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 h ighest level tested, i.e., 256.0 mu g/ml, in vitro against 24 isolates of Mycobacterium fortuitum, Mycobacterium kansasii, and Mycobacterium marinum, However, the MIC of an amoxicillin-clavulanic acid combinati on of 2:1 was less than or equal to 8.0/4.0 mu g/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 mu g/ml, respectively, Titration of amoxicillin- clavulanic acid with a fixed 2-mu g/ml concentration of ethambutol res ulted in synergistic activity against 3 of 9 isolates of M. fortuitum, 10 of 10 isolates of M. kansnsii, and 5 of 5 isolates of M. marinum, This observation was confirmed in a checkerboard analysis in which fra ctional inhibitory concentrations were less than or equal to 0.5 for 2 0 of the 24 isolates, Synergistic activity was observed against the ot her four isolates in one of two trials, On the other hand, titration o f amoxicillin-clavulanic acid in the presence of either one or two fix ed concentrations of isoniazid, rifampin, cycloserine, tetracycline, o r amikacin failed to result in synergism.