Objectives: To evaluate a novel method, the colistin-MAC test, for phenotypic screening of acquired colistin resistance mediated by transferable mcr-1 resistance determinants, based on colistin MIC reduction in the presence of dipicolinic acid (DPA). Methods: The colistin-MAC test consists in a broth microdilution method, in which colistin MIC is tested in the absence or presence of DPA (900 \u3bcg/mL). Overall, 74 colistin-resistant strains of Enterobacteriaceae (65 Escherichia coli and nine other species), including 61 strains carrying mcr-1-like genes and 13 strains negative for mcr genes, were evaluated with the colistin-MAC test. The presence of mcr-1-like and mcr-2-like genes was assessed by real-time PCR and end-point PCR. For 20 strains, whole-genome sequencing data were also available. Results: A  658-fold reduction of colistin MIC in the presence of DPA was observed with 59 mcr-1-positive strains, including 53 E. coli of clinical origin, three E. coli transconjugants carrying MCR-1-encoding plasmids, one Enterobacter cloacae complex and two Citrobacter spp. Colistin MICs were unchanged, increased or at most reduced by twofold with the 13 mcr-negative colistin-resistant strains (nine E. coli and four Klebsiella pneumoniae), but also with two mcr-1-like-positive K. pneumoniae strains. Conclusions: The colistin-MAC test could be a simple phenotypic test for presumptive identification of mcr-1-positive strains among isolates of colistin-resistant E. coli, based on a  658-fold reduction of colistin MIC in the presence of DPA. Evaluation of the test with a larger number of strains, species and mcr-type resistance determinants would be of interest

Antonelli, A.

Baccani, I.

Cannatelli, A.

Coppi, Marco

Di Pilato, V.

Giani, T.

Rossolini, G. M.

Sennati, S.

Clinical Microbiology and Infection

English

M. Coppi

A. Cannatelli

A. Antonelli

I. Baccani

V. Di Pilato

S. Sennati

T. Giani

G.M. Rossolini

Crossref

A simple phenotypic method for screening of MCR-1-mediated colistin resistance

Objectives: To evaluate a novel method, the colistin-MAC test, for phenotypic screening of acquired colistin resistance mediated by transferable mcr-1 resistance determinants, based on colistin MIC reduction in the presence of dipicolinic acid (DPA). Methods: The colistin-MAC test consists in a broth microdilution method, in which colistin MIC is tested in the absence or presence of DPA (900 μg/mL). Overall, 74 colistin-resistant strains of Enterobacteriaceae (65 Escherichia coli and nine other species), including 61 strains carrying mcr-1-like genes and 13 strains negative for mcr genes, were evaluated with the colistin-MAC test. The presence of mcr-1-like and mcr-2-like genes was assessed by real-time PCR and end-point PCR. For 20 strains, whole-genome sequencing data were also available. Results: A ≥8-fold reduction of colistin MIC in the presence of DPA was observed with 59 mcr-1-positive strains, including 53 E. coli of clinical origin, three E. coli transconjugants carrying MCR-1-encoding plasmids, one Enterobacter cloacae complex and two Citrobacter spp. Colistin MICs were unchanged, increased or at most reduced by twofold with the 13 mcr-negative colistin-resistant strains (nine E. coli and four Klebsiella pneumoniae), but also with two mcr-1-like-positive K. pneumoniae strains. Conclusions: The colistin-MAC test could be a simple phenotypic test for presumptive identification of mcr-1-positive strains among isolates of colistin-resistant E. coli, based on a ≥8-fold reduction of colistin MIC in the presence of DPA. Evaluation of the test with a larger number of strains, species and mcr-type resistance determinants would be of interest

