Application of the Etest to the antimicrobial susceptibility testing of Mycobacterium marinum clinical isolates

Mycobacterium marinum, a well-recognized cutaneous pathogen, is usually treated by chemotherapy without available standardized in vitro susceptibility testing information. In this study, we have attempted to apply the stable-gradient method (Etest; AB Biodisk, Solna, Sweden) to susceptibility testing of M. marinum in order to assess the activities of eight antimicrobial agents against 60 recent clinical strains of M. marinum collected from 10 geographic sites within the United States. Two plated media (5% sheep blood Mueller-Hinton agar and Middlebrook 7H11 agar) were compared, and 7H11 agar was found to be superior in supporting the growth of all strains. Four reference strains of M. marinum were tested on five occasions with eight drugs (160 tests) in order to evaluate Etest reproducibility. Results were observed to be within 1 log2 dilution of the all-test median MIC for 97.5% of the Etests. Our MIC results for the 60 strains clearly demonstrate the best in vitro potency against M. marinum isolates to be as follows (rank order): trimethoprim-sulfamethoxazole (MIC at which 90% of the isolates are inhibited [MIC90], 0.25 and 4.25 microg/ml, respectively) = ethambutol > clarithromycin (MIC90, 1 microg/ml) > minocycline = doycycline (MIC90, 4 microg/ml) > amikacin (MIC90, 8 microg/ml). Rifampin was only marginally active against the M. marinum strains tested (MIC90, at the National Committee for Clinical Laboratory Standards) breakpoint of 1 microg/ml), and ciprofloxacin was not active (MIC90, 8 microg/ml). These data should enhance the empiric drug selection for contemporary M. marinum infections and also provide evidence that the Etest can be utilized to guide chemotherapy with alternative agents.