Development of provisional disc diffusion breakpoints for testing quinupristin/dalfopristin

Area of Science:

• Microbiology
• Antimicrobial Susceptibility Testing
• Pharmacology

Background:

• Quinupristin/dalfopristin is a streptogramin antibiotic effective against Gram-positive bacteria.
• Limited data exists on disc diffusion testing for quinupristin/dalfopristin.
• Standardized methods are needed to assess bacterial susceptibility to this antibiotic.

Purpose of the Study:

• To evaluate the efficacy of disc diffusion testing for quinupristin/dalfopristin against Gram-positive bacteria.
• To determine optimal disc ratios and zone diameters for predicting susceptibility.
• To establish criteria for identifying bacteria susceptible to quinupristin/dalfopristin.

Main Methods:

• Disc diffusion tests were performed using various quinupristin:dalfopristin ratios (7.5:0, 0:7.5, 10:5, 5:10, 5:2.5, 2.5:5) and total disc masses of 7.5 and 15 µg.
• Minimum Inhibitory Concentrations (MICs) were determined in parallel for 44 staphylococci, 47 pneumococci, and 64 enterococci isolates.
• Tentative susceptibility breakpoints were defined as MIC ≤ 2 mg/L and resistance as MIC ≥ 4 mg/L.

Main Results:

• Discs containing ratios of both quinupristin and dalfopristin effectively separated susceptible from resistant organisms.
• The 5 µg quinupristin:10 µg dalfopristin disc demonstrated the best correlation between zone diameter and MIC.
• A zone diameter of ≥ 18 mm with the 5:10 µg disc predicted susceptibility (MIC ≤ 2 mg/L).

Conclusions:

• Disc diffusion testing can be a valuable tool for predicting quinupristin/dalfopristin susceptibility in Gram-positive bacteria.
• The 5:10 µg disc ratio with an 18 mm zone diameter provides reliable criteria for presumptive susceptibility.
• These findings support the clinical utility of disc diffusion for guiding quinupristin/dalfopristin therapy.