The Effect of Colistin Resistance-Associated Mutations on the Fitness of Acinetobacter baumannii

Abstract

Acinetobacter baumannii had emerged as an important nosocomial and opportunistic pathogen worldwide. To assess the evolution of colistin resistance in A. baumannii and its effect on bacterial fitness, we exposed five independent colonies of A. baumannii ATCC 17978 to increasing concentrations of colistin in agar (4/5) and liquid media (1/5). Stable resistant isolates were analyzed using whole genome sequencing. All strains were colistin resistant after exposure to colistin. In addition to the previously reported lpxCAD and pmrAB mutations, we identified four novel putative colistin resistance genes: A1S_1983. hepA. A1S_3026, and rsfS. Lipopolysaccharide (LPS) loss mutants exhibited higher fitness costs than those of the pmrB mutant in nutrient-rich medium. The colistin-resistant mutants had a higher inhibition ratio in the serum growth experiment than that of the wild type strain in 100% serum. Minimum inhibitory concentration (MIC) results showed that the LPS-deficient but not the pmrB mutant had an altered antibiotic resistance profile. The compensatory mutations partially or completely rescued the LPS-deficient’s fitness, suggesting that compensatory mutations play an important role in the emergence and spread of colistin resistance in A. baumannii.

Acquisition of Colistin Resistance Links Cell Membrane Thickness Alteration with a Point Mutation in the lpxD Gene in Acinetobacter baumannii

“…Colistin resistance acquisition resulted in the alteration of the antibiotic resistance profiles. This may be attributed to gene mutations responsible for the displayed resistance, leading to variations in the permeability of the outer membrane [39]. This was suggested by the changes observed in the cell morphology due to alteration of LPS, as confirmed by our investigation and other studies that linked the inactivation of LpxD to colistin resistance [40]., Knowledge concerning colistin resistance mechanisms in A. baumannii isolates of clinical origin remains limited.…”

Genomic epidemiology of multidrug‐resistant Gram‐negative organisms

“…102 Interestingly, in both K. pneumoniae and A. baumannii, genomic sequencing studies have found mutations in a common regulatory pathway controlling LPS modification systems, indicating that LPS modification is key to resistance in both species. [103][104][105] This genomederived hypothesis that altered LPS modification underlies resistance was ultimately confirmed for both organisms by comparing LPS modifications in susceptible and resistant isolates. 103,106 An important caveat in studying resistance evolution in individual patients is that the larger epidemiological significance of observed resistance mutations or mechanisms cannot necessarily be inferred.…”

“…The prospect of widespread colistin resistance is of great concern, as there are limited treatment options beyond colistin for the treatment of infections caused by carbapenemase‐producing Gram‐negatives, such as K. pneumoniae and A. baumannii . Interestingly, in both K. pneumoniae and A. baumannii , genomic sequencing studies have found mutations in a common regulatory pathway controlling LPS modification systems, indicating that LPS modification is key to resistance in both species . This genome‐derived hypothesis that altered LPS modification underlies resistance was ultimately confirmed for both organisms by comparing LPS modifications in susceptible and resistant isolates …”

Clonal expansion of colistin-resistant Acinetobacter baumannii isolates in Cape Town, South Africa

“…To date only chromosomally encoded colistin resistance mechanisms have been reported in A. baumannii. Mutations in the pmrB/pmrA/pmrC operon and genes encoding lipid A biosynthesis (lpxA, lpxC, lpxD) confer colistin resistance through modification of the lipid A component of the lipopolysaccharide (LPS) and through complete loss of LPS, respectively (Moffatt et al, 2010;Adams et al, 2009;Mu et al, 2016). The presence of the insertion sequence ISAba11 in lpxC or lpxA also causes inactivation of LPS production (Moffatt et al, 2011).…”