Impact of therapeutic treatment with beta-lactam on transfer of the bla(CTX-M-9) resistance gene from Salmonella enterica serovar Virchow to Escherichia coli in gnotobiotic rats.

The conjugative transfer of the plasmid carrying the bla(CTX-M-9) gene from Salmonella enterica serovar 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 or without selective pressure due to therapeutic doses of cefixime. The transfer of the bla(CTX-M-9) gene of S. enterica serovar 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 not affected 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.