Impact of therapeutic treatment with {beta}-lactam on transfer of the blaCTX-M-9 resistance gene from Salmonella enterica Virchow to Escherichia coli in gnotobiotic rats.

Abstract : Conjugative transfer of the plasmid carrying the blaCTX-M-9 gene from Salmonella enterica Virchow isolated from a chicken farm to a recipient Escherichia coli strain, was evaluated in vitro and in axenic rats inoculated with both strains, with and without selective pressure due to therapeutic doses of cefixime. The transfer of the blaCTX-M-9 gene of S. enterica Virchow to E. coli was confirmed in vitro, at a low frequency of 5.9 x 10(-8) transconjugants/donors. This transfer rate was higher in gnotobiotic rats and reached approximately 10(-5) transconjugants/donors without selective pressure. This frequency was unaffected by the addition of therapeutic doses of cefixime. Thus, estimates of in vitro transfer underestimated potential transfer in the digestive tract, and therapeutic doses of cefixime did not increase the selection for transconjugants.