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

Objectives: To investigate the occurrence of the newly described transferable colistin resistance gene mcr-9 in Extended Spectrum Beta-lactamase (ESBL) producing Enterobacteriaceae clinical isolates from horses in Sweden. Methods: Sixty-four Enterobacteriaceae clinical isolates (fifty-six from horses and eight from other sources) previously whole-genome sequenced were subjected to in-silico detection of genes encoding antibiotic resistance genes and identification of plasmid replicons types. For isolates positive for a mcr-gene minimum inhibitory concentration for colistin was determined using microdilution susceptibility testing. Relatedness between Enterobacteriaceae carrying mcr-genes was checked using seven allele multi locus sequence typing (MLST) and core-genome MLSTs.Results: Out of 63 ESBL-producing Enterobacteriaceae isolates, 30 were shown to be positive for the transferable colistin resistance gene mcr-9. The 30 isolates belonged to the bacterial species Enterobacter cloacae group, Escherichia coli, Klebsiella oxytoca and Citrobacter freundi, and belonged to a diverse set of 7-MLSTs. All mcr-9 containing isolates originated from horses, with two isolates from the same horse. Except for one isolate that carried only blaTEM-1B, all were characterized to carry carried blaSHV-12 and blaTEM-1B. The isolates were all considered multi-drug resistantresistant as they carried genes encoding resistance to aminoglycosides, macrolides, sulfonamides, chloramphenicol, trimethoprim, fosfomycin, quinolones and tetracycline.tetracyclines. The plasmid replicon type incHI2/A was detected in all mcr-9 positive isolates.Conclusion: Occurrence of mcr-9 is common among clinical ESBL-producing Enterobacteriaceae isolates from horses in Sweden, and the occurrence appear linked to the ESBL-encoding genes blaSHV-12 and blaTEM-1B, and plasmids belonging to replicon type incHI2/A. The occurrence of mcr-9 appears mainly due to the dissemination of an incHI2/A-plasmid but some clonal dissemination could also be observed.