Molecular characterization of Mycobacterium abscessus subspecies isolated from patients attending an Italian Cystic Fibrosis Centre

Mycobacterium abscessus (MABS) infection represents significant management challenge in cystic fibrosis (CF) patients. This retrospective study (2005-2016) aims to determine the prevalence of the subspecies of MABS isolated from CF patients, to evaluate the persistence over the years of a single subspecies of MABS and to correlate mutations responsible for macrolides and amikacin resistance with MIC values. We investigated 314 strains (1 isolate/patient/year) isolated from the lower respiratory tract of 51 chronically infected CF patients. Sequencing of rpoB gene was performed to identify the MABS subspecies. The erm(41) gene was sequenced to differentiate the strains with and without inducible macrolide resistance. Regions of 23S and 16S rRNA were sequenced to investigate mutations responsible for constitutive resistance to macrolides and aminoglycosides, respectively. Antibiotic susceptibility, using commercial microdilution plates, was evaluated according to CLSI. M. abscessus subsp. abscessus accounted for 64% of the isolates, bolletii subspecies for 16% and massiliense subspecies for 20%. All the massiliense strains presented truncated erm(41) gene while 12 abscessus strains presented the mutation T28->C in the erm(41) gene, which makes it inactive. The 23S rRNA analysis did not show constitutive resistance to macrolides in any strain. Mutation of the 16S rRNA gene was highlighted in 2 strains out of 314, in agreement with high MIC values. The correct identification at the subspecies level and the molecular analysis of 23S rRNA, 16S rRNA and erm gene is useful to guide the treatment strategy in patients with M. abscessus lung infection.