Development of provisional disc diffusion breakpoints for testing quinupristin/dalfopristin

Quinupristin/dalfopristin is an injectable streptogramin with broad activity against many Gram-positive bacteria, including Streptococcus pneumoniae, Staphylococcus aureus and Enterococcus faecium. Although a number of studies have reported the MICs of this compound against a variety of bacteria, there are no published reports of disc diffusion testing. We tested total disc masses of 7.5 and 15 micrograms with varying ratios of the component compounds, quinupristin and dalfopristin, combined in the following quinupristin: dalfopristin ratios (in microgram): 7.5:0, 0:7.5, 10:5, 5:10, 5:2.5 and 2.5:5. Zone diameters and MICs were determined in parallel for 44 isolates of staphylococci, 47 isolates pneumococci and 64 isolates of enterococci. Control strains were included for each species tested, including a strain of Enterococcus faecalis with known resistance to quinupristin/dalfopristin. Using tentative definitions for quinupristin/dalfopristin of < or = 2 mg/L as susceptible and > or = 4 mg/L as resistant, each of the four discs containing ratios of both component compounds separated presumptive susceptible organisms from resistant ones better than either quinupristin or dalfopristin alone. The best correlation of zone sizes and MICs for predicting susceptibility to quinupristin/dalfopristin at an MIC of < or = 2 mg/L was achieved with the 5 micrograms:10 micrograms disc and a zone diameter of > or = 18 mm. These criteria may be useful for identifying organisms that are presumptively susceptible to quinupristin/dalfopristin.