COPPI, MARCO

Archivio istituzionale della ricerca - Università di Genova

Accepted ManuscriptA simple phenotypic method for screening of MCR-1-mediated colistin resistanceMarco Coppi, Antonio Cannatelli, Alberto Antonelli, Ilaria Baccani, Vincenzo Di Pilato,Samanta Sennati, Tommaso Giani, Gian Maria RossoliniPII: S1198-743X(17)30457-3DOI: 10.1016/j.cmi.2017.08.011Reference: CMI 1043To appear in: Clinical Microbiology and InfectionReceived Date: 13 May 2017Revised Date: 11 August 2017Accepted Date: 12 August 2017Please cite this article as: Coppi M, Cannatelli A, Antonelli A, Baccani I, Di Pilato V, Sennati S, Giani T,Rossolini GM, A simple phenotypic method for screening of MCR-1-mediated colistin resistance, ClinicalMicrobiology and Infection (2017), doi: 10.1016/j.cmi.2017.08.011.This is a PDF file of an unedited manuscript that has been accepted for publication. As a service toour customers we are providing this early version of the manuscript. The manuscript will undergocopyediting, typesetting, and review of the resulting proof before it is published in its final form. Pleasenote that during the production process errors may be discovered which could affect the content, and alllegal disclaimers that apply to the journal pertain.MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT1  A simple phenotypic method for screening of MCR-1-mediated colistin resistance   Marco Coppi1, Antonio Cannatelli2, Alberto Antonelli1, Ilaria Baccani1, Vincenzo Di Pilato3, Samanta Sennati2, Tommaso Giani2, and Gian Maria Rossolini1,4* 1Department of Experimental and Clinical Medicine, University of Florence, Italy 2Department of Medical Biotechnologies, University of Siena, Italy 3Department of Surgery and Translational Medicine, University of Florence  4Clinical Microbiology and Virology Unit, Florence Careggi University Hospital, Florence, Italy  *Corresponding author. Mailing address:  Department of Experimental and Clinical Medicine, University of Florence, SOD Microbiologia e Virologia Azienda Ospedaliera Universitaria Careggi, I-50134 – Florence, Italy. Phone: +39-055-7949239; fax: +39-055-7949289; e-mail: gianmaria.rossolini@unifi.it  Running title: Colistin-MAC test for phenotypic screening of MCR-1  MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT2  Abstract Objectives: To evaluate a novel method, named Colistin-MAC test, for phenotypic screening of acquired colistin-resistance mediated by transferable mcr-1 resistance determinants, based on colistin MIC reduction in the presence of dipicolinic acid (DPA).  Methods: The Colistin-MAC test consists in a broth microdilution method, in which colistin MIC is tested in absence or presence of DPA (900 µg/mL). Overall, 74 colistin-resistant strains of Enterobacteriaceae (65 Escherichia coli and 9 of other species), including 61 strains carrying mcr-1-like genes and 13 strains negative for mcr genes, were evaluated with the Colistin-MAC test. The presence of mcr-1-like and mcr-2-like genes was assessed by Real-Time-PCR and end-point PCR. For 20 strains, Whole-Genome Sequencing data were also available. Results: A ≥8-fold reduction of colistin MIC in the presence of DPA was observed with 59 mcr-1-positive strains, including 53 E. coli of clinical origin, three E. coli transconjugants carrying MCR-1-encoding plasmids, one Enterobacter cloacae complex, and two Citrobacter spp. Colistin MICs were unchanged, increased, or at most reduced by two-fold with the 13 mcr-negative colistin-resistant strains (nine E. coli and four Klebsiella pneumoniae), but also with two mcr-1-like-positive K. pneumoniae strains.  Conclusions: The Colistin-MAC test could be a simple phenotypic test for presumptive identification of mcr-1-positive strains among isolates of colistin-resistant E. coli, based on a ≥8-fold reduction of colistin MIC in the presence of DPA. Evaluation of the test with a larger number of strains, species, and mcr-type resistance determinants would be of interest.    