Antibiotic susceptibility pattern of Mycobacterium marinum

In vitro activities of 17 antibiotics against 53 clinical strains of Mycobacterium marinum, an atypical mycobacterium responsible for cutaneous infections, were determined using the reference agar dilution method. Rifampin and rifabutin were the most active drugs (MICs at which 90% of the isolates tested were inhibited [MIC(90)s], 0.5 and 0.6 mug/ml, respectively). MICs of minocycline (MIC90, 4 mug/ml), doxycycline (MIC90, 16 mug/ml) clarithromycin (MIC90, 4 mug/ml), sparfloxacin (MIC90, 2 mug/ml), moxifloxacin (MIC90, 1 mug/ml), imipenem (MIC90, 8 mug/ml), sulfamethoxazole (MIC90, 8 mug/ml) and amikacin (MIC90, 4 mug/ml) were close to the susceptibility breakpoints. MICs of isoniazid, ethambutol, trimethoprim, azithromycin, ciprofloxacin, ofloxacin, and levofloxacin were above the concentrations usually obtained in vivo. For each drug, the MIC50, geometric mean MIC, and modal MIC were very close, showing that all the strains had a similar susceptibility pattern. Percent agreement (within +/-1 log, dilution) between MICs yielded by the Etest method and by the agar dilution method used as reference were 83, 59, 43, and 24% for minocycline, rifampin, clarithromycin, and sparfloxacin, respectively. Reproducibility with the Etest was Low, in contrast to that with the agar dilution method. In conclusion, M. marinum is a naturally multidrug-resistant species for which the agar dilution method is more accurate than the Etest for antibiotic susceptibility testing.