the mutator phenotype described in the literature (34,35).…”

“…Population analysis profiles. PAPs are used as the reference method to define antibiotic heteroresistance (9). The analysis was performed among 231 carbapenem-nonsusceptible isolates, based on previous analyses (23).…”

“…Colistin heteroresistance has been described (6). Multiple studies have indicated that the presence of this phenotype may account for unexplained treatment failures (7)(8)(9).…”

“…Recently, ParRS, CprRS, and ColRS TCSs have also been found to play a role in colistin homogeneous resistance in P. aeruginosa (15)(16)(17). Moreover, colistin heteroresistance mechanisms include activation of PmrAB and PhoPQ TCSs (Acinetobacter baumannii, Klebsiella pneumoniae, and Enterobacter cloacae) (9,18,19), soxRS-regulated overexpression of the acrAB-tolC efflux pump (Enterobacter asburiae and E. cloacae) (20), biofilm formation (Klebsiella pneumoniae) (21), and putrescine/YceI communication (Burkholderia cenocepacia) (22). However, only sporadic cases of colistin heteroresistance in P. aeruginosa have been reported (23), and their mechanisms of heteroresistance to colistin have not been investigated.…”