Prevention of High-Level Daptomycin-Resistance Emergence In Vitro in Streptococcus mitis-oralis by Using Combination Antimicrobial Strategies.

Among the viridans group streptococci, S. mitis-oralis strains are frequently resistant to multiple β-lactams and tolerant to vancomycin (VAN). This scenario has led to the proposed clinical use of newer agents, like daptomycin (DAP) for such S. mitis-oralis strains. However, recent recognition of the rapid and durable emergence of high-level DAP-resistance (DAP-R; DAP MICs > 256 µg/ml) induced by DAP exposures in vitro and in vivo has dampened enthusiasm for such approaches. In this study, we evaluated a broad range of DAP combination regimens in vitro for their capacity to prevent emergence of high-level DAP-R in a prototype S. mitis-oralis strain (351) during serial passage experiments, including DAP + either gentamicin (GEN), rifampin (RIF), trimethoprim-sulfamethoxazole (TMP-SMX), imipenem (IMP), ceftaroline (CPT), tedizolid (TDZ), or linezolid (LDZ). In addition, we assessed selected DAP combination regimens for their ability to exert either an early bactericidal impact and/or synergistically kill the S. mitis-oralis study strain. During serial passage, three of the eight antibiotic combinations (DAP + GEN, CPT, or TMP- SMX) exhibited significantly reduced DAP MICs (≈ by 8-40 fold) vs serial exposure in DAP alone (DAP MICs > 256 µg/ml). In addition, combinations of DAP + GEN and DAP + CPT were both bactericidal and synergistic in early time-kill curve interactions.