Drug susceptibility patterns of Mycobacterium abscessus and Mycobacterium massiliense isolated from respiratory specimens

In this study, we aimed to retrospectively investigate and compare the drug susceptibility patterns of two major Mycobacterium abscessus complex (MABC) species; M. abscessus and M. massiliense. A total of 546 MABC respiratory isolates (277 M. abscessus and 269 M. massiliense) from 2011 to 2016 were analyzed in this study. We estimated minimum inhibitory concentrations (MICs) using the broth microdilution method, and we calculated MIC ₅₀ and MIC ₉₀ values from the MIC distribution. Both M. abscessus and M. massiliense were highly susceptible to amikacin and linezolid. For M. abscessus, the proportions of inducible and acquired resistance to clarithromycin were 68.6% and 12.3%, respectively. Only 15.2% of M. abscessus remained susceptible at day 14. On the other hand, none of the M. massiliense showed inducible resistance and 6.3% showed acquired resistance to clarithromycin. A total of 92.6% of the M. massiliense remained susceptible at day 14. The resistance rate of M. abscessus to moxifloxacin (90.3%) was significantly higher than that of M. massiliense (83.3%; p = 0.016). These susceptibility differences may explain the divergent treatment outcomes between patients with pulmonary disease caused by these two species.