Tiamulin activity against fastidious and nonfastidious veterinary and human bacterial isolates: Initial development of in vitro susceptibility test methods

Tiamulin is a pleuromutilin derivative used in veterinary practice for the control and specific therapy of infections in swine. This report summarizes studies to establish standardized susceptibility testing methods, interpretive criteria, and reagent details for use in veterinary methods recently developed by the National Committee for Clinical Laboratory Standards (NCCLS) (standards M31-A and M37-A, NCCLS, Wayne, Pa., 1999). A total of 636 fastidious and nonfastidious animal and human pathogens were processed by using media and procedures described by the NCCLS. Tiamulin disk diffusion tests used a 30-mug disk concentration, and the proposed MIC breakpoints corresponding to levels achievable in animal target tissues (lung) were :54 mug/ml for susceptibility and greater than or equal to32 mug/ml for resistance. Correlate zone diameters for specific nonfastidious species were as follows: for Pasteurella multocida and staphylococci tested on Mueller-Hinton agar, susceptibility at greater than or equal to19 mm and resistance at less than or equal to11 mm, and for Actinobacillus suis, Etysipelothrix rhusiopathiae, and Streptococcus suis tested on enriched chocolate Mueller-Hinton agar, susceptibility at greater than or equal to16 mm and resistance at less than or equal to8 mm. When Actinobacillus pleuropneumoniae was tested, a susceptibility breakpoint of less than or equal to16 mug/ml (greater than or equal to9 mm) was suggested for veterinary fastidious medium broth and enriched chocolate Mueller-Hinton agar. Absolute categorical agreement between NCCLS dilution and disk diffusion test results with these criteria ranged from 90.5 to 96.2%. Tiamulin susceptibility testing methods appear to be accurate in their categorical classification for indicated species, and their availability will allow immediate testing of animal isolates to guide therapy via appropriate levels of dosing and to monitor the development of resistance for agents in this unique class.