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Objectives: Colistin often remains the last resort antibiotic active against carbapenemase-producing Enterobacteriaceae. However, while in vitro inoculum effect has been reported, therapeutic relevance of this phenomenon remains questioned., Methods: Ten E. coli strains were used that included the wild-type CFT073 and its transconjugant CFT073-MCR-1 and eight susceptible clinical isolates. Mice with peritonitis were treated for 24 h with colistin sulfate. Bacterial loads were determined in peritoneal fluid (PF) and spleen and colistin-resistant mutants were detected., Results: MICs of colistin against the eight susceptible clinical strains and CFT073 ranged from 0.125 to 0.5 mg/L with an inoculum of 10 5 CFU/mL and from 2 to 4 mg/L with a 10 7 CFU/mL inoculum; 5/9 strains with an MIC of 4 mg/L were considered resistant according to EUCAST breakpoint (resistance, > 2 mg/L). When the bacterial load of wild-type CFT073 inoculated in mice increased from 10 7 to 10 8 CFU: i) mean log 10 CFU reduction generated by colistin in PF and spleen decreased from 5.8/mL and 3.1/g, respectively, (p < 0.01) to 0.9/mL and 0.8/g, respectively (NS); ii) mice survival rate decreased from 15/15 (100%) to 6/15 (40%) (p = 0.017); and iii) proportion of mice with selection of colistin-resistant mutants increased from 4/15 to 15/15 (p < 0.01). These results were comparable to those obtained when peritonitis was produced with a 10 7 CFU bacterial load of E. coli CFT073 expressing mcr-1, for which the mean log 10 CFU reductions were 3.5/mL and 0.6/g in PF and spleen, respectively (NS), and survival rate was 8/15 (53%) (p < 0.01 versus survival of mice infected with wild-type CFT073)., Conclusions: Phenotypic colistin resistance in wild-type E. coli due to an increase in inoculum size had a therapeutic impact in mice with peritonitis that was comparable to that observed when the mcr-1 gene was expressed., (Copyright © 2019 European Society of Clinical Microbiology and Infectious Diseases. Published by Elsevier Ltd. All rights reserved.)

Background: Beyond plasmid-encoded resistance (mcr genes) prevalence in strain collections, large epidemiological studies to estimate the human burden of colistin-resistant Escherichia coli gut carriage are lacking., Objectives: To evaluate the prevalence of colistin-resistant E. coli carriage in inpatients and decipher the molecular support of resistance and the genetic background of the strains., Methods: During a 3 month period in 2017, we prospectively screened patients in six Parisian hospitals for rectal carriage of colistin-resistant E. coli using a selective medium, a biochemical confirmatory test and MIC determination. WGS of the resistant strains and their corresponding plasmids was performed., Results: Among the 1217 screened patients, 153 colistin-resistant E. coli strains were isolated from 152 patients (12.5%). The mcr-1 gene was identified in only seven isolates (4.6%) on different plasmid scaffolds. The genetic background of these MCR-1 producers argued for an animal origin. Conversely, the remaining 146 colistin-resistant E. coli exhibited a phylogenetic distribution corresponding to human gut commensal/clinical population structure (B2 and D phylogroup predominance); 72.6% of those isolates harboured convergent mutations in the PmrA and PmrB proteins, constituting a two-component system shown to be associated with colistin resistance., Conclusions: We showed that the occurrence at a high rate of colistin resistance in human faecal E. coli is the result of two distinct evolutionary pathways, i.e. the occurrence of chromosomal mutations in an endogenous E. coli population and the rare acquisition of exogenous mcr-1-bearing strains probably of animal origin. The involved selective pressures need to be identified in order to develop preventative strategies., (© The Author(s) 2019. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

The commercial Rapid Polymyxin NP test was evaluated to detect colistin-resistant Enterobacteriaceae. A total of 223 enterobacterial isolates corresponding to 136 resistant (including 38 MCR-like producers), 19 heteroresistant, and 78 colistin-susceptible isolates were tested. The test was performed according to the manufacturer's instruction, and the color of the wells was read after 2 and 3 hours of incubation. The results were compared with those of the homemade Rapid Polymyxin NP test, and manual broth microdilution according to EUCAST guidelines was used as the reference method to determine the performance of the test. Excellent performance of the commercial Rapid Polymyxin NP test was found with a very major error rate, a major error rate, a sensitivity, and a specificity of 1.9%, 5.1%, 98.1%, and 94.9%, respectively. The performance of the homemade Polymyxin NP test was similar, with a slightly better value for the very major error (1.2 %)., (Copyright © 2018 Elsevier Inc. All rights reserved.)

A collection of 126 pigs was screened for carriage of colistin-resistant Enterobacteriaceae in a farm in Minas Gerais, Brazil. Out of this collection, eight colistin-resistant Escherichia coli isolates were recovered, including one from Minas Gerais State producing a new MCR-3 variant (MCR-3.12). Analysis of the lipopolysaccharide revealed that MCR-3.12 had a function similar to that of MCR-1 and MCR-2 as a result of the addition of a phosphoethanolamine group to the lipid A moiety. Genetic analysis showed that the mcr-3.12 gene was carried by an IncA/C 2 plasmid and was embedded in an original genetic environment. This study reports the occurrence of the MCR-3-like determinant in South America and is the first to demonstrate the functionality of this group of enzymes as a phosphoethanolamine transferase., (Copyright © 2018 American Society for Microbiology.)

The plasmid-mediated mcr-1 gene encodes a phosphoethanolamine transferase that confers resistance to polymyxins. The mcr-1 gene is associated with insertion sequence IS Apl1 (IS 30 family). In vitro mobilization assays demonstrated the functionality of the composite transposon structure IS Apl1-mcr-1 -IS Apl1 Transposition generated a 2-bp duplication and occurred in AT-rich DNA regions. This is the first report demonstrating the mobility of the mcr-1 gene by transposition., (Copyright © 2017 American Society for Microbiology.)

A series of colistin-resistant Escherichia coli clinical isolates was recovered from hospitalized and community patients in South Africa. Seven clonally unrelated isolates harbored the mcr-1 gene located on different plasmid backbones. Two distinct plasmids were fully sequenced, and identical 2,600-bp-long DNA sequences defining a mcr-1 cassette were identified. Promoter sequences responsible for the expression of mcr-1, deduced from the precise identification of the +1 transcription start site for mcr-1, were characterized., (Copyright © 2016, American Society for Microbiology. All Rights Reserved.)