The effects of exposure at constant (1 h) or exponentially decreasing concentrations of quinupristin/dalfopristin on biofilms of Gram-positive bacteria

Using a biofilm model, infections by Staphylococcus aureus, coagulase-negative staphylococci and enterococci were studied. The biofilms were exposed to quinupristin/dalfopristin and five comparator antibiotics: ciprofloxacin, vancomycin, teicoplanin, flucloxacillin and erythromycin. Two methods of exposure to the drugs were used: constant for 1 h, and exponentially decreasing, with the rate of dilution being matched to the half-lives of the antibiotics. The effects of antibiotic exposure were monitored by performing viable counts on the cells eluted from the biofilms. The results are presented as the inhibitory or bactericidal effect (the reduction in numbers of bacteria eluted from the biofilms) and recovery times (the time taken for the number of cells eluted from the biofilms to return to the number eluted before the drug exposure). Quinupristin/dalfopristin was the most effective of the six antibiotics studied, especially against enterococci. However, there were no significant differences in the effects, inhibitory/ bactericidal or recovery times, produced by the two methods of exposure. The two glycopeptides showed a surprising lack of activity.