Oral β-lactam combinations are effective <i>in vitro</i> against <i>Mycobacterium avium</i> , regardless of clarithromycin susceptibility

The global incidence and prevalence of pulmonary disease caused by the <i>Mycobacterium avium</i> complex (MAC), mainly comprising <i>M. avium</i> and <i>Mycobacterium intracellulare</i>, is increasing. However, treating MAC pulmonary disease is challenging in cases of clarithromycin (CLR)-resistant MAC or where the patients experience adverse effects or drug interactions with the few available antibiotics. Therefore, developing novel and highly effective antibiotics against MAC is crucial. Although the efficacy of dual β-lactams against <i>Mycobacterium abscessus</i> has been receiving attention, the efficacy of dual β-lactams against MAC remains unclear. Here, we used MAC type strains and clinical isolates to determine whether dual β-lactams were effective against MAC and which combinations synergistically inhibited bacterial growth using a broth microdilution checkerboard assay with 6 oral and 22 intravenous antibiotics. The combination effect and antibacterial activity differed between <i>M. avium</i> and <i>M. intracellulare</i>. Five combinations of oral β-lactams and 78 combinations of intravenous β-lactams showed a synergistic effect against the <i>M. avium</i> type strain. Among the <i>M. avium</i> clinical isolates, faropenem combined with cefuroxime showed the highest synergistic effect, and amoxicillin combined with tebipenem showed the lowest minimum inhibitory concentration. There was no significant difference in the combination effects between the CLR-susceptible and CLR-resistant <i>M. avium</i> clinical isolates in these pairs. In conclusion, regardless of CLR susceptibility, the oral β-lactam combinations were effective against <i>M. avium</i>. Thus, when treating MAC pulmonary disease, it is crucial to determine whether <i>M. avium</i> or <i>M. intracellulare</i> is the cause. IMPORTANCE <i>Mycobacterium avium</i> complex causes chronic respiratory infections, but treatment is often limited by drug resistance, intolerance, or interactions. As new therapeutic strategies are urgently needed, we focused on β-lactam antibiotics, which are widely used and well tolerated. Although dual β-lactams are effective against <i>Mycobacterium abscessus</i>, their utility against <i>Mycobacterium avium</i> complex has remained largely unexplored. Our <i>in vitro</i> study revealed that several β-lactam combinations are effective against <i>Mycobacterium avium</i>, regardless of drug resistance, indicating potential for clinical use. In contrast, <i>Mycobacterium intracellulare</i> showed lower susceptibility to β-lactams. Given this difference in drug susceptibility, we emphasize the clinical need to distinguish <i>Mycobacterium avium</i> and <i>Mycobacterium intracellulare</i> to optimize treatment of <i>Mycobacterium avium</i> complex pulmonary disease.