Differences in Colistin-resistantAcinetobacter baumanniiClinical Isolates Between Patients With and Without Prior Colistin Treatment

Abstract

BackgroundThe 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.MethodsWe 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.ResultsDespite 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.ConclusionsAlthough 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.

Emergent Polymyxin Resistance: End of an Era?

“…Combination therapy utilizing synergetic effects of multi-antibiotics also may be a practical treatment option for polymyxin-resistant bacteria. Colistin in combination with the following antibiotics have been reported to be synergetic with in vivo or in vitro trials against CoRB infection: chloramphenicol, netropsin, meropenem, rifampicin, gentamicin, and resveratrol [32, 71, 84, 96, 97]. In vitro synergy of triple-drug combinations of polymyxin B, aztreonam, and amikacin also was confirmed by Bulman et al [98].…”

“…Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) can be employed for detection of lipid A modifications that confer colistin resistance. Park et al recommended to use methods that can decipher phenotypical changes, such as MALDI-TOF MS or lipid A construction, because of the mask effect of population heterogeneity [84]. Lipid A structure analysis is quick and accurate in predicting resistance phenotypes caused by lipid A modifications, regardless of strains or mechanisms.…”

Colistin as A Good Monotherapy to Restrain the Pathogenicity of Acinetobacter baumannii In vivo and In vitro

“…Heterogeneous and complete resistance to colistin has been clinically reported 39,40 . The increasing use of colistin is believed to possess a risk of the emergence of new isolates that are resistant to this compound.…”

One System for All: Is Mass Spectrometry a Future Alternative for Conventional Antibiotic Susceptibility Testing?

“…The mass spectral analysis requires an adapted extraction and hydrolyzation procedure (El Hamidi et al, 2005; Larrouy-Maumus et al, 2016; Liang et al, 2019) and the MALDI-TOF MS analysis has to be performed preferably in negative ion extraction mode, generally in the range of m/z 1000–3000 (Dortet et al, 2018a). First, clinical studies with larger sets of clinical isolates of E. coli (Dortet et al, 2018a), A. baumannii (Park et al, 2018), and Acinetobacter spp. (Leung et al, 2018) showed promising results in terms of sensitivity and specificity but with some inconsistencies that raised questions on possible hetero-resistance (Park et al, 2018).…”

“…First, clinical studies with larger sets of clinical isolates of E. coli (Dortet et al, 2018a), A. baumannii (Park et al, 2018), and Acinetobacter spp. (Leung et al, 2018) showed promising results in terms of sensitivity and specificity but with some inconsistencies that raised questions on possible hetero-resistance (Park et al, 2018). Due to the high variability of lipid A structures among bacterial species of clinical relevance (Trent et al, 2006), for a mass spectral assay target molecules need to be identified for individual species.…”