In-vitro activity of ceftriaxone combined with newer agents against MRSA

In this study, in vitro synergism in combinations of agents as ceftriaxone/dalbavancin, ceftriaxone/linezolid and ceftriaxone/daptomycin against MRSA strains were investigated. Thirty clinical MRSA strains were tested. The minimum inhibitory concentrations of all antibiotics were determined using reference broth microdilution method. In-vitro activities of antibiotics combined against the strains were tested using two-dimensional checkerboard microdilution method. Results were interpreted as follows: synergy = FICI ≤0.5; ‘no interaction’ effect = FICI ˃0.5-≤4; antagonism = FICI ˃4. The MIC50, MIC90 and MICrange of ceftriaxone, daptomycin, dalbavancin and linezolid were found as 128, 1024 and 16-2048 mg/L; 1, 1 and 0.5–1 mg/L; 0.12, 0.12 and 0.03–0.12 mg/L; and 1, 2 and 1–2 mg/L, respectively. Our results showed that the frequency of synergistic effects (FICI: ≤0.5) of three combinations were all at the same rate of 77% (23/30). No in vitro antagonism (FICI >4) was observed.