A link between the newly described colistin resistance gene mcr-9 and clinical Enterobacteriaceae isolates carrying blaSHV-12 from horses in Sweden.

Abstract Objectives To investigate the occurrence of the newly described transferable colistin resistance gene mcr-9 in Extended Spectrum Beta-Lactamase (ESBL) producing clinical Enterobacteriaceae isolates from horses in Sweden. Methods 56 whole-genome-sequenced Enterobacteriaceae isolates from horses were subjected to in-silico detection of genes encoding antibiotic resistance and identification of plasmid replicons types. For isolates positive for a mcr-gene colistin minimum inhibitory concentration was determined using microdilution susceptibility testing. Relatedness between Enterobacteriaceae carrying mcr-genes was checked using multi locus sequence typing (MLST) and core-genome MLSTs. Results Thirty ESBL-producing Enterobacteriaceae isolates from horses were positive for the colistin resistance gene mcr-9. These isolates belonged to the bacterial species Enterobacter cloacae, Escherichia coli, Klebsiella oxytoca or Citrobacter freundi, and belonged to a diverse set of 7-MLSTs within each species. All mcr-9 containing isolates originated from horses, with two isolates from the same horse. All mcr-9 isolates showed MICs of colistin below EUCAST epidemiological cut-off values, >2 mg/L and were negative for the two potential regulatory genes qseB and qseC. Except for one isolate that carried only blaTEM-1B, all carried blaSHV-12 and blaTEM-1B, and were all considered multi-drug resistant as they carried genes encoding resistance to aminoglycosides, chloramphenicol, fosfomycin, macrolides, quinolones, sulfonamides, trimethoprim and tetracyclines. The plasmid replicon types IncHI2 and IncHI2A was detected in all mcr-9 positive isolates. Conclusion Occurrence of mcr-9 was common among clinical ESBL-producing Enterobacteriaceae isolates from horses in Sweden, and was linked to the ESBL-encoding gene blaSHV-12, and plasmid replicon types IncHI2 and IncHI2A.