Spread of multidrug-resistant IncHI1 plasmids carrying ESBL gene bla and metabolism operon of prebiotic oligosaccharides in commensal Escherichia coli from healthy horses, France.

The objective of the study was to identify the genetic determinants and characteristics of expanded-spectrum cephalosporin (ESC) resistance in commensal Escherichia coli from healthy horses in France in 2015. Faecal samples from 744 adult horses were screened for ESC-resistant E. coli isolates. The extended-spectrum beta-lactamase (ESBL)/AmpC resistance genes were identified using polymerase chain reaction (PCR) and sequencing. ESC phenotypes were horizontally transferred by conjugation or transformation. Plasmids carrying ESBL/AmpC genes were typed by PCR-based replicon typing, restriction fragment length polymorphism (RFLP), and plasmid multilocus sequence typing (pMLST). The ESC-resistant E. coli isolates were typed by XbaI macrorestriction analysis. Sixteen of 41 stables harboured at least one horse carrying ESC-resistant E. coli. The proportion of individually tested horses carrying ESC-resistant E. coli was 8.5% (28/328). Fifty non-redundant ESC-resistant E. coli isolates showing a great diversity of XbaI macrorestriction profiles belonged mainly to phylogroup B1, and were negative for major E. coli virulence genes, indicating they are commensal isolates. ESBL bla genes were dominant (bla, n=34; bla, n=8; bla, n=2) and located on conjugative plasmids belonging to various incompatibility groups (IncHI1, IncI1, IncN, IncY, or non-typeable). Among these, the multidrug-resistant IncHI1-pST9 plasmids were dominant and simultaneously harboured the bla genes and an operon enabling the metabolism of short-chain fructo-oligosaccharides (scFOS). In conclusion, commensal E. coli of French horses displayed a significant distribution of IncHI1-pST9 plasmids carrying both the bla gene and the fos metabolism operon. This finding highlights the risk of co-selection of multidrug-resistant IncHI1 plasmids carrying ESBL genes possibly mediated by the use of scFOS as prebiotic in horses.