Methods: Paired antibiotic combinations were tested in vitro for synergy against the Mab reference strain ATCC 19977 at 106 CFU/mL, per CLSI guidelines. Combinations included two β-lactams, a β-lactam and a β-lactamase inhibitor, or a β-lactam and a rifamycin. The minimum inhibitory concentration (MIC) of each drug was initially confirmed via broth microdilution assay. A validated checkerboard assay was used to determine the fractional inhibitory concentration index (FICI) for each combination to identify pairs that exhibit synergistic activity against Mab.
Results: Of the initial 227 combinations screened, 18 pairs exhibited a high level of synergy (FICI ≤0.5). Half of these were combinations of two β-lactams. The average reduction in MIC for each drug in combination was at least fourfold, with 8/18 combinations exhibiting reductions greater than eightfold. Although MIC breakpoints against Mab have not been established for all of the antibiotics tested in this study, the MICs of at least seven combinations were within the therapeutic range.
Conclusion: Comprehensive inhibition of essential enzymes involved in PG synthesis requires more than one β-lactam antibiotic, and this phenomenon is hypothesized to be the basis for observed synergy between β-lactams. Some of the combinations reduced MICs to within therapeutically-achievable levels, potentially leading to vital new treatment options against drug-resistant Mab.