Comparative analysis of phenotypic and genotypic antibiotic susceptibility patterns in Mycobacterium avium complex.

OBJECTIVE: For M. avium complex (MAC) phenotypic (Sensititre Myco, pDST) and genotypic drug susceptibility testing (GenoType NTM DR, gDST) have become available as standardised assays, but comparable data is needed. The aim of this study was to investigate the phenotypic and genotypic drug susceptibility patterns in MAC clinical isolates. METHODS: Overall, 98 isolates from 85 patients were included. pDST and gDST was performed on all isolates and results compared regarding to specificity and sensitivity using pDST as a reference method. The impact of drug instability on pDST results was studied using a biological assay over a period of 14 days. Furthermore, evolution of antimicrobial resistance was investigated in sequential isolates of 13 patients. RESULTS: Macrolide resistance was rare with 1.2% (95% CI 0.7-7.3) of isolates in the base cohort. No aminoglycoside resistances were found, but 14.1% of the studied isolates (95% CI 7.8-23.8) showed intermediate susceptibility. The GenoType NTM DR identified two out of four macrolide resistant isolates. Antibiotic stability was demonstrated to be poor in rifampicin, rifabutin and doxycycylin. CONCLUSIONS: pDST results in NTM for unstable antibiotics have to be interpreted with care. Combination of pDST and gDST will be useful for the guidance of antimicrobial therapy in MAC-disease.