Standardization of broth microdilution and disk diffusion susceptibility tests for Actinobacillus pleuropneumoniae and Haemophilus somnus: Quality control standards for ceftiofur, enrofloxacin, florfenicol, gentamicin, penicillin, tetracycline, tilmicosin, and trimethoprim-sulfamethoxazole

Quality control (QC) standards for the in vitro antimicrobial susceptibilit y testing of two fastidious veterinary pathogens,Actinobacillus pleuropneum oniae and Haemophilus somnus, were developed in a multilaboratory study acc ording to procedures established by the National Committee for Clinical Lab oratory Standards for broth microdilution and disk diffusion testing. The m edium recommended for the broth microdilution testing is cation-adjusted Mu eller-Hinton broth supplemented with 2% lysed horse blood, 2% yeast extract , and 2% supplement C. This medium has been designated veterinary fastidiou s medium. The medium recommended for the disk diffusion testing is chocolat e Mueller-Hinton agar. The recommended QC organisms are A. pleuropneumoniae ATCC 27090 and H. somnus ATCC 700025. The QC MICs of ceftiofur, enrofloxac in, florfenicol, gentamicin, penicillin, tetracycline, tilmicosin, and trim ethoprim-sulfamethoxazole were determined for each isolate, as were the zon e size ranges. Of the results from the participating laboratories, 94.0% of the zone diameter results and 97.0% of the MIC results fell within the sug gested QC ranges for all compounds. These QC guidelines should allow greate r accuracy in interpreting results when testing these antimicrobial agents against fastidious pathogens.