Keywords: DPA-colistin synergy, phenotypic screening, colistin resistance, mcr-1, Colistin-MAC test MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT3  Introduction Polymyxins have lately regained a key role as last-resort antibiotics for treatment of infections caused by multidrug-resistant and extremely drug-resistant Gram-negative pathogens [1]. In Enterobacteriaceae, acquired resistance to polymyxins is mostly mediated by modification of the Lipid A target (by addition of 4-amino-4-deoxy-l-arabinose and/or phosphoethanolamine residues) that impair polymyxin binding [2]. Target modification can be dependent on chromosomal mutations upregulating the endogenous lipopolysaccharide modification systems, or by exogenous phosphoethanolamine transferases encoded by acquired mobilized colistin resistance (mcr) genes. Discovery of the latter mechanisms has raised considerable concern in view of their potential for spreading among clinical pathogens [3]. Indeed, after the first description [4], mcr-1 and variants thereof have been detected worldwide [5], while additional types of mcr genes (e. g. mcr-2, mcr-3 and mcr-4) have recently been identified [6-8].  Since MCR-1 is a zinc enzyme [9], exposure to chelators could reduce colistin resistance in MCR-1-producing strains. Indeed, EDTA was recently reported to potentiate the activity of colistin against MCR-1-producing strains [9]. Here we describe a novel test (named Colistin-MAC test), based on colistin MIC reduction in the presence of dipicolinic acid (DPA), that could be useful for phenotypic screening of mcr-1-positive colistin-resistant E. coli strains.  Methods The Colistin-MAC test is based on colistin MIC testing, in absence or presence of DPA, in a broth microdilution format according to the CLSI standard [10]. Colistin sulphate (Sigma-Aldrich, St Louis, MO, USA) concentrations tested ranged from 0.125 to 8.0 µg/mL, alone or in combination with DPA at a fixed concentration of 900 µg/mL. DPA was chosen since it was reported to have greater selectivity for zinc ions [11], and exhibited a better performance than EDTA in phenotypic MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT4  assays for detection of zinc-β-lactamases [12]. The DPA (Sigma-Aldrich) stock solution was prepared in dimethyl sulfoxide (DMSO) (SERVA, Heidelberg, Germany) at a concentration of 100 mg/mL and stored at -20°C. Cation-adjusted Mueller Hinton broth (MHB) (Becton Dickinson, Milan, Italy) was used as the medium for susceptibility testing. MHB containing DPA and DMSO at final concentrations of 900 µg/mL and 0.9% (v/v), respectively, was used for growth control. Susceptibility testing was carried out in 96-well microtiter plates (Sarstedt, Nümbrecht, Germany). Results were recorded after 20 hours of incubation at 35±2°C. Susceptibility to colistin was interpreted according to the EUCAST clinical breakpoints (≤ 2 µg/mL) [13]. All tests were carried out in duplicate. The two repetitions were considered concordant when a ≥8-fold reduction or a ≤2-fold reduction of colistin MIC in the presence of DPA was observed in both replicates. In case of discordant results, a third replicate was carried out and the modal value was considered.  The Colistin-MAC test was evaluated with 74 colistin-resistant strains from our Laboratory collection (Table S1), including: i) 61 strains carrying mcr-1-like genes, in which the genes had been detected by PCR or Whole-Genome Sequencing (WGS); ii) 13 strains in which the absence of mcr-1-like and mcr-2 genes had been confirmed by RT-PCR and end-point PCR (Table S2); in nine of these strains the absence of all known mcr genes mcr-1 to mcr-4 had also been confirmed by WGS analysis. Most strains were E. coli (N=65), and a few belonged to other enterobacterial species including Klebsiella pneumoniae (N=6), Citrobacter braakii (N=1), Citrobacter freundii complex (N=1) and Enterobacter cloacae complex (N=1). The tested strains included 70 isolates of clinical