Introduction: Mycobacterium abscessus  complex (MABC) is a group of important infectious agents that are highly associated with drug resistance, and antibiotic treatment is usually ineffective. This study investigated the characteristics of antimicrobial susceptibility of MABC isolates and the synergy between certain β-lactam combinations against MABC infection.
Methods: We collected 129 MABC isolates from patients with lower respiratory tract infections and categorized them into three subspecies. The minimum inhibitory concentrations (MICs) of 15 antimicrobials for the MABC isolates were determined using commercial Sensititre RAPMYCOI MIC plates and the broth microdilution method, as recommended in the CLSI (M24-A2). In addition, the MICs of imipenem, alone and with ceftazidime and/or avibactam, were assessed in vitro for all isolates. The erm (41) and rrl  genes were also sequenced.
Results: The MABC isolates exhibited > 80% resistance to 11 of the 15 antimicrobials. Regarding the remaining four antimicrobials, the isolates were least resistant to tigecycline (12.4%) and amikacin (3.9%), and only partially resistant to two cefoxitin (39.5%) and imipenem (40.3%). Compared with M. massiliense  isolates, M. abscessus  and M. bolletii  isolates were more resistant to amikacin and imipenem, whereas M. abscessus  was significantly less resistant to tigecycline relative to M. massiliense  and M. bolletii  isolates. The clarithromycin inducible resistance rate was 68.4% and 74.3% among M. bolletii  and M. abscessus  isolates. Furthermore, 88.7% of the M. abscessus  isolates carried a T at position 28 of erm (41), which is associated with inducible clarithromycin resistance. In addition, compared to imipenem with avibactam only, the MIC₅₀ and MIC₉₀ values of imipenem after adding ceftazidime plus avibactam were decreased fourfold.
Conclusion: The antimicrobial resistance rates and the characteristics of the erm (41) gene associated with inducible clarithromycin resistance were different among the three MABC subspecies. There was also synergy between imipenem and 100μg/mL ceftazidime against MABC isolates.
Keywords: Mycobacterium abscessus  complex, resistance, erm (41), synergy, dual β-lactam therapy