Rapid macrolide and amikacin resistance testing for Mycobacterium abscessus in people with cystic fibrosis

Introduction. Mycobacterium abscessus complex (MABSC) is an environmental organism and opportunistic pathogen. MABSC pulmonary infections in people with cystic fibrosis are of growing clinical concern. Resistance data guide the use of macrolides and amikacin in MABSC pulmonary disease treatment. MABSC can acquire resistance against macrolides or amikacin via 235 or 165 rRNA gene mutations, respectively. Gap Statement. Current culture-based methods for MABSC detection and antibiotic resistance characterization are typically prolonged, limiting their utility to directly inform treatment or clinical trials. Culture-independent molecular methods may help address this limitation. Aim. To develop real-time PCR assays for characterization of key 235 or 16S rRNA gene mutations associated with constitutive resistance in MABSC. Methodology. We designed two real-time PCR assays to detect the key 23S and 16S rRNA gene mutations. The highly conserved nature of rRNA genes was a major design challenge. To reduce potential cross-reactivity, primers included non-template bases and targeted single-nucleotide polymorphisms unique to MABSC. We applied these assays, as well as a previously developed real-time PCR assay for MABSC detection, to 968 respiratory samples from people with cystic fibrosis. The results from the molecular methods were compared to those for gold standard culture methods and 23S and 16S rRNA gene sequencing. Results.The real-time PCR MABSC detection assay provided a sensitivity of 83.8% and a specificity of 97.8% compared to culture. The results from the real-time PCR resistance detection assays were mostly concordant (>77.4%) with cultured isolate sequencing. The real-time PCR resistance detection assays identified several samples harbouring both resistant and susceptible MABSC, while culture-dependent methods only identified susceptible MABSC in these samples. Conclusion. Using the molecular methods described here, results for health care providers or researchers could be available days or weeks earlier than is currently possible via culture-based antibiotic susceptibility testing.