origin (blood or urine cultures, or surveillance rectal swabs), one isolate of environmental origin, and three laboratory-derived E. coli J53 transconjugants harboring different mcr-1-carrying plasmids (Table S1). Some of the investigated strains have been previously reported, while others are part of an ongoing study. All E. coli strains were confirmed to be clonally unrelated by random amplification of polymorphic DNA PCR (Table S1). The K. pneumoniae strain carrying mcr-1-like gene was ST1 and presented a capsular wzi-19 allele, while that carrying mcr-1.2 was ST512 and presented a capsular wzi-154 allele.   MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT5  The effect of DPA in increasing colistin susceptibility of mcr-1-like positive strains was also tested in a disc-diffusion format [14], using colistin discs (10 µg, Thermo Scientific™ Oxoid™, Waltham, MA USA) added with either 7.5 µL or 10 µL of the DPA stock solution (i. e. 750 or 1000 µg per disc, respectively) before placing the disc onto the inoculated medium. Results The tested strains exhibited a colistin MIC ranging from 4 to >8 µg/mL (Table 1), and all of them grew well in the presence of DPA and DMSO at the concentrations used in the test.  In presence of DPA at 900 µg/mL, all the 53 mcr-1 positive E. coli strains exhibited a reduction of colistin MIC of at least an 8-fold dilution (range, 8 – ≥128 fold). A similar behavior was also observed with the three E. coli J53 transconjugants carrying different mcr-1 plasmids, with the mcr-1-positive Enterobacter cloacae complex strain and with the two mcr-1-positive Citrobacter strains (Table 1). Discordant results between the two replicates were only observed with three mcr-1 positive E. coli strains, which resulted in 4-fold reduction of colistin MIC in one of the two repetitions. In these cases, a third replicate yielded a >8-fold colistin MIC reduction (Table S1) and the median value was considered. Colistin MIC was increased, unchanged or at most decreased by a two-fold dilution with the mcr-negative E. coli and K. pneumoniae strains, but also with the two mcr-1-like positive K. pneumoniae strains (Table 1).  In the disc-diffusion format, no significant differences were detected in inhibition zones between mcr-1-positive and mcr-negative colistin-resistant strains (data not shown), probably due to the low and variable diffusibility of colistin from discs [15]. Discussion The Colistin-MAC test described in this work could be a simple method for screening of mcr-1-positive strains among colistin-resistant E. coli and, possibly also of other species of Enterobacteriaceae. In fact, considering as a criterion for mcr-1 positivity/negativity a ≥8-fold /≤2-fold reduction of colistin MIC in the presence of DPA, the test was able to correctly categorize all MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT6  the 65 colistin-resistant mcr-1-positive E. coli (including 62 clinical strains and three laboratory-derived transconjugants), and also a few mcr-1-positive strains of other species. The lack of inhibitory effect observed with the mcr-1-positive K. pneumoniae could be due to a reduced permeability to DPA and/or to the presence of additional unknown mechanisms of colistin resistance in those strains. We encountered only three results falling in the intermediate range (4-fold colistin MIC reduction by DPA), that were confirmed as positive (≥8-fold colistin MIC reduction) in two additional replicates. In case of an intermediate result, we suggest repeating the test and, if the intermediate result is confirmed, reporting the result as indeterminate.    