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-negat ive staphylococci and enterococci were stud led. 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 decrea sing, with the rate of dilution being matched to the half-lives of the anti biotics. The effects of antibiotic exposure were monitored by performing vi able counts on the cells eluted from the biofilms. The results are presente d as the inhibitory or bactericidal effect (the reduction in numbers of bac teria 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 bef ore the drug exposure). Quinupristin/dalfopristin was the most effective of the six antibiotics studied, especially against enterococci. However, ther e were no significant differences in the effects, inhibitory/bactericidal o r recovery times, produced by the two methods of exposure. The two glycopep tides showed a surprising lack of activity.