P19 | Determination of optimal ratios for three sulfonamides/trimethoprim combinations against E. coli using checkerboard and time‐kill assays

Introduction: Due to the restrictive use of critical antibiotics by veterinarians in Europe, the use of first-line antibiotics such as the combination of sulfonamides (S) and trimethoprim (TMP) is likely to increase. The usual S:TMP formulations used in almost all veterinarian products on the market aims to produce an in vivo ratio of plasma concentration of 20:1, which was determined as the most effective 40 years ago. However, it is not always reached in the different animal species due to PK differences between sulfonamides and animal species. Therefore, it is necessary to determine the optimal range of S:TMP ratio to characterize properly the extent of the synergy for each sulfonamide.Objectives: The aim of this study is to determine the most synergistic ratio of three S/TMP combinations widely used in veterinary medicine: sulfamethoxazole (SMX)/TMP, sulfadimethoxine (SDMX)/TMP and sulfadiazine (SDZ)/TMP on several Escherichia coli strains (n = 9) with various susceptibility profiles to these drugs.Materials and Methods: Different ratios of SMX/TMP, SDMX/TMP and SDZ/TMP combinations were tested over a range of S and TMP concentrations (relevant with in vivo PK profiles), using checkerboard assay in conventional media. The Fractional Inhibitory Concentration Index (FICI) was calculated to identify the most synergistic ratios (FICI