This study was conceived as a preliminary proof of concept evaluation of the Colistin-MAC test and, as such, has a number of limitations including: i) the small sample size of investigated strains; ii) the limited number of investigated species, with a very small number of strains of species other than E. coli; iii) the small number of strains with (putative) chromosomal mechanisms of resistance, as negative controls; iv) the lack of strains carrying mcr genes other than mcr-1. The performance of this test should be confirmed with a larger collection of strains representative of different species and different colistin resistance mechanisms. The Colistin-MAC test has also some inherent limitations: i) the test is apparently not working with K. pneumoniae; ii) the execution requires the preparation of test-specific BMD panels, which can be labor-intensive and could increase costs in case of commercial production. In this perspective, it could be interesting to evaluate the performance of the test carried out with the addition of DPA directly to wells of a commercial panel.  Nevertheless, the possibility to presumptively detect E. coli strains with plasmid-mediated colistin resistance due to acquisition of mcr-1 genes (which overall appear to be the most prevalent strains with transferable colistin resistance worldwide) with a test that can be set up also in basic laboratories not equipped with facilities for molecular testing, could be of remarkable importance for surveillance purposes, especially in low-income settings.   MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT7  Transparency Declaration   No external funding was received for this study. Dr. Antonelli reports grants and non-financial support from ELITech group, outside the submitted work. Dr. Coppi reports non-financial support from ELITech group, outside the submitted work. Dr. Giani reports grants from Astra-Zeneca, personal fees from Alifax, personal fees from bioMérieux, personal fees from ThermoFisher Scientific, personal fees from Accelerate Diagnostics, outside the submitted work. Dr. Rossolini reports grants from bioMérieux, grants from Becton-Dickinson, non-financial support from Beckman Coulter, grants and non-financial support from Alifax, grants from Check-points, grants, personal fees and non-financial support from Accelerate Diagnostics, personal fees from Curetis, grants and personal fees from Cepheid, grants and personal fees from ELITech group, personal fees from ThermoFisher Scientific, non-financial support from Biomedical Service, personal fees from Achaogen , personal fees from Angelini, grants and personal fees from Astra-Zeneca, grants and personal fees from Basilea Pharmaceutica, grants from Biotest, personal fees from Menarini, grants and personal fees from Merck , grants and personal fees from Nordic Pharma, personal fees from Pfizer, grants from Rempex, grants and personal fees from Zambon, grants and personal fees from Angelini ACRAF, grants from Arrow diagnostics, grants from DID Diagnostics, grants from Estor, grants from Liofilchem, grants and non-financial support from Novartis, non-financial support from Copan, outside the submitted work.  References 1. Nation RL, Li J. Colistin in the 21st century. Curr Opin Infect Dis 2009; 22:535-43.  2. Baron S, Hadjadj L, Rolain JM, Olaitan AO. Molecular mechanisms of polymyxin resistance: knowns and unknowns. Int J Antimicrob Agents 2016; 48:583-91.  MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT8  3. Schwarz S, Johnson AP. Transferable resistance to colistin: a new but old threat. J Antimicrob Chemother 2016; 71:2066-70.  4. Liu YY, Wang Y, Walsh TR, Yi LX, Zhang R, Spencer J et al. Emergence of plasmid-mediated colistin-resistance mechanism MCR-1 in animals and human beings in China: a microbiological study and molecular biology study. Lancet Infect Dis 2016; 16:161-8.  5. Poirel L, Jayol A, Nordmann P. Polymyxins: Antibacterial Activity, Susceptibility Testing, and Resistance Mechanisms Encoded by Plasmids or Chromosomes. Clin Microbiol Rev 2017; 30:557-96. 6. Xavier BB, Lammens C, Ruhal R, Kumar-Singh S, Butaye P, Goossens H et al. Identification of a novel plasmid-mediated colistin-resistance gene, mcr-2, in Escherichia coli, Belgium, June 2016. Euro Surveill 2016; 7:21-7. 