Minimum inhibitory concentrations of sulfonamides and trimethoprim for veterinary pathogens: New data for old antibiotics

Targeted interpretation of antimicrobial susceptibility testing (AST) raw data is highly dependent on the availability of appropriate interpretative criteria, such as clinical breakpoints (CBP) or at least epidemiological cut-off values (ECOFF) as potential surrogates for CBPs. However, these criteria have not yet been defined for important first line antibiotics used to treat infections with veterinary pathogens. Therefore, the aims of our study were, (1) to produce minimum inhibitory concentration (MIC) distributions for important veterinary pathogens with trimethoprim - sulfamethoxazole 1:19 combination and with sulfamethoxazole, sulfadiazine, sulfadimethoxine, and trimethoprim alone, furthermore, (2) to estimate the proportion of microbiological resistance to sulfonamides and trimethoprim in the selected bacterial species, and lastly, (3) to propose presumptive quality control (QC) ranges for potential QC strain candidates. MIC determination was carried out by broth microdilution according to the recommendations of the European committee on antimicrobial susceptibility testing (EUCAST). For the majority of the veterinary pathogens analysed, MIC distributions for trimethoprim - sulfamethoxazole 1:19, sulfamethoxazole, and trimethoprim met the EUCAST criteria and presumptive ECOFFs could be proposed. In contrast, for sulfadiazine and sulfadimethoxine the tested concentration ranges (> 256 mg/L) were too low for generating data acceptable for estimation of presumptive ECOFFs. The presented MIC distributions form the basis for an inter-laboratory study with the goal to generate aggregated MIC data to be submitted to the EUCAST steering committee for setting missing ECOFFs for sulfonamides and trimethoprim and thereby supporting the use of these first-line antibiotics.