Comparison of drug-susceptibility patterns and gene sequences associated with clarithromycin and azithromycin resistance in complex isolates and evaluation of the accumulation of intrinsic macrolide resistance.

complex (MABC) is an infectious agent associated with macrolide resistance and treatment failure. Despite drug-susceptibility testing for MABC isolates including clarithromycin (CAM), long-term treatment with azithromycin (AZM) for MABC disease is recommended. We compared phenotypic and genotypic resistance to AZM and CAM in clinical isolates and evaluated the accumulation of intrinsic macrolide resistance (AIM) and morphological changes by macrolides exposure. Forty-nine isolates were characterized regarding (41) sequevars. Sequencing data were compared to the nucleotide sequence of and . The AIM MIC was performed in three reference strains and 15 isolates were randomized [each set of five isolates with subsp. (MAA) T28, MAA C28 and subsp. (MAM)]. The 49 isolates were distributed as 24 MAA T28, 5 MAA C28 and 20 MAM. The MIC values to CAM at day 3 in MAA T28, C28 and MAM were 1, 0.12 and 0.12 µg ml, while those at day 14 were 32, 0.5 and 0.12 µg ml, respectively. The AZM-MIC values at day 3 of the above isolates were 4, 0.25 and 0.5 µg ml, while those at day 14 were >64, 0.5 and 0.5 µg ml, respectively. Neither mutations in of MAA T28 with acquired resistance nor deletions in of MAA T28 without inducible resistance were observed . For AIM MIC, MAA T28 showed that the time-to-detection of AZM resistance was significantly faster over that of CAM (<0.05). Morphological changes were not determined in all isolates. Our findings did not support the suggestion for the preferential use of AZM for, at least, MAA T28 disease due to the high-level MIC value and the increased AIM. The long duration of AZM-based treatment eventually may favour the emergence of isolates with a high-level of intrinsic resistance.