ACTIVITY OF QUINUPRISTIN DALFOPRISTIN AND ITS COMPONENTS AGAINST HAEMOPHILUS-INFLUENZAE/

Quinupristin/dalfopristin is an injectable streptogramin antibiotic th at is constituted in a 30:70 (w/w) ratio of the two components. Quinup ristin and dalfopristin are thought to act synergically by binding to two separate sites on the bacterial 50S ribosomal subunit. The in-vitr o activities of the two components separately and together in differen t ratios were determined for a collection of 100 Haemophilus influenza e strains representing various antimicrobial resistance phenotypes. Th e NCCLS microdilution susceptibility testing procedure incorporating H aemophilus test medium (HTM) broth was used to determine MICs of quinu pristin, dalfopristin and seven other antimicrobial agents. The MIC50 and MIC90 values were 4 and 8, 4 and 16, and 64 and 128 mg/L for quinu pristin/dalfopristin (30:70), dalfopristin and quinupristin, respectiv ely. MICs of quinupristin and dalfopristin were also determined in Mue ller-Hinton lysed horse blood broth and by HTM agar dilution testing. Compared with HTM broth-derived results, the MICs of quinupristin/dalf opristin and its components were the same or one dilution higher in ly sed horse blood and HTM agar incubated in air, and were equivalent or one dilution lower in HTM agar incubated in a CO2 atmosphere. The MICs of quinupristin and dalfopristin separately or together were directly proportional to erythromycin MICs, but were otherwise unaffected by a ny of the resistance mechanisms represented in these strains. MICs of quinupristin and dalfopristin combined in ratios of 10:90, 70:30 and 9 0:10 did not differ significantly from those of the 30:70 ratio. Thus, unlike the synergic activity noted against Gram-positive bacteria, th e activity of quinupristin/dalfopristin against H. influenzae appears to be due almost entirely to the dalfopristin component of the combina tion.