Susceptibilities of nontuberculosis mycobacterial species to amoxicillin-clavulanic acid alone and in combination with antimycobacterial agents.

Neither amoxicillin nor clavulanic acid used alone was active at the highest level tested, i.e., 256.0 micrograms/ml, in vitro against 24 isolates of Mycobacterium fortuitum, Mycobacterium kansasii, and Mycobacterium marinum. However, the MIC of an amoxicillin-clavulanic acid combination of 2:1 was < or = 8.0/4.0 micrograms/ml for 50 percent of the isolates tested, with all isolates being inhibited in the range of 4.0/2.0 to 32.0/16.0 micrograms/ml, respectively. Titration of amoxicillin-clavulanic acid with a fixed 2-micrograms/ml concentration of ethambutol resulted in synergistic activity against 3 of 9 isolates of M. fortuitum, 10 of 10 isolates of M. kansasii, and 5 of 5 isolates of M. marinum. This observation was confirmed in a checkerboard analysis in which fractional inhibitory concentrations were < or = 0.5 for 20 of the 24 isolates. Synergistic activity was observed against the other four isolates in one of two trials. On the other hand, titration of amoxicillin-clavulanic acid in the presence of either one or two fixed concentrations of isoniazid, rifampin, cycloserine, tetracycline, or amikacin failed to result in synergism.