In-vitro bactericidal activity of quinupristin/dalfopristin against adherent Staphylococcus aureus

A simple in-vitro assay was developed to test antibiotics for bactericidal activity against Staphylococcus aureus adhering to an artificial biomaterial. The activity of the semisynthetic injectable streptogramin quinupristin/dalfopristin combination was evaluated with this assay and compared with that of vancomycin. Fibronectin-coated nylon membranes were used to mimic implanted biomaterial. To obtain adherent bacteria, 400 mu L of a suspension of S. aureus strain Wood (5 x 10(6) cfu/ml) were placed for 1.5 h at 37 degrees C in contact with the membranes. After elimination of non-adherent bacteria by successive rinses, membranes were exposed to antibacterial agents at concentrations of 5, 10 and 50 x MIG. Drug-free controls were also prepared. After various incubation times, bacteria on both treated and control membranes were removed by sonication and subcultured on to agar medium. After incubation, the number of cfu on treated membranes was compared with that on t(0) control membranes. The criterion for bactericidal activity was a 99.9% reduction of the t(0) control inoculum. Under these experimental conditions, both quinupristin/dalfopristin and vancomycin, at concentrations achievable in blood (6.25 and 20 mg/L, respectively), demonstrated a potent bactericidal activity against S. aureus adhering to a support. The bactericidal effect of quinupristin/dalfopristin was more rapid than that of vancomycin. Quinupristin/dalfopristin may therefore be a useful alternative to vancomycin in the treatment of implant-associated S. aureus infections.