APPLICATION OF THE ETEST TO THE ANTIMICROBIAL SUSCEPTIBILITY TESTING OF MYCOBACTERIUM-MARINUM CLINICAL ISOLATES

Mycobacterium marinum, a well-recognized cutaneous pathogen, is usuall y treated by chemotherapy without available standardized in vitro susc eptibility testing information, In this study, we have attempted to ap ply the stable-gradient method (Etest; AB Biodisk, Solna, Sweden) to s usceptibility testing of M. marinum in order to assess the activities of eight antimicrobial agents against 60 recent clinical strains of M. maximum collected from 10 geographic sites within the United States, Two plated media (5% sheep blood Mueller-Hinton agar and Middlebrook 7 H11 agar) were compared, and 7H11 agar was found to be superior in sup porting the growth of all strains, Four reference strains of M. maximu m were tested on five occasions with eight drugs (160 tests) in order to evaluate Etest reproducibility. Results were observed to be within 1 log(2) 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 vit ro potency against M. maximum isolates to be as follows (rank order): trimethoprim-sulfamethoxazole (MIG at which 90% of the isolates are in hibited [MIC90], 0.25 and 4.25 mu g/ml, respectively) = ethambutol > c larithromycin (MIC90, 1 mu g/ml) > minocycline = doycycline (MIC90, 4 mu g/ml) > amikacin (MIC90, 8 mu g/ml). Rifampin was only marginally a ctive against the M. maximum strains tested (MIC50, at the National Co mmittee for Clinical Laboratory Standards) breakpoint of 1 mu g/ml), a nd ciprofloxacin was not active (MIC90, 8 mu g/ml). These data should enhance the empiric drug selection for contemporary M. marinum infecti ons and also provide evidence that the Etest can be utilized to guide chemotherapy with alternative agents.