Medium and supplement effects on the antimicrobial activity of quinupristin/dalfopristin tested by agar dilution and Etest methods.

Quinupristin/dalfopristin (also known as RP-59500 and Synercid), a streptogramin combination has been documented as active against a wide variety of Gram-positive species. Broad ranges of MIC results have been observed with some species using acceptable susceptibility testing methods. To address the contribution of testing medium and supplements to this disagreement, 15 medium or supplement alterations were studied in a reference agar dilution format and three variations of Mueller-Hinton agar were used for Etest (AB Biodisk, Solna, Sweden). One-third of the agar dilution variations produced a consistent elevation (twofold) in the quinupristin/dalfopristin MIC. The greatest changes were secondary to additions of serum and blood, pH increases, and possibly charcoal. Similarly, the Etest MICs for quinupristin/dalfopristin were 0.5 to 2.5 log2 dilutions higher with media supplemented with 5% sheep erythrocytes or when chocolated. Additional quinupristin/dalfopristin stability experiments in three agars indicate a two- to fourfold decline in potency over 21 days of storage at 2 to 5 degrees C. These variations in the streptogramin MIC results would be observed most frequently when testing fastidious species or in laboratories routinely testing all Gram-positive bacteria in or on blood-supplemented media.