Antimicrobial susceptibility and molecular characterization of Mycobacterium intracellulare in China

Mycobacterium avium complex (MAC) is the most common non-tuberculosis mycobacterial pathogen isolated from respiratory samples, mainly including two species, Mycobacterium avium (M. avium) and Mycobacterium intracellulare (M. intracellulare). Although these two species belong to the same group, M. avium and M. intracellulare reveal significantly differences in pathogenicity and biology. Nevertheless, little is known regarding the drug resistant details profile of M. avium or M. intracellulare instead of MAC. Here, we examined the antimicrobial susceptibility profiles of 52 clinical M. intracellulare isolates against fourteen antimicrobial agents, which are widely selected for the treatment of nontuberculous mycobacteria (NTM) infection. The drug susceptibility test revealed that clarithromycin (47/52, 90.4%), rifampicin (41/52, 78.8%) and capreomycin (40/52, 76.9%) revealed highly antimicrobial activities against M. intracellulare isolates in vitro. Furthermore, all clarithromycin resistant isolates harbored mutations in the 23S rRNA gene, and the percentage of amikacin resistant ones with mutation in the rrs gene is 62.5% (10/16). The Hunter-Gaston Discriminatory Index (HGDI) value for the 16-loci Variable Number of Tandem Repeat (VNTR) typing of M. intracellulare isolates was 0.994, and M. intracellulare resistance to moxifloxacin was significantly more commonly found in clustered strains than in nonclustered strains (χ(2)=5.551, P=0.040). In conclusion, our data demonstrated that clarithromycin and capreomycin revealed highly antimicrobial activities against M. intracellulare isolates, and clarithromycin and amikacin resistance could be detected more readily and rapidly using molecular scanning of corresponding drug target than conventional drug susceptibility testing. We also found that infection by clustered strains was significantly associated with resistance to moxifloxacin.