Jh. Jorgensen et Ml. Mcelmeel, ACTIVITY OF QUINUPRISTIN DALFOPRISTIN AND ITS COMPONENTS AGAINST HAEMOPHILUS-INFLUENZAE/, Journal of antimicrobial chemotherapy, 39, 1997, pp. 69-73
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.