Comparative activity of quinupristin/dalfopristin and RPR 106972 and the effect of medium on in-vitro test results

Quinupristin/daliopristin and RPR 106972 were active in vitro against a wid e range of aerobic Gram-positive organisms including Enterococcus faecium. However, most isolates of Enterococcus faecalis were resistant or of interm ediate sensitivity. Against Staphylococcus aureus quinupristin/dalfopristin was more active but for all other species the range of activity of the two drugs was the same or RPR 106972 was more active. RPR 106972 was also more active against the respiratory pathogens Haemophilus influenzae and Moraxe lla catarrhalis. Quinupristin/dalfopristin MICs for isolates of H. influenz ae (1-8 mg/L) clustered around the breakpoint. There were differences in th e quality of growth, but little difference in MICs or zone diameters was ob tained on three different media: Mueller-Hinton (MHA), Iso-Sensitest (ISA), and Diagnostic Sensitivity Test (DST) agars. The addition of blood to the medium increased MICs 2- to 4-fold, with MHA showing the greatest increase, and reduced zone diameters around quinupristin/dalfopristin discs by 3-4 m m, with the greatest effect on ISA.