Abstract
BackgroundThe recent description of the first plasmid-mediated colistin-resistant gene mcr-1, conferring transferable and low-level resistance to colistin, raised concern about the need to implement a rapid and reliable screening method to detect colistin-resistant clinical isolates. The only valid method to assess the MIC of colistin is the broth microdilution according to the joint CLSI-EUCAST Polymyxin Breakpoints Working Group. UMIC Colistine is a ready-to-use broth microdilution kit developed to easily assess colistin MIC by proposing unitary polystyrene strips containing 11 concentrations of dehydrated colistin. Here, we evaluated the UMIC Colistine kit on 235 Gram-negative rods (176 Enterobacterales, including 70 harboring a mcr gene, and 59 non-fermentative), through comparison to the reference broth microdilution method prepared in accordance with EN ISO 20776-1:2006 standard. Reproducibility of the UMIC Colistine was assayed with the three recommended quality control strains E. coli ATCC 25922, E. coli NCTC 13846 (mcr-1 positive), and P. aeruginosa ATCC 27853, as for stability testing.ResultsCategorical agreement was 100% with 63.4% (n = 149) of colistin-resistant strains, and 36.6% (n = 86) of colistin-susceptible strains with both methods (S ≤ 2 μg/mL and R > 2 μg/mL). No major error or very major error was reported. Essential agreement was 94.0% (n = 221), and 100% for detection of colistin-resistant strains as compared to the reference method. Pearson’s correlation between UMIC Colistine and the reference method was 0.98. Reproducibility of the UMIC Colistine system was 97.8% with MICs of the quality control strains within the target ranges. However, some isolates had lower MIC with UMIC Colistine, but that did not change their categorization as colistin-susceptible, and this phenomenon should be further explored.ConclusionsThe UMIC Colistine kit is an easy to perform unitary device that showed excellent results when compared to the reference method. The UMIC Colistine system is a rapid and reliable broth microdilution method that is suitable to assess the colistin MIC of clinical isolates in clinical microbiology laboratories.
Highlights
The recent description of the first plasmid-mediated colistin-resistant gene mcr-1, conferring transferable and low-level resistance to colistin, raised concern about the need to implement a rapid and reliable screening method to detect colistin-resistant clinical isolates
Bacterial strains A total of 235 bacterial strains were used in this study, including 162 Enterobacteriaceae (77 Escherichia coli, 50 Klebsiella pneumoniae, 4 Klebsiella oxytoca, 18 Enterobacter cloacae, 4 Enterobacter aerogenes, 4 Enterobacter asburiae and 5 Salmonella enterica), 14 intrinsic colistin-resistant genera of Enterobacterales (9 Hafnia alvei, 1 Proteus mirabilis, 1 Morganella morganii, 1 Providencia alcalifaciens, 1 Providencia rettgeri and 1 Serratia marcescens), and 59 non-fermentative isolates (31 Pseudomonas sp., 18 Acinetobacter sp. and 10 Stenotrophomonas maltophilia) (Table 1) [18,19,20,21,22,23]
Fourteen strains classified as susceptible presented a lower Minimal Inhibitory Concentrations (MIC) with UMIC Colistine (Fig. 1), and were distributed into different species: 1 E. coli, 1 K. pneumoniae, 1 K. oxytoca, 5 E. cloacae, 5 A. baumannii and 1 A. pitti (Table 1). Those strains were tested in triplicate, resulting in the reproducibility of the discrepancies, giving MICs of 1 or 2 μg/mL with reference method and 0.25 μg/mL with UMIC Colistine, except for K. oxytoca TH44 which gave a MIC of 0.5 or 1 μg/mL with reference method and 0.125 μg/mL with UMIC Colistine
Summary
The recent description of the first plasmid-mediated colistin-resistant gene mcr-1, conferring transferable and low-level resistance to colistin, raised concern about the need to implement a rapid and reliable screening method to detect colistin-resistant clinical isolates. We evaluated the UMIC Colistine kit on 235 Gramnegative rods (176 Enterobacterales, including 70 harboring a mcr gene, and 59 non-fermentative), through comparison to the reference broth microdilution method prepared in accordance with EN ISO 20776-1:2006 standard. The transferable mcr-1 gene has been detected in samples from all over the world and from various human and animal origins [4] This discovery was followed by the description of other mcr genes: mcr-2 to mcr-8 [5,6,7,8,9,10,11,12]. This mobile colistin resistance that confers low levels of resistance with Minimal Inhibitory Concentrations (MIC) of colistin around 4 μg/mL [3] raised concerned
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