7. Yin W, Li H, Shen Y, Liu Z, Wang S, Shen Z, et al. Novel Plasmid-Mediated Colistin Resistance Gene mcr-3 in Escherichia coli. MBio 2017; 27:8.  8. Carattoli A, Villa L, Feudi C, Curcio L, Orsini S, Luppi A et al. Novel plasmid-mediated colistin resistance mcr-4 gene in Salmonella and Escherichia coli, Italy 2013, Spain and Belgium, 2015 to 2016. Euro Surveill 2017; 22: 31. 9. Hinchliffe P, Yang QE, Portal E, Young T, Li H, Tooke CL et al. Insights into the mechanistic basis of plasmid-mediated colistin resistance from crystal structures of the catalytic domain of MCR-1. Sci Rep 2017; 7:39392.  10. Clinical and Laboratory Standards Institute. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard—Tenth Edition. CLSI document M07-A10. Wayne, PA, USA; 2015. 11. Larson AA, Kitto KF. Chelation of zinc in the extracellular area of the spinal cord, using ethylenediaminetetraacetic acid disodium-calcium salt or dipicolinic acid, inhibits the antinociceptive effect of capsaicin in adult mice. J Pharmacol Exp Ther 1999; 288:759-65. MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT9  12. Giske CG, Gezelius L, Samuelsen Ø, Warner M, Sundsfjord A, Woodford N. A sensitive and specific phenotypic assay for detection of metallo-β-lactamases and KPC in Klebsiella pneumoniae with the use of meropenem disks supplemented with aminophenylboronic acid, dipicolinic acid and cloxacillin. Clin Microbiol Infect 2011; 17:552-6.  13. The European Committee on Antimicrobial Susceptibility Testing. Breakpoint tables for interpretation of MICs and zone diameters. Version 7.1, 2017. http://www.eucast.org 14. Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Disk Susceptibility Tests—Twelfth Edition: Approved Standard M2-A12. Wayne, PA, USA; 2015. 15. Lo-Ten-Foe JR, de Smet AM, Diederen BM, Kluytmans JA and van Keulen PH. Comparative evaluation of the VITEK 2, disk diffusion, Etest, broth microdilution, and agar dilution susceptibility testing methods for colistin in clinical isolates, including heteroresistant Enterobacter cloacae and Acinetobacter baumannii strains. Antimicrob Agents Chemother 2007; 51:3726-30.    MANUSCRIPT ACCEPTEDACCEPTED MANUSCRIPT10    Table S1. List of tested strains with mechanisms of colistin resistance, colistin MIC values in absence and presence of DPA, and references.       Table S2. Oligonucleotides and probes used for the RT-PCR and end-point-PCR for detection of mcr-1-like and mcr-2-like genes.  Table 1. Bacterial strains tested in this study, and colistin MIC values measured by broth microdilution in absence or presence of 900 mg/mL DPA (Colistin-MAC test). For a detailed description of tested strains and results see Table S1. Species Mechanism of Colistin resistancea No. of strains MIC Colistin (µg/mL) (median value) MIC Colistin (µg/mL) + DPA 900 µg/ml (median value) Fold of MIC reduction Escherichia coli mcr-1/ mcr-1-like 53 4 - >8 (8) ≤0.125 - 1 (≤0.125) 8 - ≥128 mcr-NEG, n.d. 9 4 - 8 (8) 4 - >8 (8) ≈b Escherichia coli J53AZIR Transconjugants mcr-1-like 3 4 (4) ≤0.125 - 0.5 (0.25) 8 - ≥32 Klebsiella pneumoniae mcr-1-like/mcr-1.2 2 8 - >8 >8 ≈b mcr-NEG  PmrB mutant/inactivated mgrB/n.d. 4 >8 >8 ≈b Citrobacter braakii mcr-1 1 8 0.5 16  Citrobacter freundii complex mcr-1 1 8 ≤0.125 ≥64 Enterobacter cloacae complex mcr-1 1 >8 ≤0.125 ≥128 a mcr-1-like indicates that the gene was amplified with primers for mcr-1 but was not entirely sequenced; mcr-NEG indicates that the strain was negative for mcr-1-like and mcr-2-like genes as assessed by RT-PCR and end-point PCR. In some cases, WGS data confirmed negativity for all known mcr-genes; n.d., indicates that the colistin resistance mechanism remained not determined.  b ≈ indicates that colistin MIC, in presence of DPA, was increased, unchanged or at most decreased by a two-fold dilution.  

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A simple phenotypic method for screening of MCR-1-mediated colistin resistance

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