1385. Mechanism-Based, In Vitro Inhibition of Mycobacterium abscessus: Assessing β-Lactam Therapy

BACKGROUND: M. abscessus (Mab) is an emerging pathogen, a highly drug-resistant rapidly-growing nontuberculous mycobacteria. Mab L, D transpeptidases (Ldt(Mab 1–5)), D,D carboxypeptidase and Bla(Mab) β-lactamase are important targets. Herein, we tested the susceptibility of ceftaroline (TAR) and imipenem (IMI) alone and in combinations with two diazabicyclooctanone β-lactamase inhibitors (BLI), relebactam (REL) and avibactam (AVI), against representative clinical isolates belonging to the Mab complex and assessed the mechanism of inhibition using mass spectrometry (QTOF-MS) METHODS: Minimum inhibitory concentrations (MICs) of TAR and IMI with or without AVI and REL and a TAR-IMI combination with and without REL were determined using microdilution. Approximately 5 x 10(5) colony-forming units (CFU) per milliliter were inoculated into Middlebrook 7H9 broth supplemented with 10% (vol/vol) oleic albumin dextrose catalase and 0.05% (vol/vol) Tween 80. AVI or REL were added at fixed concentration of 4 µg/mL to serial dilutions of TAR or IMI. For the TAR-IMI combinations, IMI at 1 µg/mL, and serial dilutions of TAR were used. Mab isolates were incubated with test agents at 30°C for 48 h, and MIC was defined as lowest antibiotic concentration that prevented visible bacterial growth. (QTOF-MS) was used to assess intermediates of Bla(Mab,) Ldt(Mab1) and Ldt(Mab2) with TAR, IMI, AVI, and REL RESULTS: In-vitro susceptibility testing on representative clinical Mab strains (table). MIC(90) was > 128 μg/mL for TAR and 8 μg/mL for IMI. Combination of TAR and IMI lowered MIC’s of all clinical isolates to <0.06 μg/mL. Addition of REL or AVI lowered TAR MICs but had minimal or no impact on IMI or TAR-IMI MICs. Mass spectrometry analyses of Bla(Mab), Ldt(Mab (1–2)) alone and incubated with IMI, TAR, REL and AVI (figure). Bla(Mab) β-lactamase bound the AVI and REL, but acyl complexes with TAR or IMI were not detected. Ldt(Mab (1–2)) form stable acyl complexes with AVI, REL, TAR, and IMI. CONCLUSION: Addition of IMI to TAR lowers MICs of TAR against Mab to therapeutically achievable concentrations. It would be welcome news for clinicians who are treating patients with highly resistant Mab infection that the combination of TAR and IMI is commercially available and thus might be considered as part of a rescue regimen. [Image: see text] [Image: see text] DISCLOSURES: All authors: No reported disclosures.