Background
Drug susceptibility test (DST) of the Mycobacterium abscessus group (MAG) and other rapidly growing nontuberculous mycobacteria by conventional microplate techniques is complicated due to inducible resistance to clarithromycin and other technical factors. This study evaluated the application of the BACTEC MGIT 960/Epicenter TB eXiST for DST of MAG clinical isolates.
Methods
M. abscessus ATCC19977 was used as the reference strain for the standardizing the DST by MGIT 960 and as the internal control for testing of 31 clinical isolates tests submitted to a reference laboratory for DST and confirmed as MAG. Clarithromycin genotyping was performed for the loci in the rrl and erm (41) genes known to impact resistance phenotype.
Results
The 31 MAG isolates included 14 M. abscessus , 8 M. massiliense , and 9 M. bolletii . Using conventional microplate technique according to CLSI guideline, the isolates had a high percentage of resistance for cefoxitin (93.5%) and imipenem (100%), and sensitivity for amikacin (96.7%). Comparing microplate and MGIT 960 results across those 93 pairs of results (31 isolates x 3 antibiotics), 73 (80.6%) were concordant and the remaining 18 (19.4%) represented minor errors; there were no major or very major errors. Concordance was 100% for amikacin, 84% for imipenem and 58% for cefoxitin. Clarithromycin DST by microplate identified 14 isolates as susceptible (all susceptible by MGIT 960), 3 isolates as resistant after 3 days incubation, and 14 isolates demonstrating inducible resistance from Day 5 through 14. Among the latter isolates, MGIT 960 reported 8 as resistant and 6 as intermediate, without modifications to the protocol developed. For all isolates, the observed clarithromycin susceptibility phenotypes were consistent with the genotypes.
Conclusion
The present study is the first description of a DST protocol for MAG isolates using the MGIT 960/Epicenter TB eXiST system. The protocol developed provided highly reliable results based on direct comparison with the conventional microplate method, including, without further modification, detection of isolates with inducible-resistance to clarithromycin. Laboratories using MGIT 960 for DST of other mycobacteria may find benefit to incorporating MAG into their routine.