Ann Lab Med.  2018 Nov;38(6):545-554. 10.3343/alm.2018.38.6.545.

Differences in Colistin-resistant Acinetobacter baumannii Clinical Isolates Between Patients With and Without Prior Colistin Treatment

• Park YJ
• Hong DJ
• Yoon EJ
• Kim D
• Choi MH
• Hong JS
• Lee H
• Yong D
• Jeong SH

• KMID: 2429120
• DOI: http://doi.org/10.3343/alm.2018.38.6.545

Abstract

BACKGROUND
The increasing morbidity and mortality rates associated with Acinetobacter baumannii are due to the emergence of drug resistance and the limited treatment options. We compared characteristics of colistin-resistant Acinetobacter baumannii (CR-AB) clinical isolates recovered from patients with and without prior colistin treatment. We assessed whether prior colistin treatment affects the resistance mechanism of CR-AB isolates, mortality rates, and clinical characteristics. Additionally, a proper method for identifying CR-AB was determined.
METHODS
We collected 36 non-duplicate CR-AB clinical isolates resistant to colistin. Antimicrobial susceptibility testing, Sanger sequencing analysis, molecular typing, lipid A structure analysis, and in vitro synergy testing were performed. Eleven colistin-susceptible AB isolates were used as controls.
RESULTS
Despite no differences in clinical characteristics between patients with and without prior colistin treatment, resistance-causing genetic mutations were more frequent in isolates from colistin-treated patients. Distinct mutations were overlooked via the Sanger sequencing method, perhaps because of a masking effect by the colistin-susceptible AB subpopulation of CR-AB isolates lacking genetic mutations. However, modified lipid A analysis revealed colistin resistance peaks, despite the population heterogeneity, and peak levels were significantly different between the groups.
CONCLUSIONS
Although prior colistin use did not induce clinical or susceptibility differences, we demonstrated that identification of CR-AB by sequencing is insufficient. We propose that population heterogeneity has a masking effect, especially in colistin non-treated patients; therefore, accurate testing methods reflecting physiological alterations of the bacteria, such as phosphoethanolamine-modified lipid A identification by matrix-assisted laser desorption ionization-time of flight, should be employed.

Keyword

Colistin; Population heterogeneity; Acinetobacter baumannii; Resistance; Lipid A analysis; Pathogenesis

MeSH Terms

Acinetobacter baumannii*
Acinetobacter*
Bacteria
Colistin*
Drug Resistance
Humans
In Vitro Techniques
Lipid A
Masks
Methods
Molecular Typing
Mortality
Population Characteristics
Colistin
Lipid A

Figure

• 

  Fig. 1 Dendrogram showing cluster analysis of SmaI-digested pulsed-field gel electrophoresis patterns from colistin-resistant Acinetobacter baumannii isolates. Mutations from genomic analysis of genes associated with colistin resistance are listed on the right in bold font. Note that CR-AB32, 34, 35, and 36 had the same mutation in the pmrB gene (insertion TTT at g.938_939) and were isolated from a single location (ICU-H) within 47 days (highlighted by a black rectangle).ABA, Acinetobacter baumannii; CC, clonal complex; CCU, coronary care unit; GW, general ward; ICU, intensive care unit; PFGE, pulsed-field gel electrophoresis; ST, sequence type; CS-AB, colistin-susceptible Acinetobacter baumannii; CR-AB, colistin-resistant Acinetobacter baumannii.

• 

  Fig. 2 Mass spectrometry of lipid A extracted from colistin-susceptible isolates and CR-AB. (A) ATCC 17978, wild type CS-AB. (B) CR-AB18, Group non-CT. (C) CR-AB14, Group CT. The mass (m/z) of peaks only detected in CR-AB strains is indicated in bold.Abbreviations: Hexa, hexa-acylated lipid A; Hepta, hepta-acylated lipid A; Octa, octa-acylated lipid A; PE, phosphoethanolamine; C, carbon; CS-AB, colistin-susceptible Acinetobacter baumannii; CR-AB, colistin-resistant Acinetobacter baumannii.

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