This study assesses the synergistic effect of the combination of cephalosporins and sulbactam with daptomycin against daptomycin-nonsusceptible, vancomycin-intermediate resistant Staphylococcus aureus (VISA) or heterogeneous vancomycin-intermediate S. aureus (h-VISA) isolates. The in vitro activity of daptomycin against daptomycin-nonsusceptible VISA/h-VISA isolates after adding cephalosporins with or without sulbactam was evaluated. The MIC of daptomycin against the VISA/h-VISA isolates was reduced after adding cephalosporins to daptomycin. Except for one VISA and two h-VISA isolates, the other VISA/h-VISA isolates became daptomycin-susceptible (MICs 1 mg/L). After adding sulbactam to each daptomycin/cephalosporin combination, the MIC of daptomycin against the VISA/h-VISA isolates decreased for 5 (33.3%), 6 (40.0%), 6 (40.0%), and 6 (40.0%) isolates with the cefazolin, cefmetazole, cefotaxime, and cefepime combinations, respectively. Synergism using the checkerboard method was noted in 100% of cefazolin and cefotaxime combinations and 87% and 80% of cefmetazole and cefepime combinations for all the VISA and h-VISA isolates. With the addition of sulbactam, synergism was noted in 100% of cefazolin, cefmetazole, and cefotaxime combinations and 93% of the cefepime combinations for all the VISA and h-VISA isolates. Almost all the FICs for the three-drug combinations were lower than those for the two-drug combinations. Using time-killing methods, a synergistic effect against five h-VISA isolates was observed. A synergistic effect of daptomycin, sulbactam, and each cephalosporin was observed for all VISA isolates. In conclusion, the activity of daptomycin against daptomycin-nonsusceptible VISA/h-VISA isolates can be enhanced by adding cephalosporins, and partially further promoted by sulbactam.
Keywords: cephalosporin; daptomycin; heterogeneous vancomycin-intermediate S. aureus; sulbactam; synergism test; vancomycin-intermediate resistant S. aureus.