Drug susceptibility distributions in slowly growing non-tuberculous mycobacteria using MGIT 960 TB eXiST

In general, uniform clinical antibiotic susceptibility breakpoints (CBPs) for slowly growing nontuberculous mycobacteria (NTM) have not been established. The aim of this study was to determine wild-type drug susceptibility distributions for relevant antibiotics using Bactec MGIT 960 equipped with EpiCenter TB eXiST and to derive epidemiological cut-offs (ECOFFs) from semi quantitative drug susceptibility measurements. One hundred and twenty-six NTM clinical isolates (Mycobacterium avium n=58, Mycobacterium intracellulare n=18, Mycobacterium kansasii n=50) were investigated in this study. Drug susceptibility distributions and MIC90 values were determined for clarithromycin, ethambutol, rifampicin, rifabutin, ofloxacin, moxifloxacin, and amikacin using Bactec MGIT 960/EpiCenter TB eXiST. For most species/drug combinations ECOFFs were determined. For some species/drug combinations ECOFFs were not defined as either the isolates were susceptible to the lowest drug concentration tested or because isolates, in part, had MIC levels exceeding the highest drug concentration tested. This study describes drug susceptibility distributions and MIC90 values of M. avium, M. intracellulare, and M. kansasii that may aid the definition of CBPs when correlating in vitro drug susceptibility with clinical outcomes in future studies.