In vitro synergistic effects of omadacycline with other antimicrobial agents against Mycobacterium abscessus

The clinical importance of <italic>Mycobacterium abscessus</italic> infections has been increasing. However, the standard treatment regimens recommended in the current guidelines often result in unfavorable outcomes. Therefore, we investigated the in vitro activity of omadacycline (OMC), a novel tetracycline, against <italic>Mycobacterium abscessus</italic> to explore its potential as a novel therapeutic option. The drug susceptibilities of 40 <italic>Mycobacterium abscessus</italic> subsp. <italic>abscessus</italic> (Mab) clinical strains obtained from the sputum of 40 patients from January 2005 to May 2014 were investigated. The MIC results for OMC, amikacin (AMK), clarithromycin (CLR), clofazimine (CLO), imipenem (IPM), rifabutin (RFB), and tedizolid (TZD) alone and their combined effects (with OMC), were examined using the checkerboard method. Additionally, we studied the differences in the effectiveness of the antibiotic combinations based on the colony morphotype of Mab. The MIC₅₀ and MIC₉₀ of OMC alone were 2 μg/mL and 4 μg/mL, respectively. The combinations of OMC with AMK, CLR, CLO, IPM, RFB, and TZD showed synergy against 17.5%, 75.8%, 25.0%, 21.1%, 76.9%, and 34.4% of the strains, respectively. Additionally, OMC combined with CLO (47.1% versus 9.5%, P = 0.023) or TZD (60.0% versus 12.5%, P = 0.009) showed significantly higher synergy against strains with rough morphotypes than those with smooth morphotypes. In conclusion, the checkerboard analyses revealed that the synergistic effects of OMC were observed most frequently with RFB, followed by CLR, TZD, CLO, IPM, and AMK. Furthermore, OMC tended to be more effective against rough-morphotype Mab strains.